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Position Paper
Position Paper
C.J.M. Philippart & M.J. Baptist
Colophon
An explanatory study into effective measures to
strengthen diadromous fish populations in the Wadden
Sea is produced by the Wadden Academy.
A draft version was externally reviewed by Dutch
experts Zwanette Jager (ZiltWater Advies), Peter Paul
Schollema (Waterschap Hunze en Aa's), Henk van der
Veer (NIOZ) and Erwin Winter (IMARES) and by
three independent scientific experts from ICES, i.e.
Romuald Lipcius (Virginia Institute of Marine Science,
USA), Dennis Ensing (Agri-Food and Biosciences
Institute Northern Ireland, IRL), and Joey Zydlewski
(University of Maine, USA).
This document can be referred to as:
Philippart, C.J.M. & M.J. Baptist, 2016. An explanatory
study into effective measures to strengthen diadromous
fish populations in the Wadden Sea. Leeuwarden,
Waddenacademie, Position Paper 2016-02.
Contactperson Waddenacademie
Klaas Deen
Secretaris
t 058 233 90 31
e [email protected]
Ontwerp BW H ontwerpers
Fotografie Saskia Boelsums
Druk Hollandridderkerk
ISBN 978-94-90289-36-2
Volgnummer 2016-02
© Waddenacademie February 2016
www.waddenacademie.nl
De basisfinanciering van de Waddenacademie is
af komstig van het Waddenfonds.
CONTENTS
CONTENTS3
EXECUTIVE SUMMARY4
1. INTRODUCTION
10
1.1 Recent changes in Wadden Sea fish10
1.2 Audit
11
1.3 Target fish species
12
1.4 Long-term changes in the environment
13
1.5 Long-term changes in fish landings
17
2. POSSIBLE DRIVERS OF LOCAL FISH DENSITIES
2.1 Introduction 2.2 Time series on fish abundance
2.3 Statistical analysis survey data
2.4 Results & Discussion
2.5 Conclusions 18
18
20
23
24
49
3. FINDING OPTIMAL LOCATIONS FOR FISH MIGRATION FROM MARINE TO FRESHWATER SYSTEMS 3.1 Introduction 3.2 Estuarine gradients in the Dutch Wadden Sea 3.3 Fish densities in tidal basins of the Dutch Wadden Sea 3.4 Fish densities near small freshwater sluices discharging into the Wadden Sea 3.5 Conclusions 51
51
52
54
56
72
4. NUMERICAL BUDGETS OF LOCAL STOCKS 4.1 Introduction
4.2 Factors
4.3 Stock Budgets
4.4 Discussion & Conclusions
73
73
74
76
81
5. POTENTIAL MEASURES AND COSTS 5.1 Introduction
5.2 Fisheries 5.3 Brackish water zones 5.4 Freshwater habitats 5.5 Fish passages 5.6 Concluding remarks 73
84
85
86
89
90
93
ACKNOWLEDGEMENTS94
REFERENCES95
3
EXECUTIVE SUMMARY
Aim of the report
The strong decline in Wadden Sea fish since the
1980s has called for action to strengthen local fish
stocks. Proposed measures include the reduction of
fisheries, restoration of natural dynamics in habitats
and improving connectivity between marine and
freshwater areas for migratory fish. The Wadden
Academy was invited by the Waddenfonds to make an
inventory of the most effective measures to strengthen
(migratory) fish stocks in the Wadden Sea area in
general, and on the added values of further investments
in fish passages in particular. Hereto, the following
questions were defined:
1. To what extent were local stocks of migratory fish
in the Wadden Sea influenced by the construction
of local barriers and fish passages?
2.W hich local measures would be most promising
to strengthen the stocks of migratory fish in the
Wadden Sea, and what would be the optimal
locations for these measures?
3. W hat would be the costs and benefits of different
measures to strengthen the stocks of migratory fish
in the Wadden Sea?
Based on the function of the Wadden Sea for fish
migration between marine and freshwater systems,
the audit was focussing on 12 diadromous fish
species (i.e. River Lamprey, Sea Lamprey, European
Sturgeon, European Eel, Allis shad, Twaite Shad,
European Smelt, Atlantic Salmon, Sea Trout, Houting,
Three-spined Stickleback and European Flounder)
supplemented with 2 species that had large stock sizes
in the former Zuiderzee (i.e. European Anchovy and
Atlantic Herring). For all of these species, the Wadden
Sea is (or was) an essential area to fulfil a particular
phase in their life cycle, often during a particular time
of the year.
4
Impacts of local barriers and fish
passages
To assess the consequences of creating (e.g. Afsluitdijk
in 1932) and removing (e.g. fish passages) local barriers
for fish migration for local fish stocks, long-term and
consistent survey data sets of before and after the time
which these interventions took place are required. The
data should have the proper spatial scales and include
quantitative information on other factors that may
have additionally influenced local fish densities (e.g.,
fisheries, predation). Since such data do not exist,
available data and additional information was used to
at least qualitatively explore such impacts.
Developments in local fish stocks were compared with
large-scale trends (e.g. in the NE Atlantic) to explore
impacts of large-scale and local impacts, such as the
effect of the closure of former estuaries in the Wadden
Sea. Because scientific fish surveys in the Wadden Sea
did not start before the 1960s, data on registrations of
commercial fisheries that started around 1900 were
used (“Landings”). Within a shorter time frame (< 50
yrs), time series of surveys were analyzed to further
explore the nature of large-scale and local drivers, and
subsequently the present options for managing local
fish stocks (“Surveys”).
Landings
In the NE Atlantic, all of the 14 species targeted
in this study were landed (as catch or by-catch) by
commercial fishing activities between 1903 and 2013.
At this scale, migratory fish have strongly declined,
often to historic lows. Exploitation is a major factor,
on top of other human-induced (e.g. pollution, habitat
loss, blocked migration) and natural factors (e.g.
hydrographical fluctuations).
In Lake IJssel, the closure in 1932 was followed by
decrease in landings of Anchovy, Flounder, Herring
and Smelt, and a temporal increase in the landings of
European Eel. Eel possibly got stuck in the area
and/or no longer needed to migrate further upstream
the river. The decline in Eel since the 1950s remains
so far unexplained. Landings of Smelt reached
comparable levels in the 1980s as before the closure.
Ongoing fishing pressure contributed, however, to a
depletion of the local Smelt stocks in the 2000s.
In the western Wadden Sea, the closure of the
Afsluitdijk was followed by a temporal increase in the
landings of Herring and Flounder. This might have
been the result of an increase in fish abundances, but
possibly also of enhanced fishing efforts as fishermen
from the former Zuiderzee continued to fish in the
Wadden Sea after 1932.
In Dutch freshwater systems including main rivers,
landings of Allis Shad, Atlantic Salmon, Houting and
Sturgeon strongly declined between the 1890s and
the 1940s. These declines are attributed to habitat
destruction (e.g., river engineering, removal of sand
and gravel), deteriorated water quality (i.e. pollution)
and overfishing.
Surveys
For all species for which sufficient data was available,
local stocks in the Wadden Sea and Lake IJssel appear
to be more or less influenced by large-scale longterm drivers such as fisheries, habitat destruction and
climate. The impact of these large-scale drivers versus
the influence of local drivers on fish stocks varies,
however, between species and areas.
For European Eel, for example, synchrony in time series
from North Sea and the Wadden Sea suggest a largescale driver within the marine waters of the study area.
Such possible large-scale drivers include atmospherically
driven dispersal of juvenile Eel by ocean currents and
changes in hydrology and exploitation. Within Lake
IJssel, the effects of these large-scale climate-driven
influences may have been obscured by local impacts,
including the influence of the Afsluitdijk in 1932 and
changes in fishery pressure and restocking hereafter.
For Twaite Shad, stock dynamics appear to be
synchronised in the western and eastern part of the
Dutch Wadden Sea and the adjacent coastal zone.
As has been observed for the Scheldt estuary, stock
dynamics of Twaite Shad in estuarine tidal basins
such as the Marsdiep and the Ems might be driven by
environmental conditions at their freshwater spawning
grounds. Improvement of these conditions might then
enhance stocks in the Wadden Sea.
For European Smelt and European Flounder,
synchronisation appears to occur between the North
Sea and the westernmost (Marsdiep) and easternmost
(Ems estuary) tidal basins of the Dutch Wadden Sea.
Both basins are characterised by a strong salinity
gradient, suggesting freshwater discharges as a driving
factor for local stocks.
Conclusions on local barriers and fish
passages
Present data are insufficient to quantitatively analyse
the impacts of barriers on local stocks. Long-term
information on fish landings of the Wadden Sea and
Lake IJssel clearly illustrated, however, the impacts
of the closure of the Zuiderzee in 1932. Whilst some
species were negatively impacted (e.g. Anchovy),
others (e.g. Eel) increased temporally following the
closure. Strong fishing pressure finally resulted in
historical lows (e.g. Smelt). In the Wadden Sea, drivers
of local stocks appear to be large-scale (e.g. Eel) or
local (e.g. Twaite Shad, Smelt, Flounder). If fish stocks
are strongly influenced by local short-term drivers,
then improvement of local conditions might result into
strengthening of local stocks, within the boundaries set
by the large-scale long-term drivers.
5
Measures and optimal locations
Fish passages
The Dutch Wadden Sea area comprises 65 existing and
4 planned (or under construction) estuarine gradients.
These gradients vary from small tidal creeks at the
islands to large freshwater sluices along the mainland
coast. Existing gradients include freshwater sluices,
ship locks, pumps and culverts of which several are
structurally operated (8) or equipped with technical
facilities (17) to enhance fish migration from the
sea to freshwater systems. Identifying best practices
and finding optimal locations requires a quantitative
assessment of fish passing efficiency of existing fish
passages. Despite the generally high costs of building
these facilities, however, no comparable data to
perform a quantitative evaluation of the construction
of fish passages was available.
As an approximation of spatial variation in supply of
fish at small freshwater discharges, data on densities
of several migratory fish species at 15 discharge points
were analysed. Results indicated that local densities
are related to local circumstances, including (1) the
presence of sufficient flow of freshwater to attract
fish to the entrance location, (2) the closeness of the
entrance to deeper waters that have a salinity gradient,
and (3) the presence of a deep and wave-sheltered
basin to allow fish to safely wait before passing. Present
and future fish passage facilities should, therefore,
pay attention to selecting and/or creating such local
conditions, including optimal habitats before and after
passing.
At the Afsluitdijk, fish can pass to and from Lake
IJssel via discharge sluices and shiplocks at Den Oever
and Kornwerderzand. Based on a very limited smallscaled number of tagging experiments and several
assumptions, the passing efficiency at Kornwerderzand
under present regular discharge conditions (REG)
appears to be around 15% for River Lamprey and Sea
Lamprey, 50% for Sea Trout and Atlantic Salmon, and
60% for Houting. Fish migration has been temporally
stimulated by changing the discharge regime and by
enhancing the migration of fish using the ship locks at
Den Oever and at Kornwerderzand, the so-called fishfriendly management (FFM). Compared to regular
6
discharge management (REG), fish-friendly discharge
management (FFM) appeared to at least double
the passing efficiency for European Eel, European
Smelt, Three-Spined Stickleback and Flounder. This
is underlined by an observed increase in Flounder
in Lake IJssel following fish-friendly discharge
management in the early 1990s. An additional fish
passage is planned directly west of the discharge sluices
of Kornwerderzand, the so-called Fish Migration
River (FMR). For Sea Trout and Flounder, projected
passing via Fish Migration River (FMR) is comparable
as presently observed during regular discharge
management. Compared to fish-friendly discharge
management (FFM), projected migration via the
FMR is higher for Herring, Smelt and Three-Spined
Stickleback, and lower for Eel.
Projections for the FMR were based upon a passing
efficiency of 50% of fish present in a semi-enclosed
area before the sluices of Kornwerderzand, as was done
for the environmental assessment (MER). Taking
the species-specific variation at present into account,
an average added value of 50% most probably does
not correctly addresses future passing efficiencies
under FMR conditions for all target species. The Fish
Migration River could have added value to FFM,
but only when optimally designed. Existing data and
information are presently, however, not sufficient to
support design recommendations. However, making
the design adequate for testing the factors related
to attraction and passing efficiencies could gain this
necessary knowledge. This would not only be relevant
for strengthening migratory fish in the Wadden Sea
area, but also in other coastal systems with comparable
man-made barriers world-wide.
Fisheries
In the Wadden Sea, Flounder, Twaite Shad, River
Lamprey and Smelt are reported as by-catch from
shrimp fisheries. In Lake IJssel, both European
Eel and Smelt are fished (but fishing on Smelt is
temporally banned). Such fishing activities appear
to be an important source of mortality, both in the
Wadden Sea (Flounder, River Lamprey, Smelt, Twaite
Shad) and in Lake IJssel (European Eel, Smelt). In
the Marsdiep tidal basin, the by-catch from shrimp
fishing on diadromous fish is estimated to be almost
2.5 million fish per year, comprising of Twaite Shad
(213 000 fish per year), Smelt (1 400 000 fish per year),
Flounder (840 000 fish per year) and River Lamprey
(4500 per year). In Lake IJssel, catches were estimated
to comprise approximately 6 million Eels and 200
million Smelt per year between 2000 and 2014. For
these species, this fishing mortality appears to be in the
same order of magnitude as the projected migration
via the Fish Migration River.
Reduction of fishing efforts in Lake IJssel and the
Wadden Sea might, therefore, be an alternative or
additional measure to strengthen local fish stocks
within the study area as a whole. Reduction of bycatch by shrimp fishing in the Wadden Sea as a whole
would result in a decrease of fishing mortality of
approximately 4 million fish per year, and beneficial
for their predators (birds, seals). Reduction of the
shrimp fishing efforts might increase additional natural
values of the Wadden Sea (e.g. benthic organisms
and biogenic structures such as mussel beds), whilst
the yield might remain stable. Reduction of fishing
activities in Lake IJssel is not only expected to favour
local fish stocks, but also as an important source of fish
(in particular Smelt) for local stocks in the Wadden
Sea, strengthening fish stocks in the study area as a
whole.
Habitats
Once fish has passed from the sea to the freshwater
system, it should be able to reach its final destination
and to close its life cycle. In general, this implies that
connectivity between all tributaries, polder ditches and
brooks are important to population sizes of migratory
fish. In addition, fish might require particular
conditions in freshwater habitats, e.g. to spawn.
For Twaite Shad, for example, suited conditions for
spawning can probably be found in (side channels of )
the upstream Ems, a river presently characterized by
high fluid mud concentrations and low oxygen levels
during summer. Water and habitat quality of the river
Ems needs to be improved drastically before becoming
an important spawning ground.
For Sea Lamprey, local habitat improvement of
potential spawning habitats in small streams in (sidechannels of ) larger rivers such as the Ems and IJssel
can be achieved by re-meandering. Studies into River
Lamprey show that both safeguarding of the habitat
quality and improving the connectivity between
various tributaries and the sea is required.
Within the Wadden Sea region, most estuarine
gradients are presently characterised by an abrupt
salinity gradient. In general, strongly developed
salinity gradients appear to favour the estuarine
recruitment of marine fish species. In former days,
for example, Anchovy, Herring, Smelt and Flounder
used the large open brackish water bodies with tidal
influence in this area to spawn, to grow and to mature.
In addition to protection and improvement of
remaining estuarine gradients (e.g. small creeks and
the Ems estuary), restoration of former gradients
may aid in strengthening fish stocks in the Wadden
Sea area. On the mainland bordering the Dutch
Wadden Sea, several large freshwater bodies exist that
potentially could be turned into brackish ecosystems,
i.e. Lauwersmeer (Marnewaard), Amstelmeer
(including the Balgzandkanaal) and the northern part
of Lake IJssel. Alternatively, the southern part of the
Marsdiep tidal basin could be made more permanently
under the influence of freshwater discharges.
Construction of brackish zones within freshwater
systems should be preceded by feasibility studies,
however, including an assessment of the behaviour
of salt water, and possible consequences for other use
of the water (e.g. agricultural, drinking). To come
up with an optimal design, additional measurements
on flows, tides, salinity gradients and fish passage
rates under present conditions might be required. If
a construction of a more permanently brackish zone
at the northern side of the Afsluitdijk is considered,
potential impacts of pumps on migrating fish from
freshwater to the sea should be kept to a minimum
(e.g. by means of using fish-friendly pumps) and an
inventory of possible effects of additional freshwater
discharges on local species and habitats in this zone
should be made.
7
Conclusions on measures and optimal
locations
Most promising potential measures to strengthen local
fish stocks and other natural values of the Wadden
Sea region include the reduction of fishing efforts, the
provisioning of suitable habitats (such as brackish zones
and freshwater spawning grounds) and the facilitation
of fish migration.
Reduction of fishing efforts in the Wadden Sea and in
Lake IJssel is expected to be beneficiary for migratory
fish stocks, their predators and bottom life in the
whole Wadden Sea region. Present natural estuarine
gradients should be safeguarded and, if necessary (e.g.
Ems), be improved for provisioning suitable habitats
for migratory fish. The Lauwersmeer (Marnewaard),
Amstelmeer (including the Balgzandkanaal), the
northern part of Lake IJssel and the southern part
of the Marsdiep tidal basin are potentially suited
for turning into large brackish habitats, but actual
suitability still needs to be checked by means of
feasibility studies.
Fish migration could be further facilitated by means
of improving the connectivity with freshwater
systems, and between freshwater and the sea. Potential
measures include fish-friendly discharge management
and fish passages, ranging from relatively simple (e.g.
fish ladder) to very complex (e.g. Fish Migration
River) solutions. At present, the efficiency of such fish
passages cannot be quantified due to a lack of data.
The set-up of a Migratory Fish Testing Facility, e.g.
within the area as presently reserved for the FMR,
would enable exploring the factors related to attraction
and passing efficiencies of various types of technical
solutions in the Wadden Sea region as a whole.
8
Benefits and costs
With regard to benefits and costs in relation to
strengthen local fish stocks only indications can be
given. Mainly because the effectivity of the proposed
measures like the reduction of fisheries, restoration
of natural dynamics in habitats and improving
connectivity between marine and freshwater areas for
migratory fish are only know within large margins of
uncertainty and the costs of the various measures also
show a great variety. Subsequently, it is very difficult
to estimate the cost effectiveness of the reduction of
fisheries, restoring habitats and for different options for
fish passing the Afsluitdijk (see “Measures and optimal
locations”) and the same holds for monetary valuation
of the improvement in fish stock.
However, several types of costs and benefits can be
distinguished and for some a monetary indication can
be given. First of all, an increase in fish stocks has an
intrinsic value in terms of a more natural ecosystem.
In addition to that, this may in the medium and long
run also have positive effects for the potential stocks of
fish available for fishing but maybe at a smaller scale.
The reduction of fisheries would deprive fishing and
processing companies of a source of income, with
annual foregone earning being estimated to be around
25 million Euro per year for shrimps in the Wadden
Sea and approximately 2 million Euro per year for Eel
and Smelt in Lake IJssel.
As far as could be derived from present information,
the initial one-time investment costs of the technical
solutions to create large brackish zones would be
approximately 3 million Euro for Marnewaard
(Lauwersmeer) and 40 million Euro for the northern
part of Lake IJssel. One time investment costs
(excluding maintenance costs) of technical solutions
for fish passages range between approximately 20
thousand Euro for simple technical solutions (e.g. flaps
and gates in sluice doors), 50 thousand Euro for more
complicated facilities (e.g. fish ladders and spill ways),
500 thousand Euro for fish-friendly by-pass pumps, 4
million Euro for fish passages with salt water return
flow systems, and 52 million Euro for the large and
complex Fish Migration River. As already indicated,
however, the efficiency of the existing fish passages
and subsequently the identification of best practices
(“value for money”) could not be quantified due to
lack of information for the Wadden Sea region.
Migratory Fish Testing Facility,
Monitoring & Modelling
Due to the lack of data, at present a quantitative
assessment of the effectivity of existing fish passages
in the Wadden Sea was not possible. With regard to
the investment costs of building these structures, it is
therefore strongly recommended to not only invest in
the strengthening of fish migration by construction of
fish passing facilities, but also to invest in appropriate
testing and monitoring over a longer time period.
Testing and monitoring is required for identification
of best practices by means of quantitative evaluation
of functioning of present facilities, for potentially
allowing modifications in original designs to optimize
the investments made, and for more adequate future
investments.
under various environmental conditions such as the
creation of large brackish zones, climate change and
sea level rise. Given the great interest and considerable
investments in fish passages at a global scale, the
insights derived from the Fish Migration Testing
Facilities will lead to more efficient and effective
investments in fish passages in the Wadden Sea and the
rest of the world.
Testing and monitoring programs should include
measurements of (1) the numbers and characteristics
(e.g., species, life phases, condition of fish passing),
(2) the efficiency of the attraction flow, (3) the
efficiency of the fish passage and, for larger fish
passages, (4) the habitat use of the local area. Proper
testing of the effectivity of different technical
solutions can be done by means of experiments under
set environmental conditions (e.g., indoor climatecontrolled basins), or by experiments under natural
conditions. The latter approach would require more
replicates (i.e. the number of distinct and in principle
similar experimental units) than the first approach
to be able to extract the signal of treatment from the
environmental noise.
To estimate the added value of a new fish passage,
monitoring should be performed at all other
surrounding fish passages before and after a new
passage is constructed. This would require an
integrated monitoring scheme for migratory fish in the
Wadden Sea. With standard techniques, equipment
and protocols, and addressing the preferred pathways
of migratory fish species at various spatial (e.g.,
freshwater discharge points, tidal basins) and temporal
(tide, season, year-to-year) scales. Incorporating the
migratory behavior of fish into hydrodynamic models
could produce biophysical models of fish movements
through the Wadden Sea. Such models would allow
for building scenarios for optimizing investments to
strengthen (migratory) fish stocks in the Wadden Sea
9
1. INTRODUCTION
1.1 R
ecent changes in Wadden
Sea fish
Many fish species rely on shallow coastal habitats for
at least one of their life stages. A suite of marine fish
species reaches these areas as postlarvae and spend
their juvenile phase here (Zijlstra 1972, 1978; Elliott
et al. 2007; Van der Veer et al. 2000, 2015). Other
species temporarily inhabit the area or pass it on
route to either marine or freshwater spawning sites
(diadromous species), during certain times of the year
or occasionally (Elliott et al. 2007). In addition to such
temporary visitors, some species spend (almost) their
entire life in the shallow waters (Elliott & Hemingway
2002). Naturally such coastal areas support large
numbers of fish that make use of the suitable habitats
characterised by a high food availability and shelter
from predators (Gibson 1994; Elliott & Hemingway
2002, Tulp et al. 2015).
Structural monitoring of the fish fauna in the Dutch
Wadden Sea takes place since 1960-1970 by two major
monitoring programs: a fyke program in the western
Wadden Sea and an annual beam trawl survey (DFS)
covering the entire Dutch Wadden Sea. Results from
these surveys show that total fish biomass increased
from 1970 to 1980, had a peak in the mid-1980s that
was followed by a strong decline especially from 19802000, with a stable trend since then in all tidal basins
(Bolle et al. 2009; Jager et al. 2009; Tulp et al. 2015).
The trends in the Dutch coastal zone differ from those
in the Wadden Sea, especially in the past 10 years,
with an ongoing decline in the Dutch Wadden coast
and an increase along the mainland North Sea coast
(Van der Veer et al. 2015; Tulp et al. 2015). The strong
decline in Wadden Sea fish since the 1980s has called
for action to strengthen local fish stocks. Proposed
measures include the reduction of fisheries, wise sand
nourishments, restoration of natural dynamics in
habitats and improving connectivity between marine
and freshwater areas for migratory fish (Van der Veer
& Tulp, 2015).
Legenda
Ondergrens transitie duurzame energiehuishouding NH
Waddengebied
Provinciegrenzen (2010)
Gemeentegrenzen (2006)
Gemeentegrenzen (2010)
Schier
nikoog
mon
Ameland
Eemsmond
Delfzijl
Terschelling
De Marne
Dongeradeel
Winsum
Ferweradiel
het
Bildt
Groningen
d
lan
e
Vli
Franekeradeel
Harlin
gen
Oldambt
Texel
Friesland
Sudwest
Fryslân
Den
Helder
Anna
Paulowna
n
ier
W
e
ing
Wieringermeer
Hollands
Kroon
Zijpe
Schagen
Harenkarspel
Andijk
Noord-Holland
Langedijk
Wervershoof
Wognum
Obdam
Heerhugowaard
Koggenland
Hoorn
Artikel 2
Het oorspronkelijke werkingsgebied zoals gedefinieerd in de Wet op het Waddenfonds blijft na decentralisatie van de middelen uit het Waddenfonds gehandhaafd,
te weten: de Waddenzee, de Waddeneilanden, de zeegaten tussen de eilanden, de Noordzeekustzone tot 3 zeemijl uit de kust, alsmede het grondgebied van de
aan de Waddenzee grenzende vastelandsgemeenten. Voor de transitie naar een duurzame energiehuishouding omvat het werkingsgebied tevens de provincies Fryslân,
Groningen en de kop van Noord-Holland voor zover het betreft de gemeenten Hoorn, Wognum, Opmeer, Opdam, Heerhugowaard, Langedijk, Harenkarspel, Zijpe en
het gebied ten noorden daarvan. In geval van gemeentelijke herindeling geldt voor de toepassing van deze regeling het grensbeloop zoals dat ten tijde van het inwerkingtreden
van de Wet op het Waddenfonds van kracht was.
Bestuursakkoord Decentralisatie Waddenfonds, Harlingen, 14 september 2011, pagina 6.
25.000
Fig. 1.1 Boundaries of the area covered by the Waddenfonds (red), i.e. the
Wadden Sea, the islands, the tidal inlets, the North Sea coastal zone (up to
3 nautical miles from the coastline) and the municipalities at the mainland
bordering the Wadden Sea (www.waddenfonds.nl).
10
m
Datum: 6 september 2012
Papier: A3
Bron: provincie- en gemeentegrenzen, Topografische Dienst Nederland
BEL/ K & B: C_201111_2238
1.2 Audit
The Wadden Academy was invited by the
Waddenfonds to make an inventory of the most
effective measures to strengthen (migratory) fish
stocks in the Wadden Sea. Many parties are coming
forward to request funding for building fish passages,
including a large fish passage in the Afsluitdijk (52
MEuro in total, of which a part requested from the
Waddenfonds). With already several projects on fish
migration funded, the Waddenfonds requested advice
on the added value of additional investments in fish
passages compared to other measures that could be
taken to strengthen migratory fish populations to
improve the ecological quality of the Wadden Sea.
More particularly, the following questions were
defined:
1. To what extent were local stocks of migratory fish
in the Wadden Sea influenced by the construction
of local barriers and fish passages?
2.W hich local measures would be most promising
to strengthen the stocks of migratory fish in the
Wadden Sea, and what would be the optimal
locations for these measures?
3. W hat would be the costs and benefits of different
measures to strengthen the stocks of migratory fish
in the Wadden Sea?
Measures in consideration are restricted to the area
covered by the Waddenfonds, being the Wadden Sea,
the islands, the tidal inlets, the North Sea coastal zone
(up to 3 nm from the coastline) and the municipalities
at the mainland bordering the Wadden Sea (www.
waddenfonds.nl; Fig. 1.1). This implies that potential
measures in other areas (e.g. the North Sea and main
rivers such as the Rhine and Meuse) will not be
addressed in this report. Measures at the boundaries of
the Waddenfonds area (e.g., Lake IJssel) will only be
discussed if they might have a direct influence on the
fish stocks within the study area.
Local experts and scientists from ICES (International
Council for the Exploration of the Sea) have reviewed
the draft report in November 2015. The first results
were presented in December 2015 to the Executive
Board of Waddenfonds. The final report was presented
to the EB of the Waddenfonds in February 2016.
COMMON NAME
River Lamprey
Sea Lamprey
European Sturgeon
European Eel
European Anchovy
Atlantic Herring
Alis shad
Twalte shad
European Smelt
SCIENTIFIC NAME
Lampetra fluviatilis
Petromyzon marinus
Acipenser sturio
Anguilla anguilla
Engraulis encrasicolus
Clupea harengus
Alosa alosa
Alosa fallax
Osmerus eperlanus
FAMILY
Petromyzontidae
Petromyzontidae
Acipenseridae
Anguillidae
Engraulidae
Clupeidae
Clupeidae
Clupeidae
Osmeridae
GUILD
Diadromous
Diadromous
Diadromous
Diadromous
Marine seasonal
Marine juvenile
Diadromous
Diadromous
Diadromous
MONTH
10 – (12) – 3
3 – (4 – 5)
4 – (6 – 7) – 9
2 – (3 – 5) – 6
4 – (5) – 6
3 – (4 – 5) – 7
4 – (5) – 6
3 – (4 – 5) – 6
2 – (3 – 4) – 5
M.
0.22
0.14
0.05
0.23
1.00
0.60
0.35
0.31
0.22
SOURCE
Winter et al. 2014
Winter et al. 2014
Winter et al. 2014
Winter et al. 2014
Redeke (1939)
Redeke (1939)
Winter et al. 2014
Winter et al. 2014
Winter et al. 2014
Atlantic Salmon
Sea Trout
Salmo salar
Salmo trutta
Salmonidae
Salmonidae
Diadromous
Diadromous
5 – (6 – 11) – 12
5 – (6 – 11) – 12
0.44
0.28
Winter et al. 2014
Winter et al. 2014
Houting
Three-spined
Stickleback
European Flounder
Coregonus oxyrinchus
Salmonidae
Diadromous
10 – (11) – 12
0.57
Winter et al. 2014
Gasterosteus aculeatus
Gasterosteidae
Diadromous
2 – (3 – 4) – 5
2.96
Winter et al. 2014
Platichthys flesus
Pleuronectidae
Diadromous
3 – (4 – 6) – 7
0.41
Winter et al. 2014
Table 1.1 Overview of fish species targeted in
this report, their main migration window (see
last column for references) and mortality rate
(www.fishbase.se).
11
1.3 Target fish species
Based on the function of the Wadden Sea for fish
migration between marine and freshwater systems,
the audit is focussing on 12 diadromous fish species
supplemented with 2 species that had large stock sizes
in Lake IJssel, i.e. European Anchovy and Atlantic
Herring (Table 1.1). For all of these species, the
Wadden Sea is (or was) an essential area to fulfil a
particular phase in their life cycle, often at a particular
time of the year. For the diadromous fish species,
this shallow coastal sea functions as a corridor
between freshwater systems and the open North Sea
when adults and juveniles migrate to and from their
spawning areas (Winter et al. 2014; Heessen et al.
2015).
YEAR
CHANGE
DIKE (KM)
The former Zuiderzee was a major spawning area
for Anchovy (Petitgas et al. 2012), and the Anchovy
larvae stayed in the area until October after hatching
(Redeke 1939). In 1994, eggs of this fish species were
found north of the Afsluitdijk (Boddeke & Vingerhoed
1996). In May 2010 and May 2011, schools of Anchovy
were found in a pelagic survey in the Marsdiep area.
Their mean length (14.5 cm) corresponded to age-1
fish that are likely to spawn. Hence, for the Wadden
Sea area is important both as nursery as well as for
spawning (Couperus et al 2016). Before 1932, the
Zuiderzee might have harboured a subpopulation
of Zuiderzee Herring, which entered the Zuiderzee
from March until July for spawning (Redeke 1939).
In 2006, Dutch fishermen landed 800 kg of Herring
of which the morphology resembled that of Zuiderzee
Herring (Witte, NIOZ), implying that a remnant
stock might still be present.
NEW
HABITAT
NAME
AREA
(KM2 )
Lake
Polder
Polder
Lake
Amstelmeer
Andijk
Wieringermeer
IJsselmeer
7
<1
200
3440
Tidal basins
Marsdiep, Eijerlandse Gat &
Vlie
1513
Polder
Polder
Polder
Lake
Lake
Noordoostpolder
Eastern Flevoland
Southern Flevoland
Markermeer
IJsselmeer
480
540
430
700
1100
Lake
Polder
Lauwersmeer
Lauwersmeer
Tidal basins
Lauwers & Eilander Balg
24
67
183
Polder
Polder
Carel Coenraad
Breebaart
Zuiderzee (4953 km 2 before 1924)
1924
1927
1930
1932
1942
1957
1967
1975
Dike built
Land reclaimed
Land reclaimed
Dike built
Land reclaimed
Land reclaimed
Land reclaimed
Dike built
3
2
18
32
55
90
70
28
Lauwerszee (274 km 2 before 1969)
1969
Dike built
13
Ems estuary (460 km at present)
1924
Land reclaimed 15
1979
Land reclaimed 3
2
Table 1.2 Overview of major fixed changes in former estuaries Zuiderzee
and Lauwerszee and the present Ems estuary during the past 100 years
(1916-2015). Sources: Herman et al. (2009), www.britannica.com,
https://en.wikipedia.org/wiki/Zuiderzee_Works, https://en.wikipedia.
org/wiki/Lauwersmeer, Baptist & Philippart (2015).
12
15
<1
Fig. 1.2 Maps of the western part of the study area. Left:
Navigational map of the Zuiderzee in 1921, as made
by M.H. Blokpoel of the Royal Dutch Meteorological
Institute (originally printed by Roeloffzen-Hübner en Van
Santen, Amsterdam; sold at the J.L. Willem Seyffardt
bookshop, Amsterdam). Right: Map of the Wadden Sea,
Lake IJssel and Markermeer in 2015 as derived from
Open Street Map, layout Humanitarian Data Model,
with present-day coastline, tidal basins, dikes and polders
(see Table 1.3 for details).
1.4 Long-term changes in the
environment
As the result of anthropogenic pressures such as
fisheries, the construction of physical barriers between
the marine and fresh waters, river channelization,
deterioration of water quality and habitat destruction,
fish stocks in coastal ecosystems including the Wadden
Sea have been degrading from the medieval time
onwards, with acceleration during the last 150 to 300
years (Wolff 2000; Jackson et al. 2001; Lotze et al.
2005, Jager et al. 2009).
During the past 100 years, for example, the Dutch
part of the Wadden Sea was irreversibly impacted
by damming and land reclamation (Table 1.2).
Parts of former estuaries, such as the Zuiderzee
and Lauwerszee, were closed off from the sea by
means of dams and turned into freshwater lakes (i.e.,
Amstelmeer, IJsselmeer, Lauwersmeer). After the
closure in 1932, large parts of Lake IJssel (IJsselmeer
in Dutch) were reclaimed, whilst the lake itself was
further subdivided into two freshwater basins in 1975
(Fig. 1.2; Table 1.2). This reduction and changes in
habitat has most probably affected diadromous fish
stocks. If considered to be irreversible (e.g. as the
result of their role in safety against flooding or the
high economic values of agriculture and freshwater
supply), this limits the potential for strengthening local
diadromous fish stocks.
13
300
Sea Lamprey
0
1930
1950
1970
1990
2010
1890
1910
1930
1950
1970
1990
2010
1890
1910
1930
1950
1970
1990
2010
1890
1910
1930
1950
1970
1990
2010
1890
1910
1930
1950
1970
1990
2010
1890
1910
1930
1950
1970
1990
2010
1890
1910
1930
1950
1970
1990
2010
European Eel
3000
1000
0
0
0
1910
1930
1950
1970
1990
2010
1890
1910
1930
1950
1970
1990
2010
1890
1910
1930
1950
1970
1990
2010
1890
1910
1930
1950
1970
1990
2010
1890
1910
1930
1950
1970
1990
2010
1890
1910
1930
1950
1970
1990
2010
Twaite Shad
8
6
4
2
0
0
0
1000
500
0
0
0
350
20
Sea Trout
Houting
0
0
European Flounder
3000
5000
100
150
20000 30000
3−Spined Stickleback
14
1000
0
0
0
0
5000
50
10000
10000 15000 20000
0
50
5
500
150
10
1000
250
15
1500
2000
0
500
2000
5000
Atlantic Salmon
2000
1500
2500
3500
6000
10000
European Smelt
15000
25000
0
50
50
50
150
250
150
250
100 150 200
350
250
0
500000
1000000
Atlantic Herring
1500000
1890
10
40000
0
350
Allis Shad
0
2000
4000
6000
0e+00 1e+06 2e+06 3e+06 4e+06
European Anchovy
80000
120000
0
5
5
5000
10
10
10000
Atlantic Sturgeon
15
15000
1910
15
1890
5000
0
5
50
10
100
15
20
200
25
30
35
River Lamprey
Fig. 1.3 Annual landings
(tonnes live weight) of 14
fish species as caught in the
Northeast Atlantic (FAO
fishing area 27; black dots
and left y-axis) and the
North Sea (FAO area 27
subarea IV; white dots and
right y-axis) from 1903 to
2014. The dashed vertical
lines indicates the year 1932
when the Afsluitdijk was
closed, the dotted lines the
closure of the Amstelmeer
and Lauwersmeer in 1924
and 1969, respectively.
Source: ICES database on
www.ices.dk.
1.5 L
ong-term changes in fish
landings
Comparison of fish stock developments within the
study area with large-scale developments (e.g. in the
NE Atlantic) might help to unravel large-scale and
local impacts on fish stocks, such as the effect of the
closure of former estuaries in the Wadden Sea. Fish
surveys in the Wadden Sea did not start before the
1960s (see paragraph 1.1), however, which is long after
major changes in fish stocks and habitats took place.
Another source of information may be found in fish
landings, which are registered since the beginning of
the previous century. These data sets provide a longterm (> 100 years) view on the (changes in) intensity
of fishery-induced mortality on fish stocks. Long-term
changes in landings may also potentially reflect longterm changes in fish stocks or changes in the fishery
management. Commercial catch data are usually
collected systematically and consistently over long
periods, and the data tend to be less noisy than survey
data because of the much greater sampling effort.
However, proper acknowledgement of potential biases
(e.g. misreporting, interaction between fishing fleets,
technological improvements, commercial value of the
fish species etc.) is a prerequisite when performing
such interpretations (Engelhard et al. 2011).
NE Atlantic and North Sea (1903-2013)
As can be derived from catch statistics as supplied by
International Council for the Exploration of the Sea
(ICES; www.ices.dk), all of the 14 species targeted in
this study are or were landed (as catch or by-catch)
by commercial fishing activities in the North-East
Atlantic (Fig. 1.3). Within the North Sea, however,
some species were not caught (River Lamprey, Sea
Lamprey, Sturgeon, Allis Shad and Houting) or hardly
caught (Twaite Shad, Three-spined Stickleback)
(Fig. 1.3).
On both sides of the Atlantic Ocean, diadromous
fishes have strongly declined, often to historic lows
(Limburg & Waldman 2009). Exploitation is a major
factor (on top of hydrographic fluctuations influencing
larval transport, pollution, blocked migration, diseases)
in European eel decline (Dekker 2004a) and the nearly
complete demise of the once-widespread European
Sturgeon (Williot et al. 2002). In addition, habitat
loss, pollution, climate change, and replacement with
non-native species (hybridization, escapees from
aquaculture) contributed to declines in this group
(Limburg & Waldman 2009).
15
Zuiderzee (1892-2013)
For landings of fish caught in the northern and
southern parts of former Zuiderzee before and after
the closure in 1932 (Fig. 1.4), data sets from different
periods and sources were combined, i.e. southern
part in 1892-1906 from Redeke (1907), southern part
in 1925-1938 from Redeke (1939), southern part in
1966-2012 for Smelt and Flounder from De Boois et
al. (2014), southern part in 1907-1924 and in 19392013 for Eel from De Graaf & Deerenberg (2015),
and northern part and all other years for the southern
part from Fish Auction data as supplied by Jaap Quak
(Sportvisserij Nederland).
some were more extreme such as landings of Smelt
from Lake IJssel in 1968/1969 of 1588/1329 tonnes by
De Boois et al. (2014) and 103/38 tonnes as supplied
by Jaap Quak, possibly as the result of including bycatch or not.
In landings from the western Wadden Sea, a
temporally increase of Herring and Flounder was
observed (Fig. 1.4). This might be the result of an
increase in fish abundances, but also of an increase in
fishing efforts as fishermen from the former Zuiderzee
continued to fish in the Wadden Sea after 1932 (Wolff
2005). In local landings from Lake IJssel, the closure
of the Afsluitdijk in 1932 was followed by more or less
abrupt decreases in landings of Anchovy, Flounder,
Herring and Smelt, and a temporally increase in the
landings of European Eel (Fig. 1.4).
It must be noted that where data sets of different
origin overlapped, values on landings were not always
similar. Whilst most of these differences were minor,
3000
1000
0
0
1950
1970
1990
2010
1890
1910
1930
1950
1970
1990
2010
1890
1910
1930
1950
1970
1990
2010
1890
1910
1930
1950
1970
1990
2010
1500
2500
European Smelt
0
100
1000
200
2000
300
500
3000
400
European Flounder
0
0
0 2000
0 500
5000
10000
Atlantic Herring
6000
10000
1500
2500
3500
1930
0
200
150
100
50
1910
15000
1890
1890
16
5000
European Anchovy
1000 2000 3000 4000
200
600
1000
250
European Eel
1910
1930
1950
1970
1990
2010
Fig. 1.4 Annual landings (tonnes live weight) of five fish
species as caught in the northern part of the former Zuiderzee
(western Wadden Sea since 1932; black dots) and southern
part of the former Zuiderzee (Lake IJssel since 1932; white
dots). The dashed vertical lines indicates the year 1932
when the Afsluitdijk was closed, the dotted lines the closure
of the Amstelmeer and Lauwersmeer in 1924 and 1969,
respectively. Sources: Redeke (1907), De Boois et al.
(2014), De Graaf & Deerenberg (2015) and Jaap Quak.
Anchovy immediately disappeared after the closure
(Fig. 1.4). The loss of Herring spawning grounds
in 1932 led to the nearly complete disappearance of
the Herring stock within a few years (Wolff 2005).
Flounder did not fully disappear after the closure
(Fig. 1.4), possibly because of its tolerance to low
salinities (Winter 2009). Landings of Smelt reached
comparable levels as before the closure (Fig. 1.4). Smelt
has developed a land-locked population (Phung et
al. 2015). Ongoing fishing pressure on Smelt in Lake
IJssel contributed, however, to a depletion of the local
stocks (Masterplan Toekomst IJsselmeer 2014).
(Willem Dekker, pers. comm.). Alternatively, owing
to the freshening of Lake IJssel, Eel no longer needed
to move away from the brackish Zuiderzee into rivers
and inland waters (Redeke 1939). The cause of the
subsequent decline of the local stock of European Eel
in Lake IJssel is still unexplained (Dekker 2004b).
Worldwide there are multiple causes identified that
have adverse effects on eels but the relative importance
of each cause is not known yet (Kettle et al. 2011).
Several freshwater fish and fisheries benefited from the
new conditions after the closure. Landings of Pikeperch (Sander lucioperca), for example, rose from 111
kg in 1933 to more than 125.000 kg in 1938 (Redeke
1939).
The initial increase in Eel catches in Lake IJssel after
1932 might have resulted from an inability for Eel to
take advantage of selective tidal transport for migrating
upstream and, therefore, Eel got stuck in Lake IJssel
1940
1960
1980
2000
2020
1900
1920
1940
1960
1980
2000
2020
100000
40000
1920
1940
1960
1980
2000
2020
1900
1920
1940
1960
1980
2000
2020
Houting
0
10000
Rhine (kg/yr)
30000
Atlantic Salmon
1900
3000
1920
1000
1900
0
Rhine & Meuse (n/yr)
0
200
400
600
Rhine & Meuse (n/yr)
800
Allis Shad
0
The Netherlands (n/yr)
Atlantic Sturgeon
Fig. 1.5 Annual landings (number or kg) of four fish species
caught in large rivers of The Netherlands. The dashed vertical
lines indicates the year 1932 when the Afsluitdijk was closed,
the dotted lines the closure of the Amstelmeer and Lauwersmeer
in 1924 and 1969, respectively.
Source: www.compendiumvoordeleefomgeving.nl.
17
Rivers (1892-1954)
Data on landings of fish caught in Dutch tributaries
such as the rivers Rhine and Meuse (Fig. 1.5) were
taken from a Dutch governmental website supplying
basic information on the natural environment of The
Netherlands (www.compendiumvoordeleefomgeving.
nl). Landings of Allis Shad, Atlantic Salmon, Houting
and Sturgeon as caught in Dutch freshwater systems
including main rivers strongly declined between the
1890s and the 1940s (Fig. 1.5). These declines are
attributed to habitat destruction (e.g., as the result
of river engineering, removal of sand and gravel),
deteriorated quality of the water (i.e. pollution) and
overfishing (De Groot 2002).
1.5 Outline of the report
To quantify impacts of barriers and passages on
fish populations in the Wadden Sea, long-term and
consistent survey data sets of the period during which
these interventions took place (e.g., the construction
of the Afsluitdijk in 1932) are required, at the proper
spatial scales and including quantitative information
on other factors that may have influenced local fish
densities (e.g., fisheries, predation). Since such data
do not exist, we have used available data sets and
additional information to at least qualitatively explore
such impacts. Limits of using these data sets for this
purpose are addressed throughout the report.
In Chapter 2, annual fish surveys at different spatial
scales were compared to see if year-to-year densities
of fish in the tidal basins of the Wadden Sea were
dominated by local variation, possibly including local
effects such as barriers and passages, or by underlying
variation at larger spatial scales (possibly indicating
larger-scale drivers of variation).
In Chapter 3, the abundance of migratory fish species
in the Wadden Sea at a tidal basin scale and at local
scale (small freshwater sluices and fish passages) was
examined to explore the spatial variation in potential
for strengthening populations of migratory fish by
means of additional fish migration projects.
In Chapter 4, information on local densities, sources of
mortality (fisheries, predation by fish, birds and seals)
and transport via sluices, ship locks and the proposed
Fish Migration River are compared to identify the
relative contribution of these factors to changes in fish
populations in the Wadden Sea and Lake IJssel.
In Chapter 5, an overview is given of possible
measures to strengthen populations of migratory
fish in the Wadden Sea (o.a. by reducing present
threats), including an indication of costs and possible
effectiveness.
18
2. POSSIBLE DRIVERS OF LOCAL
FISH DENSITIES
2.1 Introduction
Long-term changes in fish stocks are influenced by
a suite of drivers, including fisheries pressure and
environmental factors such as sea surface temperatures,
and interactions between these drivers (Pauly et al.
2002). Correlations between simultaneously measured
time series can help to provide insight into possible
driver-response relationships. If the data were perfect
(noise-free, bias-free, infinitely long), we should be
able to detect, at least in some cases, which of the
coupled systems is the driver (or at which scale the
driver operates) and which is the response (Fig. 2.1).
Synchronization in population sizes that takes place
over large spatial scales may point to common
large-scale drivers such as climatic conditions. Even
locally regulated populations can be synchronized
by large-scale environmental shocks (Moran 1953).
Synchronous dynamics on smaller spatial scales can
result from spatial trends in regional variables. For
instance, regional trends in food availability, predation
and coastal fisheries could lead to distinct regional
synchronization patterns. Synchronous dynamics
among nearby stocks can also result from distancelimited dispersal of fish between stocks (Bjørnstad et
al. 1999; Ranta et al. 2006).
Analyses of synchrony in time series at different spatial
scales might help to unravel the nature of drivers, and
eventually the possibilities and limitations for local
management of local fish stocks, e.g. by means of
facilitating fish migrations.
Fig. 2.1 Possible spatial relationships between
drivers and synchrony in time series.
19
Table. 2.1 Overview of surveys and their
characteristics as used in this chapter.
20
Eijerlandse Gat
Vlie
Borndiep
Zoutkamperlaag
Lauwers/Schild
Ems estuary
Spijk
Oterdum
Terborg
Lake IJssel trawl
Lake IJssel 01
Lake IJssel 02
Den Oever 1
Den Oever 2
Harlingen
Lauwersmeer
Termunterzijl
Nieuwe Statenzijl
WS_T612
WS_T616
WS_T617
WS_T618
WS_T619
WS_T620
WS_SSP
WS_SOT
WS_STB
LIJ_TLIJ
LIJ_F01
LIJ_F02
WS_LD1
WS_LD2
WS_LHA
WS_LLA
WS_LTZ
WS_LNZ
Glass eel
Glass eel
Glass eel
Glass eel
Glass eel
Glass
WFD
WFD
WFD
WFD
WFD
WFD
DFS
DFS
DFS
DFS
DFS
DFS
Marsdiep
WS_T610
Dutch coastal zone
NS_T404
NSER
WS_FMD Marsdiep tidal inlet
North Sea Ecoregion
NS_TER
SURVEY
DFS
NIOZ
Fyke
DFS
NAME
CODE
Wadden Sea
Wadden Sea
Wadden Sea
Wadden Sea
Wadden Sea
Wadden Sea
Lake IJssel
Lake IJssel
Lake IJssel
Wadden Sea
Wadden Sea
Wadden Sea
Wadden Sea
Wadden Sea
Wadden Sea
Wadden Sea
Wadden Sea
Wadden Sea
Wadden Sea
Wadden Sea
North Sea
North Sea
AREA
1996-2014
1991-2014
1979-2014
1992-2014
1995-2014
1938-2014
1994-2013
1994-2013
1966-2012
2006-2014
2006-2014
2007-2014
1970-2014
1970-2014
1970-2014
1970-2014
1970-2014
1970-2014
1970-2014
1960-2014
1970-2014
1977-2013
YEARS
Juv.
9 – 10
9 – 10
5+9
N h (80m )
N net-1
N net-1
1 m 2 lift net
1 m 2 lift net
Glass eel
Glass eel
4–5
4–5
Glass eel
4–5
N net
1 m lift net
Glass eel
4–5
-1
N net
2
Glass eel
-1
1 m lift net
2
4–5
Glass eel
4–5
N net-1
Juv. + Adult
12 – 10
1 m 2 lift net
Juv. + Adult
12 – 10
-1
N net-1
Nd
Nd
Juv. + Adult
9 – 10
Juv. + Adult
Juv. + Adult
Juv. + Adult
-1
5+9
N h-1 (80m 2 ) -1
N ha-1
5+9
N h (80m )
2 -1
-1
-1
N ha-1
N ha-1
N ha
Juv.
Juv.
9 – 10
2 -1
Juv.
N ha
9 – 10
Juv.
9 – 10
-1
Juv.
9 – 10
Juv.
Juv. + Adult
3-6 + 9-10
9 – 10
Juv. + Adult
9 – 10
-1
Juv. + Adult
LIFE PHASE
8–9
MONTHS
N ha-1
N ha-1
N ha
-1
N d-1
N ha
-1
Index
UNIT
1 m 2 lift net
Eel fyke
Eel fyke
Bottom trawls
Stow net
Stow net
Stow net
Beam trawl
Beam trawl
Beam trawl
Beam trawl
Beam trawl
Beam trawl
Beam trawl
Kom-fyke
Beam trawls
Bottom trawls
GEAR
IMARES C003/15 & C006/04
IMARES C003/15 & C006/04
IMARES C003/15 & C006/04
IMARES C003/15 & C006/04
IMARES C003/15 & C006/04
IMARES C003/15 & C006/04
IMARES C164a/14
IMARES C164a/14
IMARES C060/13
RWS / ZiltWater (Z Jager)
RWS / ZiltWater (Z Jager)
RWS / ZiltWater (Z Jager)
IMARES (I Tulp)
IMARES (I Tulp)
IMARES (I Tulp)
IMARES (I Tulp)
IMARES (I Tulp)
IMARES (I Tulp)
IMARES (I Tulp)
NIOZ (H vd Veer)
IMARES (I Tulp)
IMARES (H Heessen)
SOURCE
2.2 Time series on fish abundance
Time series used for the synchronization analyses are
based on surveys only; landings are not considered
here. It is assumed that survey data are consistently
sampled in time and space and are not as biased by
changes in effort and gear as data on landings often
are (Heessen et al. 2015). The time series comprise
surveys of the North Sea (i.e. bottom-trawl surveys
of the North Sea ecoregion and of the coastal zone
north of the islands), the Wadden Sea (i.e. bottomtrawl surveys of tidal basins including the Ems estuary,
and surveys with a fyke, lift-nets and stow-nets) and
Lake IJssel (i.e. bottom-trawls and fykes) (Table 2.1,
Figure 2.2). Note that these surveys differ not only in
sampling area (North Sea, Wadden Sea, Lake IJssel)
and sampling gear (bottom-trawls, fykes, lift-nets,
stow-nets), but also in sampling period. This implies
that, although targeting similar species, time series on
densities are not fully comparable due to differences
in catchability (e.g. low catches of pelagic fish when
using bottom-trawls) and in life-phases (e.g. surveys of
1x1m lift nets select for small species and juveniles of
larger species).
Fig. 2.2 Locations of the fish surveys. Blue circle: Marsdiep Fyke, Pink
circles: Glass Eel survey, Orange circles: Ems estuary stow net survey, Green
circles: Lake IJssel fyke survey, Red lines: locations of trawls in Lake IJssel
trawl survey, Black lines: locations of trawls in Demersal Fish Survey
(the direction of the line does not indicate trawl direction), Brown: Boundaries
of Demersal Fish Survey (DFS) areas and codes. North Sea Ecoregion
surveys are not shown on this map.
21
North Sea Ecoregion surveys (NS_TER)
The survey index for the North Sea Ecoregion is
based upon a combination of data from a variety of
national and international surveys in the North Sea,
which use otter trawls and beam trawls (Heessen et
al. 2015). All data were scrutinized (e.g. for reporting
of less common species, misidentifications), corrected
(e.g. for differences in coding and fish lengths), and
standardized (Heessen et al. 2015).
It should be noted that the sampling was targeted
at commercial species (such as Herring, Cod and
Plaice) and not geared to get a representative index for
Lampreys, Salmon and Trout (Henk Heessen,
IMARES, pers. comm.). This might imply that trends
for the latter group of fish species contain relatively
low signal to noise ratio.
Beam-trawl surveys North Sea & Wadden
Sea (NS_T404, WS_T612, WS_T616, WS_T617,
WS_T618, WS_T619 & WS_T620)
The Demersal Fish Survey (DFS) is part of an
international coastal and inshore survey, which
started in 1970, is performed in autumn (Sep-Oct)
and aiming at monitoring young plaice and sole in
their nursery grounds in the North Sea coastal zone,
Wadden Sea and South Western Delta. The Dutch
survey areas included in this analysis comprise the
coastal zone of the North Sea north of the Wadden Sea
islands (DFS404), the Wadden Sea (from west to east:
DFS610, DFS612, DFS616-619), and the Ems-Dollard
estuary (DFS620) (Figure 2.2).
Within the Wadden Sea and Ems estuary, fishing with
a 3-m beam trawl was restricted to the tidal channels
and gullies deeper than 2 m because of the draught
of the research vessel, whereas the North Sea coastal
zone was fished with a 6-m beam trawl. The mean
abundance per area was calculated for all subareas in
the period 1970-2013 weighed by surface area for
five depth strata (intervals of 5 m) within the subareas
(Tulp et al. 2015).
As result of the sampling gear, the DFS results are
considered to reflect the densities of demersal fish,
such as Flounder, best and less those of the other target
species (Ingrid Tulp, IMARES, pers. comm.). The
combination of low fishing speed (2-3 knots) and fine
22
mesh size (20 mm) results in selection of the smaller
fish species and younger year classes of (particularly
demersal) fish and other epibenthos. Furthermore,
the timing of occurrence in the Wadden Sea needs to
coincide with the survey period (September-October)
(Tulp et al. 2015).
Kom-fyke surveys Marsdiep (WS_FMD)
Since 1960, a passive kom-fyke trap has been operating
at the entrance of the Marsdiep basin in the western
Dutch Wadden Sea (Van der Veer et al. 1992). Fishing
normally started in March - April and lasted until
October. In winter the trap was removed because
of possible damage by ice scouring and from 1971
onwards no fishing took place during part of the
summer ( July-August) because of fouling and potential
clogging of the net by macroalgae and jellyfish. All
catches were sorted out immediately and identified
to species level (Van der Veer et al. 2015). Fixed
gears require mobility (active movement or passive
transport) of the organism. The location of the fyke
near a tidal inlet implies that in spring catches will
contain fish that were migrating from the North Sea
into the Wadden Sea and in autumn fish on their way
to migrate to the North Sea including the locally
produced young-of-the-year (Fonds 1983).
Stow-net surveys Ems estuary (WS_SSP, WS_
SOT & WS_STB)
Within the Ems estuary, WFD (Water Framework
Directive) fish monitoring is carried out at three
stations, i.e. Spijk (coded as WS_SSP in this report,
see Table 2.1), Oterdum (WS_SOT) and Terborg
(WS_STB), in the Ems at river 70.5, 52.5 and 24.5
km, and thus relates to the polyhaline, mesohaline and
oligohaline zone, respectively (BioConsult 2009; Jager
et al. 2011). The fishing gear is a stow net with a width
of 10 m and a variable vertical opening of max. 10 m.
For fishing a commercial fishing vessel is chartered in
spring (May) and autumn (September). The vessel puts
out its anchor and deploys the nets; the tidal current
flows through the net during 3.0 to 4.5 hours. One
catch per station was carried out over most of the ebb
tide phase and one catch over most of the flood tide
phase. All fish catches were standardized to annual
averages of numbers per hour per 80 m 2 surface of the
net opening per station ( Jager et al. 2011).
Lift-net surveys freshwater sluices (WS_LD1,
WS_LD2, WS_LHA, WS_LLA & WS_LTZ)
Glass Eel is caught per lift net haul at the freshwater
sluices in Den Oever (coded as WS_LD1 in this
report, see Table 2.1) in the period April-May, using
a 1 m 2 lift net with mesh size 1x1 mm, annually since
1938 (Dekker 2002). Monitoring is carried out daily
in 5 hauls every 2 hours between 22:00-5:00 h.
In addition, recruitment of Dutch waters is monitored
at twelve sites along the Dutch coast. Five of these
are located in the Wadden Sea, i.e. near Den Oever
(WS_LD2), Harlingen (WS_LHA), Lauwersoog
(WS_LLA), Termunterzijl (WS_LTZ) and Nieuwe
Statenzijl (WS_LNZ). At these stations, glass eel was
caught by 1 m 2 lift nets in 2 hauls between 18:00
and 8:00 h in the period April-May with a weekly
frequency (De Graaf & Deerenberg 2015). Sampling
efforts were more diverse than the long-term survey at
Den Oever 1 (WS_LD1), with shorter time series and
missing years (De Graaf & Deerenberg 2015).
the open waters of Lake IJssel. In 2002, the sampling
frequency was reduced from three times a year (May,
Aug, and Oct-Nov) to once a year (Oct-Nov). For
long-term trends, data are taken for the Oct-Nov
period only (Van der Sluis et al. 2014).
For most fish species, with exception of European Eel,
Smelt and Three-spined Stickleback, these gears are
selective for juveniles (Van der Sluis et al. 2014).
Bottom-trawl surveys Lake IJssel (LIJ_TLIJ)
The longest time-series in Lake IJssel stems from the
open-water monitoring program with active fishing
from a vessel. From the start of the survey in 1966 up
to 2012, sampling was performed by means of a large
(7.4 m wide, 26.9 m long, 1 m high) bottom trawl net
(in Dutch: “kuil”), which was replaced by a smaller
beam trawl (4 m wide, 19.95 m long, 1 m high) in
2013 (Van der Sluis et al. 2014). Catches of the new
gear were corrected for differences with the old gear
in catchability for biomass of Smelt and numbers of
Flounder (Van Overzee et al., 2013). Since 1989,
sampling is consistently performed at 29 locations in
Fish were caught using Eel fyke nets with a stretched
mesh size of 18-20 mm. This survey started with
33 locations in lakes and rivers but the number of
locations declined. Lake IJssel had two locations, one
at the Afsluitdijk near Kornwerderzand (IJsselmeer01,
in this report coded as LIJ_F01, see Table 2.1) and one
at the Houtribdijk (IJsselmeer02, LIJ_F02), although
in fact at this location two nearby fykes were applied
(labelled IJm02a and IJm02b in Figure 2.2). Eel
fishing, and thus registration of catches, is allowed
from December to October (Van der Sluis et al. 2014).
Fyke surveys Lake IJssel (LIJ_F01 & LIJ_F02)
This fishery survey was executed from 1994 to 2013
in order to sample freshwater fish abundance in large
water bodies of The Netherlands (Van der Sluis et
al. 2014). During this survey, professional fishermen
under scientific guidance register by-catch of fisheries.
The catch effort (number of nets per day and the
fishing duration per day) was registered since 1994
(Van der Sluis et al. 2014). Since 2014 a different
survey set-up has been followed and these surveys have
been abandoned.
23
2.3 Statistical analysis survey data
Synchronization is defined as the correlation between
the time series of normalized densities of fish species
for the different surveys. For each pair of time
series, synchronization is calculated using Pearson’s
correlation coefficient (r). Correlation coefficients
were calculated between areas and stations with nonzero densities. This resulted in symmetric correlation
matrices, of which the size differed per fish species.
For each species, the correlation matrix was visualized
by plotting ellipses shaped as contours of a bivariate
normal distribution with unit variance and correlation
r. More elliptical and more intense colour means
higher correlation; colour (red or blue) is indicating
the respective negative or positive nature of the
correlation. The points of the contour are given by
(x,y) = (cos(θ+d/2), cos(θ−d/2)) and d = arccos(r)
where θ ϵ [0, 2π] (Murdoch & Chow 1996).
In addition, the results for each fish species are
presented within schematized maps of the study area
(Fig. 2.3). Within the maps, the connectivity between
sampling areas is depicted by lines with rounded
endpoints. The colour of these lines is an indication of
the sign and value of the correlation between the times
series of the normalized abundances of fish from both
subareas. If no or not enough data were available to
calculate the correlation, then this line was not drawn.
R 2.15.2 (R Development Core Team 2009) was
used for data handling, estimation, and plotting. For
spatial data handling and statistics the R packages
sp (Pebesma & Bivand 2005; Bivand et al. 2008),
rgeos (Bivand & Rundel 2011), and rgdal (Keitt et
al. 2011) were used and for mixed modelling the R
package lme4 (Bates and others 2012). For plotting,
ellipse (Murdoch & Chow 1996, 2012) and ggplot2
(Wickham 2009) were used. Data analysis was
performed by Eelke Folmer (EcoSpace).
Fig. 2.3 Compartmentalization of the study area based upon fish abundance
as determined by means of surveys in the North Sea (NSER), the North Sea
coastal zone (DFS), and the Wadden Sea including the Ems estuary (DFS), and
Lake IJssel (LIJ). Within the Wadden Sea, the blue boxes refer to DFS areas
(MD = Marsdiep, EG = Eijerlandse Gat, VL = Vlie, BD =Borndiep, ZL =
Zoutkamperlaag / Pinkegat, LS = Lauwers / Schild, ED = Ems-Dollard) and
the white boxes to islands (TX= Texel, VL = Vlieland, TR = Terschelling,
AM = Ameland, SO = Schiermonnikoog, RP = Rottumerplaat). Blue lines
with rounded endpoints indicate (potential) corridors for migratory fish between
different compartments. Note that potential migration points to inland waters are
not indicated (e.g. all sluices along the Dutch Wadden Sea coast).
24
2.4 Results & Discussion
of densities of River Lamprey in the North Sea and the
Wadden Sea suggests an increase since the 1990s (Fig.
2.4.1.1), it cannot be excluded that misidentification
during the surveys might have caused this (Heessen
et al. 2015). On average, densities in the North Sea
coastal zone and the Wadden Sea are around 0.1 fish
ha-1, and less than 0.01 fish ha-1 in Lake IJssel (Fig.
2.4.1.1).
2.4.1 Lampreys (Petromyzontidae)
River lamprey (Lampetra fluviatilis) – River Lamprey
was caught during all surveys and in almost all
surveyed areas, with exception of the tidal basin of the
Eijerlandse Gat (Fig. 2.4.1.1). Although the time series
(n/ha)
0.02
0.04
0.06
0.030
0.020
(n/d)
0.010
1980
1990
2000
2010
0.25
1990
2000
2010
0.00
1990
2000
2010
1960
1970
1980
1990
2000
2010
1980
1990
2000
2010
Zoutkamperlaag
0.6
(n/ha)
0.0
1960
1970
1980
1990
2000
2010
1960
Spijk
1970
1980
1990
2000
2010
2000
2010
2000
2010
Oterdum
20
5
0.5
1990
2000
2010
1960
1980
1990
2000
2010
1990
2000
2010
1960
1980
1990
Flevoland
0.6
(n/d)
(n/d)
1970
0.5
0.15
0.04
0.03
0.05
(n/ha)
0.00
0.1
0.01
0.00
1970
1980
Kornwerderzand
0.02
2.5
2.0
1.5
1.0
0.5
1960
1970
Lake IJssel
0.05
3.5
3.0
1980
0.7
1970
0.4
1960
0.3
2010
0.2
2000
1960
1970
1980
1990
2000
2010
0.0
1990
Terborg
0.20
1980
0.10
1970
0
0.0
1960
15
(n/h/80m2)
10
1.5
(n/h/80m2)
1.0
0.2
0.1
0.004
(n/ha)
0.006
2.0
0.3
25
2.5
Ems estuary
0.008
1970
0.2
0.05
0.00
1960
Lauwers / Schild
0.010
1980
0.15
(n/ha)
0.15
(n/ha)
0.05
0.00
1980
1970
Borndiep
0.10
0.8
0.6
0.4
0.0
1970
1960
Vlie
0.20
1.0
1970
0.8
1960
30
2010
0.4
0.000
2000
0.10
1990
0.2
(n/ha)
0.06
(n/ha)
0.04
1980
0.002
(n/ha)
Marsdiep
0.02
0.00
1970
Eijerlandse Gat
1960
0.000
Marsdiep inlet
0.10
0.08
4
3
2
survey index (−)
1
0
1960
(n/h/80m2)
NS coastal zone
0.12
North Sea
1960
1970
1980
1990
2000
2010
1960
1970
1980
1990
Fig. 2.4.1.1 Time series of normalized densities of
River Lamprey as determined by means of surveys in
the North Sea, Wadden Sea and Lake IJssel.
25
Comparison of trends from the different surveys in
time show that year-to-year variations of fish caught
by the three stow nets in the Ems estuary are more or
less comparable with each other and with the variation
of fish caught by beam trawls in the Vlie tidal basin
(Fig. 2.4.1.2). In Lake IJssel, the variation in the bycatch of the fyke near Kornwerderzand is negatively
correlated with that in the beam trawl surveys in Lake
IJssel (Fig. 2.4.1.2).
When comparing the trends in the beam trawl
surveys in the study area, it appears that there is some
correlation between the North Sea and the Marsdiep
tidal basin, between the NS coastal zone and the Vlie
tidal basin and between the tidal basins of the Lauwers
and the Ems estuary (Fig. 2.4.1.3).
Within the Netherlands, “Gasterense Diepje” (a
branch of the small river “Drentsche Aa”) is one
of the few locations where these fish are presently
known to spawn (Winter & Griffioen 2007). River
Lampreys can reach this area via the sluices of Delfzijl
in the Ems estuary and adjacent canals in the Province
of Groningen (Winter et al. 2013). Other but still
unknown spawning areas might be reached when
River Lamprey migrates upstream in the rivers IJssel
and Vecht after accessing Lake IJssel via the Afsluitdijk
(Erwin Winter, IMARES, pers. comm.).
Fig. 2.4.1.2 Ellipse matrix of the correlations
between times series of normalized densities of
River Lamprey as determined by means of surveys
in the North Sea, Wadden Sea and Lake IJssel.
Colour indicates positive (blue) or negative (red)
correlation, colour intensity the (absolute) size of
the correlation coefficient.
Fig. 2.4.1.3 Correlations between times series of normalized abundance of River
Lamprey between adjacent basins as determined by means of surveys in the North
Sea, the Wadden Sea and Lake IJssel. Colour indicates positive (blue) or negative
(red) correlation, colour intensity the (absolute) size of the correlation coefficient.
26
Sea Lamprey (Petromyzon marinus) - Sea Lamprey
was caught in all types of surveys (except for the beam
trawl survey in Lake IJssel), but only in 10 of the 16
surveyed areas and locations (Fig. 2.4.2.1). For half
of these locations, Sea Lamprey was caught only in 1
year or in 2 years of the study period (Fig. 2.4.2.1).
Given that adult Sea Lamprey are parasitic feeders
that attach to large preys, i.e. big fish and marine
2010
2000
2010
1980
1990
2000
2010
(n/ha)
0.02
1.0
2000
1960
(n/h/80m2)
1970
1990
2000
2010
2010
1990
2000
2010
1970
1980
1990
2000
2010
2000
2010
Flevoland
0.20
0.4
2000
(n/d)
0.3
(n/d)
0.6
0.2
0.10
0.4
0.05
0.1
1960
1970
1980
1990
2000
2010
0.00
(n/ha)
1990
1980
Oterdum
1960
0.5
1.0
1980
Kornwerderzand
0.2
1980
1970
0.2
1960
Lake IJssel
0.0
1970
0.6
2010
0.8
0.3
0.2
0.1
0.0
1960
2010
0.4
(n/ha)
1990
Spijk
0.2
1970
2000
0.2
1980
0.0
1960
0.4
1970
0.8
(n/h/80m2)
0.15
(n/ha)
0.10
0.05
1990
Terborg
1990
0.0
1960
0.00
1980
0.04
0.06
2000
1.0
1990
1980
Zoutkamperlaag
0.8
1980
1970
0.8
1.0
(n/ha)
0.2
1970
0.20
0.20
0.15
0.10
0.05
1970
1960
0.0
1960
Ems estuary
0.00
1960
2010
0.0
2010
2000
0.30
2000
1990
Borndiep
0.25
1990
1980
1.0
1980
1970
0.8
0.020
0.005
0.000
1970
0.00
1960
0.6
2010
0.4
2000
0.15
1990
Vlie
Lauwers / Schild
(n/ha)
1980
0.010
(n/ha)
0.6
0.4
0.0
1960
0.04
(n/d)
0.02
1970
0.015
0.8
1.0
0.00
1960
0.6
2010
0.4
2000
0.6
1990
0.4
1980
Eijerlandse Gat
0.2
(n/ha)
0.06
(n/ha)
0.04
0.02
0.00
1970
Marsdiep
0.06
0.10
0.08
5
4
3
survey index (−)
2
1
0
1960
(n/h/80m2)
Marsdiep inlet
0.08
NS coastal zone
6
North Sea
mammals, the catchability of Sea Lamprey with the
nets used in the different surveys is very small (Erwin
Winter, IMARES, pers. comm.). Due to possible
misidentifications, the time series of densities in Sea
Lamprey for the North Sea cannot be trusted (Heessen
et al. 2015). In general, catches were too low to
perform a correlation analyses.
1960
1970
1980
1990
2000
2010
1960
1970
1980
1990
Fig. 2.4.2.1 Time series of normalized densities of
Sea Lamprey as determined by means of surveys in
the North Sea, Wadden Sea and Lake IJssel.
27
2.4.2 European Eel (Anguilla anguilla)
discharge points in the Wadden Sea). The time series
of the adults and juveniles suggest that densities were
generally higher in the 1980s than in the 1990s and
2000s except for the Ems estuary (Fig. 2.4.3.1). At
sampling stations near the Afsluitdijk, Eels appear
to have declined at both sides during the past
decades, i.e. adults and juveniles in Lake IJssel near
Kornwerderzand (Fig. 2.4.3.1) and Glass Eel at the
sampling stations near Den Oever in the Wadden Sea
(Fig. 2.4.3.4).
European Eels were caught during all surveys and in
all areas (Fig. 2.4.3.1, Fig. 2.4.3.2). It must be noted
that the surveys can be grouped according to the life
phases of the fish caught, i.e. (i) adults & juveniles (all
surveys in the North Sea and the North Sea coastal
zone, fyke catches in the Marsdiep tidal inlet and Ems
estuary, and all surveys in Lake IJssel), (ii) (mainly)
juveniles (all surveys for the tidal basins of the Wadden
Sea), and (iii) Glass Eel (lift net surveys at freshwater
Marsdiep inlet
Marsdiep
14
NS coastal zone
1960
1970
1980
1990
2000
2010
1960
1970
1980
1990
2000
2010
1970
1980
1990
2000
2010
1960
1970
Borndiep
1980
1990
2000
2010
Zoutkamperlaag
8
5
12
4
6
10
0.8
4
(n/ha)
3
(n/ha)
1990
2000
2010
1970
1980
1990
2000
2010
1960
Ems estuary
1970
1980
1990
2000
2010
1960
1980
1990
2000
2010
1970
1980
1990
2000
2010
2010
2000
2010
2000
2010
(n/h/80m2)
0.2
0.0
1960
1970
1980
1990
2000
2010
1960
1970
Kornwerderzand
1980
1990
Flevoland
10
(n/d)
10
(n/d)
5
5
1960
1970
1980
1990
2000
2010
0
0
10
1.0
20
30
(n/ha)
40
50
2.0
15
15
60
Lake IJssel
1.5
(n/h/80m2)
2000
0.8
1960
Terborg
1960
1970
1980
1990
2000
2010
1960
Fig. 2.4.3.1 Time series of normalized densities of European
Eel (juveniles and adults) as determined by means of surveys
in the North Sea, Wadden Sea and Lake IJssel.
28
1990
0.15
0.05
0.00
0.5
0.0
1970
1980
Oterdum
0.10
(n/h/80m2)
2.0
1.5
(n/ha)
1.0
2
1
0
1960
1970
Spijk
2.5
6
5
4
3
(n/ha)
0
1
1960
0.6
1980
Lauwers / Schild
0.4
1970
0.20
1960
0
0
0.0
2
0.2
2
4
2
6
(n/ha)
8
0.6
0.4
(n/ha)
8
(n/ha)
4
2
0
1960
Vlie
1.0
Eijerlandse Gat
6
30
0
0
10
1
20
2
(n/d)
3
(n/ha)
40
4
3
2
0
1
survey index (−)
4
10
50
5
12
5
60
6
North Sea
1970
1980
1990
2000
2010
1960
1970
1980
1990
The ellipse matrix of correlations suggests synchronous
dynamics in European Eel (including Glass Eel)
between the marine waters of the North Sea
(including the coastal zone) and the Wadden Sea
(including the Ems estuary) (Fig.2.4.3.2, Fig.2.4.3.3).
Correlations were particularly high between Glass
Eel catches at Den Oever, Harlingen and Lauwersoog
(Fig.2.4.3.2, Fig.2.4.3.3). At the stations Terborg
(TB) and Spijk (SP) in the Ems estuary, ( juvenile and
adult) Eel showed opposite trends compared to most
other stations and larger marine areas (Fig.2.4.3.2,
Fig.2.4.3.3).
When comparing the trends in the beam trawl surveys
in the study area, it appears that trends in many
subareas are positively correlated, in particular the
Marsdiep with its adjacent basins (Vlie, Eijerlandse
Gat) and the North Sea, and the tidal basins of
Borndiep and Zoutkamperlaag (Fig. 2.4.3.2).
Correlations were low or absent between the Marsdiep
tidal basin and Lake IJssel, and between the North Sea
coastal zone and the tidal basins of Zoutkamperlaag
and Lauwers (Fig. 2.4.3.3).
Fig. 2.4.3.2 Ellipse matrix of the correlations
between times series of normalized densities of
European Eel (juveniles and adults) as determined
by means of surveys in the North Sea, the Wadden
Sea and Lake IJssel. Colour indicates positive (blue)
or negative (red) correlation, colour intensity the
(absolute) size of the correlation coefficient.
Fig. 2.4.3.3 Correlations between times series of normalized abundance
of European Eel (juveniles and adults) between adjacent basins as
determined by means of surveys in the North Sea, the Wadden Sea and
Lake IJssel. Colour indicates positive (blue) or negative (red) correlation,
colour intensity the (absolute) size of the correlation coefficient.
29
30
120
(n/net)
0
0
5
20
10
15
20
25
100
80
60
40
(n/net)
Den Oever 2
35
Den Oever 1
1960
1970
1980
1990
2000
2010
1960
1970
1980
1990
2000
2010
100
Lauwersmeer
60
(n/net)
40
20
15
0
0
5
20
10
(n/net)
25
80
30
35
Harlingen
1960
1970
1980
1990
2000
2010
1960
1970
1980
1990
2000
2010
Nieuw Statenzijl
(n/net)
40
5
30
0
0
10
20
(n/net)
10
50
60
15
70
Termunterzijl
1960
1970
1980
1990
2000
2010
1960
1970
1980
1990
2000
2010
This large-scale synchronicity suggests a common
driver of Eel dynamics, e.g. an atmospherically driven
dispersal by ocean currents connecting the putative
spawning grounds of the European eel and the Gulf
Stream, determining the arrival of juveniles at the
European coast (Baltazar-Soares et al. 2014). An
alternative explanation is given by Kettle et al. (2011),
who point to a change in hydrology and exploitation
in the southwest corner of the geographical range
of European Eel, particularly the Atlantic coast
of Morocco. Possibly, Glass Eel trends are more
dominated by large-scale drivers (hydrographic),
whereas those of the (Yellow and Silver) Eel are
more determined by local drivers (e.g., entrance
to fresh/inland water, fisheries) (Zwanette Jager,
ZiltwaterAdvies, pers. comm.).
Dekker & Casselman (2014) showed that in Europe
recruitment of (Glass) Eel has declined by 90% –99%
since 1980. Global oceanic changes, as well as
direct and indirect impacts (barriers to migration,
contaminants, fisheries exploitation,
habitat loss, parasite introductions, and water quality
30
Fig. 2.4.3.4 Time series of normalized densities of
European Eel (Glass Eel) as determined by means of
surveys in the Wadden Sea.
deterioration), have been suggested as causes for the
decline (a.o., Castonguay 1994; Feunteun 2002;
Friedland et al. 2007; Palstra et al. 2007; Robinet &
Feunteun 2002).
Since its closure in 1932, Lake IJssel has been subject
to a suite of management regulations with regard
to Eel, including changes in fishery pressure (e.g.
fishing efforts, fishing season, engine power, and
increases in mesh size; De Leeuw et al. 2008) and
restocking (Dekker & Beaulaton 2016b). Furthermore,
the increase in Eel in Lake IJssel directly after the
closure might have resulted from the fact that Eel
was no longer able to take advantage of selective tidal
transport for swimming up rivers and got stuck in this
area (Dekker & Beaulaton 2016a). Alternatively, a new
suitable and vast habitat came available for a large part
of the Glass Eel entering Lake IJssel and therefore there
was no need for more dispersal further upstream. This
fill-up principle is also observed in French rivers at the
Bay of Biscay after barriers were taken away (Erwin
Winter, IMARES, pers. comm.).
NS coastal zone
Marsdiep
3
0.6
12
4
10
2
(n/ha)
0.4
2
(n/d)
3
(n/ha)
8
6
1960
1990
2000
2010
2010
0.06
(n/ha)
1960
1990
2000
2010
1960
1.0
0.8
0.6
2000
2010
2000
2010
1970
1980
1990
2000
2010
2000
2010
Flevoland
0.4
0.4
1990
1990
80
1980
Kornwerderzand
0.2
1980
1980
20
1970
0.0
1970
1970
Oterdum
(n/d)
0.6
1960
0.04
0.02
2010
0
1960
0.8
2010
2000
1.0
2000
0.4
(n/ha)
2000
1990
Spijk
0.8
1990
Lake IJssel
0.2
1990
1980
(n/h/80m2)
(n/h/80m2)
1980
0.0
1980
1970
50
1970
1.0
0.05
1970
0.00
1960
0
1960
0.04
0.03
0.02
0.01
0.00
1960
2010
0.08
2010
0.06
2010
2000
100
2000
0.04
(n/ha)
2000
1990
60
1990
Ems estuary
0.02
1990
Terborg
1980
40
(n/ha)
1980
0.00
1980
1970
Zoutkamperlaag
0.1
1970
0.08
1.0
0.8
0.6
0.4
0.2
1970
1960
0.0
1960
0.0
1960
2010
0.6
2010
2000
0.2
2000
1990
Borndiep
250
1990
1980
200
1980
Lauwers / Schild
1970
0.3
0.20
0.15
(n/ha)
0.10
0.05
0.00
1970
0
1960
0.25
1.0
0.8
0.6
0.4
0.2
(n/ha)
1980
Vlie
0.0
1960
(n/ha)
1970
(n/d)
2010
0.0
2000
0.4
1990
0.2
1980
Eijerlandse Gat
150
1970
100
1960
(n/h/80m2)
0.0
0
0
2
1
1
0.2
4
survey index (−)
Marsdiep inlet
5
14
North Sea
1960
1970
1980
1990
2000
2010
1960
1970
1980
1990
Fig. 2.4.4.1 Time series of normalized densities of
European Anchovy as determined by means of surveys in
the North Sea, Wadden Sea and Lake IJssel.
2.4.3 European Anchovy (Engraulis
encrasicolus)
Anchovy was caught in 11 of the 20 surveyed areas or
locations, and caught only in 1 year or in 2 years of the
study period within 6 of these 11 areas (Fig. 2.4.4.1).
The peaks in the stow-net catches in 2007 in the Ems
estuary (Spijk, Oterdum, Terborg) are in concordance
with the only catch in the beam trawl survey for this
area, and was also observed in the North Sea (Fig.
2.4.4.1; Fig. 2.4.4.2). Another peak in 1998 coincided
simultaneously in the tidal basins of Vlie, Borndiep
and Zoutkamperlaag (Fig. 2.4.4.1; Fig. 2.4.4.2).
Correlations in trends between tidal basins and
adjacent marine waters (North Sea, North Sea coastal
zone) are poor, with exception of the high correlation
between Vlie, Borndiep and Zoutkamperlaag resulting
from the simultaneous peak in 1998 (Fig. 2.4.4.3). In
general, the time series of the Wadden Sea tidal basins
31
Fig. 2.4.4.2 Ellipse matrix of the correlations
between times series of normalized densities of
European Anchovy as determined by means of
surveys in the North Sea, the Wadden Sea and
Lake IJssel. Colour indicates positive (blue) or
negative (red) correlation, colour intensity the
(absolute) size of the correlation coefficient.
Fig. 2.4.4.3 Correlations between times series of normalized
abundance of European Anchovy between adjacent basins as
determined by means of surveys in the North Sea, the Wadden
Sea and Lake IJssel. Colour indicates positive (blue) or negative
(red) correlation, colour intensity the (absolute) size of the
correlation coefficient.
32
were characterized by one (1998 or 2007) or two
years (1975 and 1998) with occurrences of Anchovy,
and zero observations for other years within the study
period (1970-2014). This can be explained by the fact
that Anchovy is a summer (May-July) visitor and is not
found in autumn in these waters when these surveys
are performed.
In the North Sea, anchovy abundance has fluctuated,
with periods of high abundance being followed by
periods of near absence (Aurich 1953). Data from trawl
surveys and commercial information landings indicate
a strong increase in Anchovy abundance since 1995
after a period of absence (Beare et al. 2004). This
increase is considered to be the result of an improved
productivity of existing populations, associated with
an expansion in thermal habitats and/or wind-induced
variations in the inflow of Atlantic water in winter,
and probably not due to a northward shift in the
distribution of a southern (Bay of Biscay) population
(Corten & Van de Kamp 1996; Petitgas et al. 2012).
The former Zuiderzee used to support the largest
spawning population of Anchovy in northwestern
Europe (Petitgas et al. 2012), but catches rapidly
declined after the closure in 1932 (see Chapter 1,
Fig. 1.4) and this species is no longer caught in Lake
IJssel (Fig. 2.4.4.1). Within the western Wadden
Sea, spawning individuals were reported up to 1962
(sequence of cold winters) and eggs were observed
again in 1994 north of the freshwater discharge sluices
of Kornwerderzand, close to former spawning areas in
the Vlieter tidal gully (Boddeke & Vingerhoed 1996).
In May 2010 and May 2011, schools of Anchovy were
found in a pelagic survey in the Marsdiep area. Their
mean length (14.5 cm) corresponded to age-1 fish that
are likely to spawn. Hence, for anchovy the Wadden
Sea area is important both as nursery as well as for
spawning (Couperus et al. 2016).
In historical times, Anchovy formed the basis of a
substantial fishery in the Ems estuary, which came
to an end before 1855 (Stratingh & Venema, 1855),
which was before the blooming of the fishery in
Zuiderzee between 1880 and 1903 (Gmelich Meijlingvan Hemert 2008). Possibly, the recent catches of
Anchovy in the (spring) stow-net survey in the middle
and outer part of the Ems estuary indicate a return of
this fish species in this formerly rich area.
33
2.4.4 Herrings (Clupeidae)
trawl catches in the North Sea and the Wadden Sea
were low in the 1970s, and increased hereafter (Fig.
2.4.5.1). The variation in stow-net catches at Terborg
appears to be positively correlated with the beam trawl
catches in tidal basins of Eijerlandse Gat, Borndiep and
Zoutkamperlaag (Fig. 2.4.5.2).
1970
1980
1990
2000
2010
5000
1970
1980
1990
2000
(n/ha)
3000
2010
500
0
1960
1970
1980
1990
2000
2010
1960
1990
2000
2010
1970
1990
2000
2010
800
0
1960
1970
1980
1990
2000
2010
1960
1970
Spijk
1980
1990
2000
2010
2000
2010
2000
2010
Oterdum
3000
Ems estuary
2000
2010
0.06
1980
1990
2000
2010
1980
1990
2000
1000
2010
1960
Kornwerderzand
1970
1980
1990
2000
2010
1970
1980
1990
Flevoland
0.4
(n/d)
0.6
(n/d)
0.2
0.0
1960
Fig. 2.4.5.1 Time series of normalized densities of
Atlantic Herring as determined by means of surveys
in the North Sea, Wadden Sea and Lake IJssel.
34
1970
0.4
0.04
(n/ha)
0.02
0.00
1970
(n/h/80m2)
1960
1.0
1990
Lake IJssel
0.08
600
500
1980
0.8
1970
400
300
200
100
0
1960
500
500
0
1960
0.6
2010
0.2
2000
0.0
1990
Terborg
1.0
1980
0.8
1970
2000
(n/h/80m2)
1000
(n/ha)
20
0
0
1960
(n/h/80m2)
40
400
200
(n/ha)
60
600
80
800
1980
2010
200
200
0
1960
2000
1500
1980
Lauwers / Schild
1990
600
(n/ha)
600
400
(n/ha)
400
(n/ha)
200
0
1970
1980
Zoutkamperlaag
800
600
120
100
80
60
20
0
1960
1970
Borndiep
1000
Vlie
40
(n/ha)
1500
4000
1960
Eijerlandse Gat
Marsdiep
400
1960
0
0
1000
100
2000
200
(n/ha)
(n/d)
300
2.0
1.5
1.0
survey index (−)
0.5
0.0
Marsdiep inlet
2000
NS coastal zone
400
North Sea
1000
Atlantic Herring (Clupea harengus) – Herring was
caught in all marine survey areas and locations, and
during two years (1991, 1992) in the trawl catches in
Lake IJssel (Fig. 2.4.5.1). On average, marine bottom-
1960
1970
1980
1990
2000
2010
1960
1970
1980
1990
Fig. 2.4.5.2 Ellipse matrix of the correlations
between times series of normalized densities of
Atlantic Herring as determined by means of surveys
in the North Sea, the Wadden Sea and Lake IJssel.
Colour indicates positive (blue) or negative (red)
correlation, colour intensity the (absolute) size of the
correlation coefficient.
Fig. 2.4.5.3 Correlations between times series of
normalized abundance of Atlantic Herring between adjacent
basins as determined by means of surveys in the North Sea,
the Wadden Sea and Lake IJssel. Colour indicates positive
(blue) or negative (red) correlation, colour intensity the
(absolute) size of the correlation coefficient.
35
When comparing the trends in the beam trawl surveys
in the study area, it appears that trends in the tidal
basins are positively correlated, and that variation
in Herring densities in the eastern tidal basins is
correlated with that in the North Sea coastal zone
(Fig. 2.4.5.3).
In the North Sea, numbers increased steadily in the
1980s reflecting a period of recovery after the collapse
of the Herring stock as the result of overfishing in
the 1970s and remained more or less stable hereafter
(Dickey-Collas et al. 2010). Relatively low stocks in
the 2000s are ascribed to low recruitment (2002–2010)
caused by an increase in temperature, a regime shift in
the ecosystem and/or predation by the large stock of
adult Herring on its own larvae (Corten 2013).
These large-scale changes are, however, not
synchronised with changes in the Dutch coastal
zone nor in the tidal basins of the Wadden Sea (Fig.
2.4.5.2). A critical factor for the survival of the larvae
of Herring born in the central and northern North
Sea appears to be their transport in winter across the
North Sea towards the shallow coastal nursery areas
in the east, including the Wadden Sea (Corten 2013).
36
There is evidence that this winter transport of larvae
across the North Sea was interrupted for a number of
years prior to 1980 as the result of a reduced inflow
from the Atlantic Ocean (Corten 2013), which might
explain the relatively low densities during that time in
the Marsdiep tidal inlet and the western tidal basins of
the Wadden Sea (Fig. 2.4.5.1).
There appears to be a synchronisation in Herring
abundances in the coastal zone and the eastern part of
the Wadden Sea, probably due to local circumstances
reducing fish numbers such as competition for food
with the invasive cnetophore since 2005 (Kellnreitner
et al. 2013), temperature-related infections by parasites
(Schade et al. 2015) or enhanced predation by fish
(Cardoso et al. 2015), birds and seals (Van der Veer et
al. 2015).
The occurrence of Herring in Lake IJssel in 1991 and
1992 might have been the result of a change in sluice
management in 1991-1993, where discharge sluices
were kept open until water levels at both sides of
the sluices were equal, as was observed for Flounder
(Winter 2009).
Allis Shad (Alosa alosa) – Allis Shad was only
caught in the North Sea bottom trawl survey, and
therefore not further considered for analyses (Fig.
2.4.6.1). In 2013, three young-of-the-year Alosa alosa,
probably originating from natural reproduction, were
documented for the first time in a period of nearly
100 years in the River Rhine. In 2014, a further
increase was observed when 57 juveniles and eight
adults were caught; seven of these eight adults were
derived from stocking (Hundt et al. 2015).
2000
2010
1.0
2000
2010
1.0
2000
2010
1990
2000
1.0
2010
1990
2000
2010
1980
1990
2000
2010
2000
2010
2000
2010
Oterdum
1960
0.6
1970
1980
1990
Flevoland
0.2
0.4
(n/d)
0.4
1980
0.6
(n/ha)
1980
Kornwerderzand
0.2
1970
0.4
0.2
1.0
(n/ha)
0.2
(n/h/80m2)
1970
0.0
1960
1970
0.0
1960
0.6
0.4
(n/ha)
1990
1960
1.0
1990
Lake IJssel
0.2
1980
2010
0.8
1980
0.0
1970
2000
0.2
1970
0.8
1.0
0.8
0.6
0.4
0.2
0.0
1960
1990
0.0
1960
2010
0.6
2010
1980
Spijk
1.0
2000
1970
0.8
1990
Terborg
2000
0.2
(n/h/80m2)
0.6
(n/ha)
0.4
0.2
0.0
1980
1990
0.0
1960
1.0
1990
Ems estuary
1980
Zoutkamperlaag
0.8
1980
1970
0.8
1.0
(n/ha)
0.2
1970
0.8
1.0
0.8
0.6
0.4
0.2
1970
1960
0.0
1960
0.0
1960
2010
(n/d)
2010
2000
0.0
2000
1990
Borndiep
1.0
1990
1980
0.8
1980
Lauwers / Schild
1970
0.8
1.0
0.8
0.6
(n/ha)
0.0
0.2
0.4
0.0
1970
0.0
1960
0.6
2010
0.4
2000
0.6
1990
Vlie
0.4
1980
Marsdiep
0.8
1.0
0.8
0.6
(n/d)
0.4
0.2
1970
0.4
0.6
0.8
1.0
0.0
1960
0.6
2010
0.4
2000
0.6
1990
Marsdiep inlet
0.4
(n/ha)
1980
0.2
(n/ha)
0.4
0.2
0.0
1970
Eijerlandse Gat
1960
(n/ha)
0.6
0.8
20
15
10
survey index (−)
5
0
1960
(n/h/80m2)
NS coastal zone
1.0
North Sea
1960
1970
1980
1990
2000
2010
1960
1970
1980
1990
Fig. 2.4.6.1 Time series of normalized densities of
Allis Shad as determined by means of surveys in the
North Sea, Wadden Sea and Lake IJssel.
37
Twaite (Alosa fallax) – Twaite Shad was caught by
most surveys in the North Sea, the coastal zone of
the North Sea, in all tidal basins of the Wadden Sea,
only not by the bottom trawl surveys in Lake IJssel
(Fig. 2.4.7.1). The year 1982 was an outlier with large
numbers of fish caught in the coastal zone (Fig. 2.4.7.1;
Heessen et al. 2015). This peak in 1982 occurred in
several tidal basins of the Wadden Sea as well (Fig.
2.4.7.1). For two of three stow net surveys (Oterdum,
Spijk), variation in Twaite Shad was positively
correlated with that in the North Sea (Fig. 2.4.7.2).
When comparing the trends in the beam trawl
surveys in the study area, the variation in Twaite
Shad appeared to be synchronised in the North Sea,
the Dutch coastal zone and the westernmost and
easternmost tidal basins of the Wadden Sea
(Fig. 2.4.7.3).
NS coastal zone
Marsdiep inlet
Marsdiep
10
(n/ha)
20
1960
1970
1980
1990
2000
2010
0
0
0
0
20
5
10
5
40
(n/d)
60
(n/ha)
15
10
survey index (−)
80
30
20
15
100
North Sea
Variation at Oterdum was, however, negatively
correlated with that of the third (oligohaline) stow
net at Terborg and with that of the (freshwater) fyke
catches at Kornwerderzand (Fig. 2.4.7.2).
1960
1970
1980
1990
2000
2010
1960
1970
Vlie
1990
2000
2010
1960
1970
Borndiep
4
1980
1990
2000
2010
Zoutkamperlaag
(n/ha)
10
4
(n/ha)
2
(n/ha)
3
2010
1960
1980
1990
2000
2010
20
5
0
1960
Ems estuary
1970
1980
1990
2000
2010
1960
15
(n/h/80m2)
60
40
(n/h/80m2)
10
(n/ha)
1970
1980
1990
2000
2010
1960
(n/d)
0.2
0.0
1980
1990
2000
2010
1960
1970
1980
1990
1990
2000
2010
1970
2000
2010
1980
1990
2000
2010
2000
2010
0.08
Flevoland
0.06
1960
1970
1980
1990
2000
2010
Fig. 2.4.7.1 Time series of normalized densities of Twaite Shad as determined by
means of surveys in the North Sea, Wadden Sea and Lake IJssel.
38
1960
(n/d)
1.0
0.6
0.4
(n/ha)
1.0
(n/h/80m2)
0.8
1970
1980
Kornwerderzand
0.6
0.4
0.2
1960
1970
Lake IJssel
0.8
1.2
1.4
0
1960
0.02
2010
2010
0.00
2000
2000
0
20
1990
Terborg
0.00 0.05 0.10 0.15 0.20 0.25 0.30
1980
1990
50
5
0
1970
1980
Oterdum
40
30
(n/ha)
20
10
0
1960
1970
Spijk
80
50
60
1970
200
2000
150
1990
100
1980
Lauwers / Schild
0.04
1970
100
1960
0
0
0
1
1
2
2
(n/ha)
15
3
6
4
20
5
1980
8
Eijerlandse Gat
1960
1970
1980
1990
Twaite Shad need tidal freshwater systems to spawn
successfully and estuaries for nursing, thus the
occasional fish in Lake IJssel at present is considered
to be have entered these waters without successful
spawning (De Groot 2002). Improvement of the
water quality, in particular with regard to the oxygen
conditions, resulted in a comeback of Twaite Shad
in the Western Scheldt since 1996 with spawning
individuals since 2010 (Peter Herman, NIOZ, pers.
comm.). In the Ems, at least in some years, successful
spawning occurs (Erwin Winter, IMARES,
pers. comm.).
Fig. 2.4.7.2 Ellipse matrix of the
correlations between times series of
normalized densities of Twaite Shad
as determined by means of surveys in
the North Sea, the Wadden Sea and
Lake IJssel. Colour indicates positive
(blue) or negative (red) correlation,
colour intensity the (absolute) size of
the correlation coefficient.
Fig. 2.4.7.3 Correlations between
times series of normalized abundance
of Twaite Shad between adjacent
basins as determined by means
of surveys in the North Sea, the
Wadden Sea and Lake IJssel. Colour
indicates positive (blue) or negative
(red) correlation, colour intensity
the (absolute) size of the correlation
coefficient.
39
2.4.5 European Smelt (Osmerus eperlanus)
those in the North Sea and in Lake IJssel (Fig. 2.4.8.3).
In addition, some correlation is found between the
North Sea coastal zone and Zoutkamperlaag tidal
basin, and between the tidal basins of Lauwers and
Ems estuary (Fig. 2.4.8.3). This suggests that drivers
of the abundance of Smelt at the edges of the Dutch
Wadden Sea occur in the North Sea, but not in the
coastal zone (Fig. 2.4.8.3). It seems, however, unlikely
that drivers in the North Sea steer Smelt abundance.
The main drivers will likely be the two large
freshwater discharges from Lake IJssel and Ems (Erwin
Winter, IMARES, pers. comm.).
Smelt was caught in all surveys in the North Sea, the
Wadden Sea and Lake IJssel (Fig. 2.4.8.1). Stow-net
catches at Spijk and Oterdum are positively correlated,
whilst the stow-net catches at Spijk are negatively
correlated with those of the bottom-trawl surveys and
the fyke catches near Kornwerderzand in Lake IJssel
(Fig. 2.4.8.2).
When comparing the trends in the beam trawl surveys
in the study area, annual variations in Smelt in the
Marsdiep tidal basin appears to be correlated with
Marsdiep
8
(n/ha)
4
(n/d)
1.0
2010
0
1960
1970
1980
2000
2010
1960
1970
1990
2000
2010
1960
1970
2000
2010
400
600
(n/ha)
1990
1990
2000
2010
1960
1970
1980
1990
2000
2010
100
1970
1980
1990
2000
2010
1960
Spijk
1990
2000
2010
2000
2010
2000
2010
Oterdum
300
(n/h/80m2)
150
100
100
2010
1960
1970
1980
1990
2000
2010
1960
1990
2000
2010
2000
2010
1960
1970
1980
1990
Flevoland
(n/d)
(n/d)
30
30
40
10000
6000
10
(n/ha)
1980
1990
4000
0
2000
0
1970
1980
Kornwerderzand
8000
1000
800
600
400
200
0
1960
1970
Lake IJssel
20
2000
10
1990
1960
1970
1980
1990
2000
2010
0
1980
20
1970
Terborg
(n/h/80m2)
1980
0
0
1960
1960
1970
1980
1990
2000
2010
Fig. 2.4.8.1 Time series of normalized densities of European Smelt as determined
by means of surveys in the North Sea, Wadden Sea and Lake IJssel.
40
1970
350
600
400
(n/h/80m2)
20
200
40
(n/ha)
300
60
500
80
60
40
20
(n/ha)
0
1960
Ems estuary
400
1980
Lauwers / Schild
250
1970
200
1960
0
0
0
2
200
200
4
400
1980
Zoutkamperlaag
800
1000
600
(n/ha)
1980
Borndiep
800
12
10
8
6
(n/ha)
1990
Vlie
300
2000
(n/ha)
1990
0
2
1980
Eijerlandse Gat
400
1970
200
(n/ha)
0.5
0.0
1
0
1960
200 400 600 800
6
4
3
2
survey index (−)
Marsdiep inlet
1200
NS coastal zone
1.5
5
North Sea
1960
1970
1980
1990
Fig. 2.4.8.2 Ellipse matrix of the
correlations between times series of
normalized densities of European
Smelt as determined by means
of surveys in the North Sea, the
Wadden Sea and Lake IJssel. Colour
indicates positive (blue) or negative
(red) correlation, colour intensity
the (absolute) size of the correlation
coefficient.
Fig. 2.4.8.3 Correlations between
times series of normalized abundance
of European Smelt between adjacent
basins as determined by means
of surveys in the North Sea, the
Wadden Sea and Lake IJssel. Colour
indicates positive (blue) or negative
(red) correlation, colour intensity
the (absolute) size of the correlation
coefficient.
Smelt was formerly abundant in the Zuiderzee. After
closure of the dike in 1932, a landlocked population
developed, that matures after only 1 year instead of
after 2-3 years as for the diadromous smelt, while the
diadromous population still inhabits the coastal waters
of the Wadden Sea. The diadromous population may
either be seeking spawning grounds in open estuaries
(e.g. Ems estuary) or may be entering Lake IJssel
through the sluices, although the contribution of
diadromous smelt is neglible in the current situation
(Tulp et al. 2013). Within Lake IJssel, the Smelt
populations have declined strongly since 1990 (Fig.
2.4.8.1), and Smelt fishery was closed during five years
in the last decade because of low biomass (De Leeuw
et al. 2008).
Barium and Strontium concentrations in the otoliths
of Wadden Sea Smelt suggest a mixture of two
populations, i.e. those who were born in the Wadden
Sea (55%) and in Lake IJssel (5%) or Markermeer
41
2.4.6 Salmonids (Salmonidae)
(45%) and migrated to the Wadden Sea (Phung et al.
2015). This implies that variation in the Wadden Sea
stocks of Smelt is at least partly driven by import from
Lake IJssel.
Atlantic Salmon (Salmo salar) - The time series
of the North Sea just shows the incidental catches
(Heessen et al. 2015; Fig. 2.4.9.1). Within all other
surveys, Salmon was only caught in the two fykes in
Lake IJssel (Fig. 2.4.9.1). This species was therefore
not considered for further analyses. Low to zero
catches are most probably due to the low catchability
of Atlantic Salmon, resulting from their strong
swimming capacity (escaping nets) and their periodic
occurrence (smolts in late spring, and adults mainly
during summer and autumn) when using the Wadden
Water Board Hunze en Aa’s observes migration of
adult Smelt through the sluice of Nieuwe Statenzijl
in the Ems estuary. Monitoring with fykes in
inland waters shows that they migrate up to 20 km
upstream. Possibly they spawn in the Westerwoldse
Aa considering the fact that juvenile 0+ individuals
are caught in local fykes (Peter Paul Schollema,
Waterschap Hunze en Aa’s, pers. comm.).
1.0
0.8
1.0
0.8
Marsdiep
0.6
(n/ha)
0.4
0.6
0.4
(n/d)
0.6
(n/ha)
6
0.4
8
10
0.8
14
12
Marsdiep inlet
0.2
0.2
2010
1.0
1990
2000
2010
1.0
0.8
(n/ha)
(n/h/80m2)
1970
(n/d)
1970
1980
1990
2000
2010
1990
2000
2010
1980
1990
2000
2010
2000
2010
2000
2010
Oterdum
1960
1970
1980
1990
Flevoland
0.05
1960
1970
1980
1990
2000
2010
Fig. 2.4.9.1 Time series of normalized densities of Atlantic Salmon as determined
by means of surveys in the North Sea, Wadden Sea and Lake IJssel.
42
1970
0.04
0.6
1960
1980
(n/d)
1.0
2010
1960
Kornwerderzand
0.4
(n/ha)
2000
2010
0.0
1960
Lake IJssel
0.2
1990
2000
0.06
1980
0.0
1980
1990
Spijk
0.2
1970
0.8
1.0
1970
1980
0.0
1960
0.8
0.6
0.4
0.2
0.0
1960
1970
0.02
2010
2010
0.01
2000
2000
0.2
(n/h/80m2)
0.6
(n/ha)
0.4
0.2
1990
Terborg
1990
0.0
1960
1.0
2000
1980
Zoutkamperlaag
0.8
1990
Ems estuary
1970
0.2
(n/ha)
1980
0.0
1980
1960
0.2
1970
0.8
1.0
0.8
0.6
0.4
1970
2010
0.0
1960
0.0
1960
2000
0.00
2010
1990
Borndiep
1.0
2000
1980
0.8
1990
1970
0.6
1.0
0.6
(n/ha)
0.4
0.2
1980
Lauwers / Schild
0.0
1960
0.4
2010
0.6
2000
0.4
1990
Vlie
0.03
1980
0.0
1970
0.2
(n/ha)
1970
0.8
1.0
0.8
0.6
(n/ha)
0.4
0.2
0.0
1960
(n/h/80m2)
0.0
1960
1.0
2010
0.8
2000
0.6
1990
0.4
1980
Eijerlandse Gat
0.6
1970
0.4
1960
0.00 0.02 0.04 0.06 0.08 0.10 0.12
0
0.0
2
0.2
4
survey index (−)
NS coastal zone
1.0
North Sea
1960
1970
1980
1990
Sea as corridor to the open ocean (Erwin Winter,
IMARES, pers. comm.).
reports have been more frequent in the 1990s than
in the last decade (Heessen et al. 2015). In contrast
to Atlantic Salmon (using the Wadden Sea only as a
corridor to the open ocean), Sea Trout is roaming the
coastal areas and estuaries directly downstream their
spawning rivers to feed. The Wadden Sea will also be
partly used as a feeding habitat, but this fast swimmer
is difficult to catch with most of the survey gears
(with exception of fykes, and possibly stow nets), and
these surveys will therefore grossly underestimate its
abundance (Erwin Winter, IMARES, pers. comm.).
Sea Trout (Salmo trutta) – Within the study area, Sea
Trout was caught by the bottom trawls in the North
Sea (irregularly) and Lake IJssel (in 1995 and 1996
only), fykes in the Marsdiep (regularly) and Lake IJssel,
and some of the stow nets in the Ems estuary (Fig.
2.4.10.1). This species was therefore not considered
for further analyses. Although the small catches in the
North Sea were quite irregular since the late 1970s,
Marsdiep inlet
4
0.8
2010
1.0
2010
(n/ha)
0.2
1970
2000
2010
1980
1990
2000
2010
(n/h/80m2)
1960
1970
1970
1980
1990
2000
2010
2000
2010
2000
2010
Oterdum
1980
1990
2000
2010
1960
1970
1980
1990
Flevoland
(n/d)
0.2
(n/d)
0.3
0.3
0.4
0.06
0.4
Kornwerderzand
1960
1970
1980
1990
2000
2010
0.0
0.0
0.00
0.1
0.1
0.02
0.1
1960
0.2
1970
0.04
(n/ha)
0.3
1990
Spijk
Lake IJssel
0.2
1980
0.0
1960
2010
0.01
(n/h/80m2)
0.6
0.4
(n/ha)
2000
2000
0.0
1960
0.2
1990
Terborg
1990
0.8
1.0
2000
1980
Zoutkamperlaag
0.05
1990
Ems estuary
1970
0.04
1980
0.0
1980
0.6
(n/ha)
0.2
1970
0.8
1.0
0.8
0.6
0.4
0.2
1970
1960
0.0
1960
0.0
1960
2010
0.00
2010
2000
0.2
2000
1990
Borndiep
1.0
1990
1980
0.8
1980
Lauwers / Schild
1970
0.8
1.0
0.8
0.6
(n/ha)
0.0
0.2
0.4
0.0
1970
0.6
(n/ha)
0.2
1960
1.0
2010
0.6
2000
0.4
1990
Vlie
0.03
1980
0.02
1970
0.4
0.6
0.8
1.0
0.0
1
1960
0.4
2010
0.6
2000
0.4
1990
0.4
(n/d)
2
(n/ha)
1980
0.2
(n/ha)
0.4
0.2
0.0
1970
Eijerlandse Gat
1960
(n/ha)
0.6
3
0.8
4
3
2
survey index (−)
1
0
1960
(n/h/80m2)
Marsdiep
1.0
NS coastal zone
1.0
North Sea
1960
1970
1980
1990
2000
2010
1960
1970
1980
1990
2000
2010
1960
1970
1980
1990
Fig. 2.4.10.1 Time series of normalized densities of Sea Trout as determined
by means of surveys in the North Sea, Wadden Sea and Lake IJssel.
43
Houting (Coregonus oxyrhinchus) - Within the study
area, Houting was caught once (2010) in the Marsdiep
tidal basin and furthermore only caught in Lake
IJssel, in the bottom trawls as well as in the fykes (Fig.
2.4.11.1). This species was therefore not considered for
further analyses. Houting was historically distributed
in the Wadden Sea extending from Jutland (Denmark)
to the Schelde delta (The Netherlands). The species
has been considered extinct in the River Rhine since
the 1940s (Fig. 1.5).
2000
2010
1.0
2000
2010
2000
2010
1960
1980
1990
2000
2010
(n/h/80m2)
1960
1970
0.7
Lake IJssel
1980
1990
2000
2010
1980
1990
2000
2010
2000
2010
1970
1980
1990
2000
2010
2000
2010
Flevoland
0.10
(n/d)
0.04
0.02
0.2
0.00
0.0
1960
1970
1980
1990
2000
2010
1960
1970
1980
1990
2000
2010
Fig. 2.4.11.1 Time series of normalized densities of Houting as determined
by means of surveys in the North Sea, Wadden Sea and Lake IJssel.
44
1990
0.08
0.6
0.4
(n/d)
0.4
(n/ha)
0.3
0.2
0.1
0.0
1970
1980
Oterdum
1960
Kornwerderzand
0.5
0.8
0.6
(n/h/80m2)
0.4
0.2
0.0
1960
1970
0.0
0.2
1970
0.10
(n/ha)
0.05
1.0
(n/ha)
1990
Spijk
0.0
1960
2010
0.2
1980
0.6
1.0
1970
0.8
1990
Terborg
2000
0.2
(n/h/80m2)
0.6
(n/ha)
0.4
0.2
0.0
1980
1990
0.0
1960
1.0
1990
Ems estuary
1980
Zoutkamperlaag
0.8
1980
1970
0.8
1.0
(n/ha)
0.2
1970
0.8
1.0
0.8
0.6
0.4
0.2
1970
1960
0.0
1960
0.0
1960
2010
0.12
2010
2000
0.06
2000
1990
Borndiep
1.0
1990
1980
0.8
1980
Lauwers / Schild
1970
0.8
1.0
0.8
0.6
(n/ha)
0.0
1970
0.00
1960
0.6
2010
0.4
2000
0.6
1990
Vlie
0.4
1980
0.2
0.4
0.0
1960
0.15
1.0
0.6
(n/d)
0.4
0.2
1970
0.4
0.6
0.8
1.0
0.0
1960
0.6
2010
0.4
2000
0.6
1990
Marsdiep
0.4
1980
Eijerlandse Gat
Marsdiep inlet
0.8
1.0
0.8
1970
0.2
(n/ha)
0.6
(n/ha)
0.2
0.0
1960
(n/ha)
NS coastal zone
0.4
0.6
0.4
0.0
0.2
survey index (−)
0.8
1.0
North Sea
A successful re-introduction programme, however, reestablished a self-reproducing population (Borcherding
et al. 2014). The River IJssel, a lower branch of the
River Rhine, and/or its tributaries serve as spawning
ground. Freyhof & Schöter (2005), however, conclude
that C. oxyrinchus is a globally extinct species and
recently re-introduced coregonids in the Rhine
catchment from Danish rivers are in fact C. maraena, a
species not native to the Rhine.
1960
1970
1980
1990
In 2010, justly hatched larvae were caught directly
upstream from Kampen, where the IJssel discharges
into Lake IJssel, most probably hatched in upstream
areas of the River IJssel (Borcherding et al. 2014). The
fish caught in the Lake IJssel survey since 2006 and
in the Marsdiep in 2010 are, therefore, most likely
originating from the newly established spawning
population in the River IJssel. Fyke monitoring
programmes and seine-net efforts to catch Houting
for telemetry experiments showed that many Houting
use Lake IJssel for feeding and that only part of
the population migrated to marine environments
(Borcherding et al. 2008, 2014).
Marsdiep
1.0
6
Marsdiep inlet
0.6
(n/ha)
0.4
3
(n/d)
2010
1.0
1980
1990
2000
2010
0.2
2000
2010
1960
(n/ha)
(n/ha)
1980
1990
2000
2010
1960
Ems estuary
1970
1980
1990
2000
2010
Zoutkamperlaag
0.0
0.2
1970
1970
1980
1990
2000
2010
1960
1970
Spijk
1980
1990
2000
2010
2000
2010
2000
2010
Oterdum
1970
1980
1990
2000
2010
1970
2000
4
2010
1970
1980
1990
2000
2010
1970
1980
1990
2000
2010
1980
1990
Flevoland
0.6
(n/d)
1
0
1960
1970
0.5
6
(n/d)
3
2
200
100
0
1960
1960
5
500
400
300
(n/ha)
1990
7
600
8
6
4
2
1980
Kornwerderzand
0
(n/h/80m2)
0
1960
Lake IJssel
0.7
1960
0.4
2010
0.3
2000
0.2
1990
Terborg
0.1
1980
0.0
1970
4
1960
0
0.0
0.0
1
1
0.2
0.2
2
3
(n/h/80m2)
3
2
(n/h/80m2)
0.6
(n/ha)
0.4
0.6
0.4
(n/ha)
5
4
0.8
0.8
6
1.0
1.0
1990
Borndiep
0.0
1960
Lauwers / Schild
1980
5
1970
1970
0.2
0.8
0.6
(n/ha)
0.0
0.2
0.4
0.2
0.0
1960
0.0
1960
1.0
2000
0.8
1990
Vlie
0.6
1980
0.4
0.6
0.8
1.0
1970
0.4
1960
7
2010
1.0
2000
0.8
1990
0.6
1980
Eijerlandse Gat
0.4
1970
0
0.0
1
0.2
2
0.4
(n/ha)
0.6
4
0.8
5
0.8
10
8
6
4
survey index (−)
2
0
1960
(n/ha)
NS coastal zone
1.0
North Sea
1960
1970
1980
1990
2000
2010
1960
1970
1980
1990
Fig. 2.4.12.1 Time series of normalized densities of Three-Spined Stickleback as
determined by means of surveys in the North Sea, Wadden Sea and Lake IJssel.
45
2.4.7 Three-spined Stickleback
(Gasterosteus aculeatus)
Within the study area, Three-Spined Sticklebacks
were caught in the bottom-trawl surveys in the North
Sea and Lake IJssel, in fykes in the tidal basins of the
Marsdiep and Lake IJssel, and in the stow-net surveys
in the Ems estuary (Fig. 2.4.12.1). Variations in stownet catches appear to be correlated with each other, as
the stow-net catches near Terborg and the fyke catches
near Kornwerderzand, and the bottom-trawls in Lake
IJssel with the fyke catches near Flevoland
(Fig. 2.4.12.2). In the North Sea, catches have
been quite variable from year to year without any
clear trend. However, bottom-trawl catches are not
representative of annual fluctuations in Three-Spined
Sticklebacks (Heessen et al. 2015).
Fig. 2.4.12.2 Ellipse matrix of the correlations between times series of
normalized densities of Tree-Spined Stickleback as determined by means
of surveys in the North Sea, the Wadden Sea and Lake IJssel. Colour
indicates positive (blue) or negative (red) correlation, colour intensity the
(absolute) size of the correlation coefficient.
46
2.4.8 European Flounder
(Platichthys flesus)
explained by a stock-recruitment relationship, i.e. high
number of spawning adults in the North Sea result in
high numbers of juvenile Flounders in the western
Wadden Sea (Zwanette Jager, ZiltwaterAdvies, pers.
comm.).
Flounder was caught in all surveys in the North
Sea, the Wadden Sea and Lake IJssel (Fig. 2.4.13.1).
When comparing trends in the bottom-trawl catches
in the study area, annual variations in Flounder in
the Marsdiep tidal basin appears to be correlated
with those in the North Sea, the Vlie tidal basin and
Lake IJssel (Fig. 2.4.13.3). This positive relationship
between the North Sea and the Marsdiep might be
Marsdiep inlet
Marsdiep
200
100
150
(n/ha)
100
(n/d)
50
4
(n/ha)
2.0
1.5
1980
1990
2000
2010
1960
1970
1980
1990
2000
2010
1960
Vlie
1980
1990
2000
2010
1960
1990
2000
2010
200
150
(n/ha)
100
50
50
50
(n/ha)
100
(n/ha)
1980
Zoutkamperlaag
150
60
50
40
30
1970
Borndiep
1980
1990
2000
2010
1960
1970
1980
1990
2000
2010
1970
1980
1990
2000
2010
1960
1970
1980
1990
2000
2010
2000
2010
2000
2010
Oterdum
(n/h/80m2)
8
10
(n/h/80m2)
100
(n/ha)
12
14
150
16
Spijk
100
80
60
0
0
4
20
5
6
50
40
(n/ha)
0
1960
Ems estuary
120
Lauwers / Schild
15
1970
10
1960
0
0
0
10
20
(n/ha)
1970
150
70
Eijerlandse Gat
100
1970
200
1960
0
0
0
0.5
50
2
1.0
survey index (−)
2.5
6
250
3.0
150
NS coastal zone
8
North Sea
Within Lake IJssel, the abundance of Flounder
increased rather suddenly from less than 5 individuals
per ha up to more 30 individuals per ha in the early
1990s (Fig. 2.4.13.1).
1970
1980
1990
2000
2010
1960
1970
1980
1990
2000
2010
1960
1970
Lake IJssel
35
Terborg
1980
1990
2000
2010
1960
1970
Kornwerderzand
1980
1990
Flevoland
5
30
(n/d)
20
(n/d)
3
4
25
20
(n/ha)
15
15
1960
1970
1980
1990
2000
2010
1960
1970
1980
1990
2000
2010
0
0
0
1
5
10
2
10
10
5
0
(n/h/80m2)
20
30
6
25
40
1960
1960
1970
1980
1990
2000
2010
1960
1970
1980
1990
Fig. 2.4.13.1 Time series of normalized densities of European Flounder as
determined by means of surveys in the North Sea, Wadden Sea and Lake IJssel.
47
This increase has been attributed to a change in sluice
management in 1991-1993, where discharge sluices
were kept open until water levels at both sides of the
sluices were equal allowing some salt water near the
bottom to enter Lake IJssel (originally sluices were
closed when water levels at the seaside were still 10 cm
lower than in Lake IJssel) (Winter 2009). In addition
to sluice management, a strong correlation with
Flounder densities and discharge rates in late summer
(Aug/Sept) was observed, with an approximate slope
of +0.3 individuals per ha per m 3 discharge for the
period between 1991 and 2000 (Winter 2009).
After an initial decrease of the proportion of Flounder
with skin ulcers in front of the drainage sluices
of the Afsluitdijk from some 30% to 10% since
1988, the situation improved further at Den Oever
and Kornwerderzand following the adapted sluice
management between 1991 and 1993 (Vethaak et al.
2004). If the diseases led to increased mortality or
weaker growth and smaller sized fish, this may have
affected the reproductive potential of part of the
population through size-specific fecundity (Rijnsdorp
1994; Van der Veer et al., 2000).
The cause of the outbreak of disease in Flounder
in front of the Afsluitdijk drainage sluices has been
attributed to cumulating stress factors affecting
the fish’ immune system, making the fish more
susceptible to a variety of diseases. The most important
stress factors include long periods of time in the
unfavourable water conditions outside the drainage
sluices, including abrupt and extreme salinity
fluctuations due to the discharge of fresh water from
Lake IJssel (Vethaak et al. 2004). Reduction of these
stress factors may result in increased fecundity and,
subsequently, in increased stock size.
Fig. 2.4.13.2 Ellipse matrix
of the correlations between
times series of normalized
densities of European
Flounder as determined by
means of surveys in the North
Sea, the Wadden Sea and
Lake IJssel. Colour indicates
positive (blue) or negative (red)
correlation, colour intensity
the (absolute) size of the
correlation coefficient.
48
A decrease, however, was found in the Marsdiep
tidal inlet that more or less coincided with changes
in the coastal zone (Fig. 2.4.13.3). The absence of
synchronicity with the other areas suggest the presence
of a local driver in the coastal zone that is reflected
in the fyke catches in the Marsdiep tidal inlet. Van
der Veer et al. (2015) found a correlation between
time series of decreasing fish numbers and increasing
volumes of sand nourishments, for example, but it
is unclear if this relationship is causal or not. As was
suggested for Smelt (Erwin Winter, IMARES, pers.
comm.), variations in stocks of Flounder might also or
additionally be driven be large freshwater discharges
from Lake IJssel and Ems. This is in line with the
positive correlation in bottom-trawl catches of the
North Sea and the Ems estuary (Fig. 2.4.13.2).
Fig. 2.4.13.3 Correlations between times series
of normalized abundance of European Flounder
between adjacent basins as determined by means
of surveys in the North Sea, the Wadden Sea
and Lake IJssel. Colour indicates positive (blue)
or negative (red) correlation, colour intensity the
(absolute) size of the correlation coefficient.
49
2.5 Conclusions
From a historical perspective (100-2000 yrs),
long-term stock dynamics of the fish species under
consideration were driven by overfishing, pollution
and habitat destruction, including the interruption of
the river continuum by barriers (Chapter 1). Within
a shorter time frame (< 50 yrs), the synchronies in
abundances suggest an influence of additional factors,
from large-scale climate variation to local import
from freshwater systems (Fig. 2.5), depending on the
species.
For River Lamprey and Sea Lamprey,
synchronisation patterns were difficult to assess as
the result of possible misidentifications and low
catchabilities, in particular for Sea Lamprey. For River
Lamprey, abundances in the Ems estuary might be
driven by spawning success in the “Gasterense Diepje”,
a branch of the “Drentsche Aa” river.
For European Eel, synchrony in time series from
North Sea (including the coastal zone) and the
Wadden Sea (including the Ems estuary) suggest a
large-scale driver within the marine waters of the
study area. Such possible large-scale drivers include
atmospherically driven dispersal by ocean currents and
change in hydrology and exploitation. Within Lake
IJssel, the effects of these large-scale climate-driven
influences may have been obscured by local impacts,
including the influence of the Afsluitdijk in 1932 and
changes in fishery pressure and restocking hereafter.
For European Anchovy, the lack of correlations
between basins and adjacent marine waters (North
Sea, North Sea coastal zone) suggest the absence of
a common driver. Since Anchovy visits the study in
summer whilst most of the surveys were performed
in autumn, however, possible existing correlations
might have been missed. It cannot be excluded that the
climate-driven increase of Anchovy stocks in the North
Sea results in time in larger stocks in the study area.
For Atlantic Herring, the synchronisation in stock
dynamics in all tidal basins suggests the existence of
a local driver at the scale of the Dutch Wadden Sea.
Fig. 2.5 Potential long-term and short-term drivers (as derived from synchrony in time
series of surveys) of stock sizes of migratory fish in the Dutch Wadden Sea.
50
The correlation structure of the stock dynamics of
the tidal basins with the coastal zone of the North Sea
further suggests that the central and eastern part of
this zone are part of the synchronisation area as well.
Possible local drivers include competition for food
with ctenophores, parasitic infections and predation by
fish, birds and seals.
For Allis Shad and Twaite Shad, only
synchronisation patterns of the latter could be
analysed. Stock dynamics of Twaite Shad appear to
be synchronised in the western and eastern part of
the Dutch Wadden Sea and the adjacent coastal zone.
As has been observed for the Scheldt estuary, stock
dynamics of Twaite Shad in estuarine tidal basins
such as the Marsdiep and the Ems might be driven by
environmental conditions at their freshwater spawning
grounds. Improvement of these conditions might then
attribute to larger stocks in the Wadden Sea.
For Atlantic Salmon and Sea Trout, the catchability
of the fishing gear used during the surveys was
generally too low to get reliable information on
variation in local densities. Whilst Atlantic Salmon
uses the Wadden Sea only as a corridor to the open
ocean, Sea Trout is roaming the coastal areas and
estuaries to feed and local stocks might therefore
benefit from available resources.
For Houting, fish numbers caught were too low
for further analysis. Incidental catches of this species
throughout the study area suggested, however, that
offspring of successful reproduction of introduced fish in
River IJssel was caught in the Marsdiep tidal basin in the
same year. This implies that successful re-introduction
programs of diadromous fish in rivers might contribute
to local stocks in the Wadden Sea area.
For Three-Spined Sticklebacks, most of the
sampling gear used in the surveys was not suited to get
reliable estimates of local densities of this species. This
species is better caught by means of 1x1 m lift nets, as
been used during a monitoring study at relatively small
freshwater discharge points along the mainland coast
of the Wadden Sea area. Results of this study will be
discussed in Chapter 3.
For European Smelt and European Flounder,
synchronisation appears to occur between the North
Sea and the westernmost (Marsdiep) and easternmost
(Ems estuary) tidal basins of the Dutch Wadden Sea.
Both basins are characterised by a strong salinity
gradient, suggesting freshwater discharges as a
driving factor. Possible mechanisms include a stockrecruitment relationship, with high adult numbers
offshore resulting in high numbers of larvae and
juveniles inshore. An alternative driver might be
supply of fish to the Wadden Sea and North Sea from
lakes and rivers via freshwater discharges, as has been
observed for Smelt from Lake IJssel to the Marsdiep
tidal basin.
Summarized, local stocks of all targeted fish species
are influenced by large-scale long-term drivers
such as fisheries, habitat destruction and climate.
For some species, however, local short-term drivers
such as re-introductions, freshwater discharges and
environmental quality of the freshwater spawning
areas might obscure the influence of these drivers. If
so, then improvement of local conditions might result
in strengthening of local stocks, within the boundaries
set by the large-scale long-term drivers.
51
3. F
INDING OPTIMAL LOCATIONS FOR FISH
MIGRATION FROM MARINE TO FRESHWATER SYSTEMS
3.1 Introduction
In order to aid in identifying optimal areas and
locations for fish migration from the Wadden Sea
to adjacent freshwater systems, the following basic
information is provided in this chapter. First, an
overview of the existing options for fish passing
(including natural estuarine gradients) is presented
as potential locations for optimisation within present
conditions. Second, the density of migratory fish
at a larger (tidal basin) scale is given as an index of
potential supply of migratory fish for local options.
When fish supply is high, a large number of fish is
able to migrate to inland freshwater systems. Third,
the spatial variation in density of migratory fish at
the seaside of existing discharge stations is statistically
analysed. These local-scale densities are compared to
large (tidal basin) scale spatial variation in densities
and discussed with regard to local conditions (e.g.
freshwater discharge).
This analysis has some strong limitations, which should
be taken into account when interpreting the results.
The supply of fish (number of fish that can potentially
migrate within one day; fish d-1) in a fish passage
sampling point is actually the product of density (e.g.,
number of fish per m 2 ), area (m 2 ) and the turnover
rate of fish (net flux of fish entering and leaving the
52
study area within a day; fish fish-1 d-1). This implies
that, for example, a relatively high passing efficiency
(with many fish migrating from the sea to freshwater
systems) may result in relatively low abundances and
densities of fish in front of the fish passage (Zwanette
Jager, ZiltwaterAdvies, pers. comm.). Unfortunately,
such information is not available. Fish densities might,
therefore, only be weakly related to fish supply (Erwin
Winter, IMARES, pers. comm.).
Furthermore, finding optimal locations requires a
quantitative assessment of fish passing efficiency of
existing fish passages in the area. Despite the general
high costs of building these structures, however, no
comparable data to perform a quantitative evaluation
of the construction of fish migration facilities were
available. So far, only one study (at Roptazijl)
compared the density of fish measured in front of the
fish passage with the number of fish that goes through
(Allix Brenninkmeijer, Altenburg & Wymenga,
pers. comm.). Interpretation of the results is not easy,
however, because different fishing gears were used
at both sides of this fish passage. In another study (at
Nieuwe Statenzijl), passage efficiency was measured
by means of mark-recapture experiments with dyed
Glass Eel, but results are not yet available (Peter
Paul Schollema, Waterschap Hunze en Aa’s, pers.
comm.). More details on these studies are given in the
discussion of this chapter.
3.2 Estuarine gradients in the Dutch
Wadden Sea
Within the Dutch Wadden Sea area there are 65
locations that can be considered as estuarine gradients
as originally put forward by De Boer & Wolff (1996)
and four more locations, which are planned or under
construction. These gradients vary from tidal creeks
in saltmarshes to large freshwater sluices that discharge
over 300 m 3 s-1 on average at an annual basis. The
discharge of freshwater is a reflection of the size of the
inland potential habitat available. We have actualised
and revised the list of estuarine gradients by De
Boer & Wolff (1996) and grouped them according to
(Figure 3.1, Table 3.1):
• Sluices – characterised by an abrupt salinity gradient,
caused by relatively large discharges of freshwater
into the sea at low tide, resulting in an almost
freshwater environment during low tide and a nearly
full saline environment during high tide;
• Pumps – characterised by an abrupt salinity gradient,
caused by inputs of freshwater into the sea during
pumping, resulting in a temporary freshwater
environment;
• Culverts – characterised by an abrupt salinity
gradient, caused by relatively small inputs of
freshwater into the sea with an almost freshwater
environment during low tide and a nearly full saline
environment during high tide;
• Locks – characterised by an abrupt salinity gradient
on either side of the ship locks. Leakage of freshwater
into the sea or salt water into the lake does not
neutralize abrupt salinity gradients;
• Salt Water (SW) inlets – found at De Bol on the
island of Texel and in Polder Breebaart. Both are
enclosed brackish areas, where seawater is taken in
during high tide;
• Fresh Water (FW) seepages – at several island
locations seepage of freshwater is resulting in
estuarine gradients near the coast. These areas can be
flooded with salt water during storm tides;
• Creeks – open connections between freshwater and
seawater characterised by a gradual salinity gradient,
caused by freshwater (rainwater) coming from the
dunes and marshes, and by saltwater flowing in at
the seaside during high tide. These include island
saltmarsh creeks that are fed by water from the
Wadden Sea, with the exception of De Slufter that is
fed by North Sea water.
• Estuaries – open connection between a river and the
sea.
Fig. 3.1 Locations of estuarine gradients in the Duch Wadden Sea.
53
Many of the estuarine gradients are freshwater sluices,
pumps, culverts or locks, some of which are equipped
with facilities to enable fish to migrate from the sea
to the freshwater system, or to prevent damage to fish
when migrating from freshwater to the sea through
discharge facilities. Some pumping stations are not
suitable for anadromous migration upstream, but
downstream migration may be possible for small fish
that migrate to the sea (George Wintermans, WEB,
pers. comm.). Within the Wadden Sea, there are five
main types of fish passage facilities installed, tested or
planned (Table 3.1):
• Tide gates or tidal flaps in culverts, or small openings
or slots made in medium-scale to large-scale locks;
• Fish friendly discharge sluice and ship lock
management (FFM). The effectivity varies
depending on the way the sluices or locks are
operated and on the amount of seawater that is
allowed to flow landwards;
• Pumps used for fish bypasses (usually with low
capacity pumps, and relatively low numbers of fish
passing) or fish-friendly pumps (usually with a high
pump capacity);
• Fish ladders (“cascade”), siphon spillways (“vishevel”
in Dutch; using freshwater discharges) or Eel gutters;
• Large-scale fish passage constructions that include a
collection basin for the inflow of salt water with the
incoming flood tide.
Table 3.1 Names, characteristics and freshwater discharge (m3 s-1) of estuarine
gradients in the Dutch Wadden Sea at the islands of Texel (TX), Vlieland
(VL), Terschelling (TS), Ameland (AM), Schiermonikoog (SC), Rottumerplaat
(RP), Rottumeroog (RO) and the coastal mainland of the Provinces NorthHolland (NH), Fryslan (FR) and Groningen (GR). Information on estuarine
gradients is an update of the overview published by De Boer & Wolff (1996),
information on fish passages as supplied by George Wintermans (WEB) and
Allix Brenninkmeijer (Altenburg & Wymenga).
54
3.3 Fish densities in tidal basins of
the Dutch Wadden Sea
A tidal basin is defined as a water body in a semi
enclosed coastal area that is subject to tides.
Tidal basins are useful for comparative research
because they are logical units from morphological,
hydrodynamic, and ecological perspectives (Kraft et
al. 2011). Within the Dutch Wadden Sea, Kraft et al.
(2011) distinguished ten tidal basins, i.e. Marsdiep,
Eijerlandse Gat, Vlie, Amelander Zeegat, Pinkegat,
Zoutkamperlaag, Eilander Balg, Lauwers, Schild, and
Ems-Dollard (from west to east).
The Dutch survey area of the Demersal Fish Survey
(DFS) comprises the coastal zone north of the Wadden
Sea islands (DFS404), the Wadden Sea (from west to
east: DFS610, DFS612, DFS616 -619), and the EmsDollard estuary (DFS620). The DFS areas within
the Wadden Sea not fully match the tidal basins as
proposed by Kraft et al. (2011). Within this report,
tidal basins are therefore based upon the DFS areas,
i.e. Marsdiep (DFS610), Eijerlandse Gat (DFS612),
Vlie (DFS616), Borndiep (DFS617), Zoutkamperlaag
(DFS618), Lauwers/Schild (DFS619) and Ems-Dollard
(DFS620), (Fig. 2.2).
Fig. 3.2 Normalized densities of migratory fish ( from top
to bottom: European Eel, Twaite Shad, Herring, Houting,
Anchovy, River Lamprey, Smelt, Flounder & Sea
Lamprey) as determined by means of the Demersal Fish
Survey (DFS) between 1970 and 2013.
55
The Demersal Fish Survey (DFS), which started in
1970, was able to fish for most years in most of these
tidal basins and the adjacent Dutch coastal zone.
On average, fish densities showed strong variations
between years and basins with many low values and
only a few high values (see Chapter 2). To reduce the
strong influence of the extreme values on the overall
patterns, annual fish densities (Ft ; numbers per ha)
were rescaled to normalised fish densities (Ftn ) for each
species, according to:
Ftn = (Ft −min(Ft ) / max(Ft )−min(Ft ))
where min(Ft ) and max(Ft ) are the lowest and highest
values within the data set.
As result of the sampling gear, the DFS results are
considered to reflect the densities of demersal fish,
such as Flounder, best and less those of the other
target species (Ingrid Tulp, IMARES, pers. comm.).
Furthermore, one should consider the timing of
occurrence in the Wadden Sea in relation to the survey
56
period (September-October) (Tulp et al. 2015).
Although densities varied from year to year (see
Chapter 2 for a more elaborate discussion on
this), some species show more or less stable spatial
distribution patterns in time (Fig. 3.2). Smelt, for
example, generally showed higher densities in
the larger tidal basins of the western Wadden Sea
(Marsdiep, Vlie) compared to the eastern part of
the Dutch Wadden Sea (“O. eperlanus” in Fig. 3.2).
Anchovy is most commonly found in the Marsdiep
tidal basin (“E. encrasicolus” in Fig. 3.2). Most other
species show temporal variation as well as spatial
patterns. Houting (“C. oxyrhinchus” in Fig. 3.2) was
observed once in the Marsdiep tidal basin in 2010
(see Chapter 2). Flounder, for example, shows high
densities in the central part of the Dutch Wadden Sea
(Borndiep, Zoutkamperlaag) during the beginning
of the study period (1970-1974), but densities were
relatively high in the Marsdiep tidal basin between
1994 and 2004 (“P. flesus” in Fig. 3.2).
3.4 Fish densities near small
freshwater sluices discharging into
the Wadden Sea
3.4.1 Introduction
As part of the project ‘Making way for fish in the
Wadden Sea area”, the northern angling federations
and regional water authorities initiated a four-year
monitoring study (2012-2015) on the density of
migratory fish at the seaside of relatively small sluices,
pumps and fish passages along the mainland coast of
the Wadden Sea. During these surveys, 15 out of the
65 transitions between sea and freshwater systems in
this area were consistently sampled during late winter
and next spring. Sampling was performed by means
of 1x1m lift net with a funnel of 75 cm and a mesh
size of 1 mm and took place within 2 hrs around local
high tide, with 3 to 5 hauls per sampling day, during
4 to 5 months per year (February - June). Freshwater
discharges were always zero during sampling.
Recent data (2012-2015; Period II) could be
compared with older data (2001-2003; Period I) on
fish densities derived by similar method at the same
locations. Sampling was performed by volunteers,
and coordinated by WEB in 2012-2015 (Wintermans
2014) and by WEB and RIKZ in 2001-2003
(Wintermans & Jager 2003).
The catch data were used to explore the spatial
variation of the density of migratory fish species along
the small sluices and fish passages. The statistical
analysis was restricted to those species (Table 3.4.1),
which were consistently sampled. Flounder was, for
example, excluded because larvae were not counted
at each station. Eel was subdivided into Glass Eel and
Yellow Eel. Herring and Sprat were lumped, because
not all volunteers could identify these fish up to species
level. Species-specific fish densities (Catch Per Unit
Effort; CPUE) was calculated as the total number of
fish of a particular species caught divided by the total
number of hauls taken during that year.
CODE
NAME
LATITUDE (°N) LONGITUDE (°E)
01_DH1
Den Helder Helsdeur
52.946
4.788
02_DH2
Den Helder Oostoever
52.933
4.793
04_ROZ
Roptazijl
53.210
5.439
05_ZWH
Zwarte Haan
53.309
5.626
06_LO1
Lauwersoog discharge
53.411
6.191
07_LO2
Lauwersoog shiplock
53.405
6.188
08_NPZ
Noordpolderzijl
53.433
6.582
09_SPP
Spijksterpompen
53.413
6.873
10_DZ1
Damsterdiep
53.329
6.925
11_DZ2
Ems channel discharge
53.328
6.927
12_DZ3
Duurswold
53.327
6.930
13_DZ4
Ems channel shiplock
53.320
6.940
14_TEZ
Termunterzijl
53.302
7.038
15_FIE
Fiemel
53.299
7.073
17_NSZ
Nieuwe Statenzijl
53.231
7.208
Table 3.4.1 Overview of small freshwater discharge sluices that were
analysed for spatial variation in densities of European Eel, Herring
(lumped with Sprat), Smelt and Three-Spined Stickleback. NB: The
originally planned sampling stations “Harlingen” (nr. 03) and “Polder
Breebaart” (nr. 16) were excluded from the analyses, because sampling was
performed differently than at the other stations.
57
3.4.2 Statistical analyses
After data exploration (Zuur et al. 2010), data
were analysed according to the following stepwise
procedure for each species (and for Eel for both age
groups as well).
Step 1: Effect of Period
The analysis was started by applying a Generalized
Linear Model (GLM) with the log-transformed
catching effort (number of hauls) as an offset (so that
in essence we model the ratio) with only Period as an
effect, assuming a Poisson distribution of the numbers
of fish caught per sampling event (Zuur et al., 2009):
M1a:
( )
Catchi ~ Poisson µi
(
)
E Catchi = µi
( )
log µi = β1 + β2 × Periodi + log(Effort i )
The results show, assuming a Poisson distribution,
if the number of fish caught significantly differed
for the two periods or not. The fit of the model was
checked for overdispersion (i.e. the observed variance
is higher than the variance of a theoretical model) and
underdispersion (i.e. less variation in the data than
predicted by the model).
If overdispersion or underdispersion occurred, then
the GLM was fitted assuming a negative binomial
distribution of the numbers of fish caught per sampling
event:
M1b:
(
Catchi ~ NB µi ,k
(
)
)
E Catchi = µi
( )
log µi = β1 + β2 × Periodi + log(Effort i )
The negative binomial distribution is a valid choice
when the variation in the data is larger than allowed
for by a Poisson distribution.
The results show, assuming a negative binomial
distribution, if the number of fish caught significantly
differed for the two periods or not. If overdispersion
or underdispersion still existed, then this could be
attributed to another distribution of the data then
Poisson or negative binomial, or as the result of a
spatial effect (i.e. differences between numbers of fish
caught at the different locations).
Step 2: Effect of Location
As a next step, the Pearson residuals of the Poisson
or negative binomial GLM (model 1a or 1b) were
checked for the presence of a pattern related to the
different sampling locations (freshwater sluices and
58
fish passages). This can either be done visually via a
boxplot of the residuals versus locations or by applying
a linear regression model in which the residuals are
modelled as a function of the categorical covariate
location. A significant covariate effect indicates that
there are residual patterns, which means that the
Poisson or negative binomial GLM is faulty. In that
case a Poisson or negative binomial GLMM that takes
into account the location effect should be applied.
M2:
(
Residualsi ~ N µi ,σ 2
(
)
)
E Residualsi = µi
µi = β1 + Passagei
The results give information on how much of the
variation in the residuals (in %) can be explained by
the differences between fish numbers at the different
locations.
Step 3: Interaction between Period and
Location
If fish numbers differed for the locations per period,
there might be an additional need to add a so-called
“interaction” term, i.e. highest density of fish was
observed at other locations during in the early 2000s
than in the early 2010s. The existence of such a
difference was checked by means of comparing the
fits of two different Generalised Linear Mixed Models
(GLMM’s), assuming a Poisson or negative binomial
distribution of the data. GLMM’s allow for the analysis
of grouped data and complex hierarchical structures
in data (e.g., different catches at the locations for the
different periods). One GLMM model included Period
as fixed term and Fish Passage as random effect (M3),
whilst the other model included Period as fixed term,
Fish Passage as random intercept and a random slope
for Period (M4):
M3:
(
Catchi ~ NB µi ,k
(
)
)
E Catchi = µi
( )
log µi = β1 + β2 × Periodi + log(Effort i )+ ai
M4:
2
a ~ N(0,σ 1,Passage
)
i
(
Catchi ~ NB µi ,k
(
)
)
E Catchi = µi
( )
log µi = β1 + β2 × Periodi + log(Effort i )+ ai + bi × Periodi
2
ai ~ N(0,σ 1,Passage
)
2
)
b ~ N(0,σ 2,Passage
i
The accuracy of the fit of these models was checked
by comparison of additional models assuming Poisson
distribution of the data and by taking the presence of
catches with no individuals of the particular species
(zero-inflated data sets) into account.
The best model (M3 versus M4) was identified by
means of the Akaike Information Criterion (AIC), a
measure of the relative quality of statistical models for
a given set of data (Akaike 1980). The AIC deals with
the trade-off between the goodness-of-fit of the model
and the complexity of the model. If the difference in
AIC between the models is smaller than 2, then the
most simple model (in this case M3) is considered to
be the best one explaining the data.
Step 4: Spatial and temporal correlation
Significant differences between fish caught at different
locations can be the result of local conditions at the
small freshwater sluices or fish passages or reflecting
a wider spatial pattern (e.g., more fish in the western
than in the eastern part of the Dutch Wadden Sea).
Significant differences between fish caught at different
periods can be the result of the periods themselves or
reflecting a dependency between years (e.g., if fish is
abundant in one year it may also be abundant in the
next and vice versa).
The existence of such spatial and temporal correlations
was analysed by means of interpretation of variograms,
i.e. graphs that visualise the autocorrelation of a variable
such as the density of fish at sluices and passages in
relationship to space (km) or time (year). The number
of unique years (n=7), however, was not sufficient to
analyse the existence of temporal correlation. It was
therefore assumed that temporal correlation, if present,
was captured by the random effect of location (the term
“Passage” in models 3 and 4).
Step 5: Identification of significantly different
stations
To include uncertainties in anticipated performance
indices (e.g., fish densities at fish passages), the
statistical analysis of the best model (model 3 or 4,
with or without spatial correlation) was performed
by means of Markov Chain Monte Carlo (MCMC)
methods.
The Markov Chain refers to a random process that
undergoes transitions from one state to another on a
state space, where the probability distribution of the
next state depends only on the current state and not
on the sequence of events that preceded it (e.g., rolling
a dice). Monte Carlo refers to running a computer
simulation to simulate this random process many
times (e.g., as rolling a dice time after time) and see
the probability as the number of a particular outcome
divided by the number of simulations performed (e.g.,
the number of times that you rolled a five divided by
the total number of rolls).
The Deviance Information Criterion (DIC), an
estimate of expected predictive error, is a measure
of how well the model fits the data (lower deviance
is better). The outcomes of the model allow for
Bayesian post-hoc test to identify which locations are
significantly different from each other with respect
to anticipated fish densities. These results can be used
to group locations according to their commonalities
(i.e., no significant differences in densities between
locations).
Statistical analyses were performed by Alain Zuur
(HighStat) using R and relevant packages (lmer).
Interpretations of the results were checked by George
Wintermans (WEB).
59
3.4.3 Results
European Eel (Anguilla anguilla)
Glass Eel stage
Summary results stepwise analysis
• No significant effect of Period
• Significant effect of Location (60% of variation
explained)
• No spatial correlation
• Densities high at ROZ, ZWH, NPZ & NSZ, and
lower at other stations
• Locations fall into 13 density groups
The best model for Glass Eel with regard to testing the
effect of Period was the model that assumed a negative
binomial distribution of the data (lowest AIC for
Model 1b compared to Model 1a; see Table 3.4.2.1),
with a non-significant effect of Period on fish densities
near fish passages (p > 0.05). The overdispersion of
this model (d = 1.88) might be due to a spatial effect.
The residuals of Model 1b showed a strong spatial
effect (p < 0.001) and explained more than 60% of
the variation. The AIC of Models 3 and 4 were not
different enough (difference < 2) to be used as a
criterion for best model, so the parsimony rule was
STEP
1
MODEL
MODEL 1A
MODEL 1B
2
MODEL 2
3
MODEL 3
4
5
MODEL 4
VARIOGRAM
MODEL 5
PARAMETER
AIC
AIC
Intercept
Period
Dispersion
SS FishPassage
Multiple r2
AIC
Intercept
Period
Dispersion
AIC
Spatial correlation
Nr of iterations
Intercept
Period
Size
Sigma site
DIC
Value
459197
1480
2.181 ± 0.277
0.202 ± 0.367
1.88
114.44
0.602
1372
0.601 ± 0.507
-0.045 ± 0.252
0.79
1369
In conclusion, the densities of Glass Eel were highest
at the fish passages of Roptazijl (ROZ) followed by
Zwarte Haan (ZWH), Noordpolderzijl (NPZ) and
Nieuwe Statenzijl (NSZ), whilst lowest densities were
found at Den Helder2 (DH2) and Fiemel (FIE) (Fig.
3.4.2; Table 3.4.2.2). This spatial distribution was
similar for both periods and not due to larger spatial
patterns (Fig. 3.4.2; Table 3.4.2.2). Locations were
significantly different with regard to density of Glass
Eel and could be divided into 13 groups with no
overlap between the first three groups (that included
the four locations with highest densities of Glass Eel)
and the remaining ten locations where densities were
significantly lower (Table 3.4.2.2).
P-VALUE
REMARKS
Better model among M11 and M1b
< 0.001
0.582
< 0.001
Best model
0.236
0.859
Not visible
50000
0.685 ± 0.596
-0.045 ± 0.255
0.828 ± 0.109
2.108 ± 0.483
1342
Table 3.4.2.1 Summary of results of stepwise statistical
analyses of European Eel (Glass Eel stage). Less than
1% of this data set consisted of zero counts.
60
applied (identifying the most simple model as best
model, which was Model 3). This implies that fish
densities over locations showed a more or less similar
pattern during the two periods. The variogram did
not show the presence of spatial correlation in the data
set. The parameter values for Intercept and Period of
the fit of Model 5 were comparable to those of Model
3 and the DIC was relatively low, indicating a good
predictive power of this model for the differences in
densities of Glass Eel between locations.
RANK
NAME
CODES
GROUPS
1
04_ROZ
A
I
2
05_ZWH
AB
II
3
08_NPZ
BC
III
4
17_NSZ
C
IV
5
10_DZ1
D
V
6
14_TEZ
DE
VI
7
01_DH1
DEFGH
VII
8
11_DZ2
DEFGH
VII
9
09_SPP
DEFGHI
VIII
10
07_LO2
EFGHI
VIII
11
13_DZ4
FGHI
IX
12
12_DZ3
GHI
X
13
06_LO1
HIJ
XI
14
15_DEF
IJ
XII
15
02_DH2
J
XIII
150
A B C D E
F
G H I
J
Table 3.4.2.2 Grouping of sampling
locations for European Eel (Glass
Eel stage) as derived from a Bayesian
post-hoc test. Locations are ranked
from high to low fish densities as
derived from Model 2. Green blocks
indicate similarity (non-significant
differences) within a column.
01_DH1
02_DH2
04_ROZ
05_ZWH
06_LO1
07_LO2
08_NPZ
09_SPP
10_DZ1
11_DZ2
12_DZ3
13_DZ4
14_TEZ
15_FIE
17_NSZ
100
50
0
150
CPUE European Eel (glass eel)
100
50
0
150
100
50
0
150
100
50
0
PeriodI
PeriodII
PeriodI
PeriodII
PeriodI
PeriodII
Fig. 3.4.2 Boxplots of densities (CPUE) of Glass Eel (Anguilla
anguilla) as caught by lift nets at various freshwater sluices and fish
passages along the mainland of the Dutch Wadden Sea during 20012003 (Period I) and 2012-2015 (Period II).
61
Yellow Eel stage
Summary results stepwise analysis
• A significant effect of Period
• Significant effect of Location (47% of variation
explained)
• The Period effect did not change over time
• No spatial correlation
• Densities high at ROZ, NSZ, ZWH & NPZ, and
lower at other stations
• Locations fall into 5 density groups
The best model for Yellow Eel with regard to testing
the effect of Period was the model that assumed a
negative binomial distribution of the data (lowest
AIC for Model 1b compared to Model 1a; see Table
3.4.3.1), with a significant effect of Period on fish
densities near freshwater sluices and fish passages
(p < 0.001). This model was slightly underdispersed
(d = 0.79), indicating that the Period effect would
be even more significant if the heterogeneity in the
data would be fully covered. The residuals of Model
1b showed a strong spatial effect (p < 0.001) and
explained almost 47% of the variation. The AIC of
Models 3 and 4 was different enough (> 3) to be
used as a criterion for best model, being Model 3.
STEP
1
MODEL
MODEL 1A
MODEL 1B
2
MODEL 2
3
MODEL 3
4
5
MODEL 4
VARIOGRAM
MODEL 5
PARAMETER
AIC
AIC
Intercept
Period
Dispersion
SS FishPassage
Multiple r2
AIC
Intercept
Period
Dispersion
AIC
Spatial correlation
Nr of iterations
Intercept
Period
Size
Sigma site
DIC
Value
1848
365
-2429 ± 0.454
-1516± 0.604
0.79
37.49
0.468
307
-6.128 ± 1277
-1250 ± 0.402
0.47
310
In conclusion, the densities of Yellow Eel were highest
at Roptazijl (ROZ) followed by Nieuw Statenzijl
(NSZ), Zwarte Haan (ZWH) and Noordpolderzijl
(NPZ), whilst lowest densities were found at Den
Helder2 (DH2) and Fiemel (FIE) (Fig. 3.4.3; Table
3.4.3.2). The densities were higher during the first
period compared to the second period, but similarly
distributed over the locations and could not be
attributed to larger spatial patterns (Fig. 3.4.3; Table
3.4.3.2). Locations were significantly different with
regard to density of Yellow Eel and could be divided
into 5 groups with no overlap between the first
three groups (that included the four locations with
highest densities of Yellow Eel) and the remaining
ten locations where densities were significantly lower
(Table 3.4.3.2).
P-VALUE
REMARKS
Best model
< 0.001
0.012
< 0.001
Best model
< 0.001
< 0.001
Weakly visible
300 000
-6.498 ± 1770
-1.259 ± 0.417
0.809 ± 0.226
4.839 ± 1.895
279
Table 3.4.3.1 Summary of results of stepwise statistical
analyses of European Eel (Yellow Eel stage). More
than 67% of this data set consisted of zero counts.
62
This implies that the Period effect on fish densities
at sampling locations did not change over time, i.e.
between the two periods. The variogram showed a
weak presence of spatial correlation in the data set.
The parameter values for Intercept and Period of the
fit of Model 5 were comparable to those of Model
3 and the DIC was relatively low, indicating a good
predictive power of this model for the differences in
densities of Yellow Eel between sampling locations.
RANK
NAME
1
04_ROZ
A B C D CODES
A
GROUPS
I
2
07_NSZ
A
I
3
05_ZWH
A
I
4
08_NPZ
A
I
5
14_TEZ
B
II
6
11_DZ3
BC
II
7
11_DZ2
CD
IV
8
01_DH1
CD
IV
9
06_LO1
CD
IV
10
07_LO2
CD
IV
11
09_SPP
CD
IV
12
10_DZ1
CD
IV
13
13_DZ4
D
V
14
15_DEF
D
V
15
02_DH2
D
V
Table 3.4.3.2 Grouping of sampling
locations for European Eel (Yellow
Eel stage) as derived from a Bayesian
post-hoc test. Locations are ranked
from high to low fish densities as
derived from Model 2. Green blocks
indicate similarity (non-significant
differences) within a column.
01_DH1
02_DH2
04_ROZ
05_ZWH
06_LO1
07_LO2
08_NPZ
09_SPP
10_DZ1
11_DZ2
12_DZ3
13_DZ4
14_TEZ
15_FIE
17_NSZ
1.5
1.0
0.5
0.0
1.5
CPUE European Eel (yellow eel)
1.0
0.5
0.0
1.5
1.0
0.5
0.0
1.5
1.0
0.5
0.0
PeriodI
PeriodII
PeriodI
PeriodII
PeriodI
PeriodII
Fig. 3.4.3 Boxplots of densities (CPUE) of Yellow Eel (Anguilla
anguilla) as caught by lift nets at various freshwater sluices and fish
passages along the mainland of the Dutch Wadden Sea during 20012003 (Period I) and 2012-2015 (Period II).
63
Hering/Sprat
Summary results stepwise analysis
• No significant effect of Period
• Weak effect of Location (21% of variation explained)
• No spatial correlation
• Densities gradually declining from LO2 to DZ2
• Locations fall into 7 (partly overlapping) density
groups
The best model for Herring/Sprat with regard to
testing the effect of Period was the model that assumed
a negative binomial distribution of the data (lowest
AIC for Model 1b compared to Model 1a; see Table
3.4.4.1), with no significant effect of Period on fish
densities near freshwater sluices and fish passages
(p > 0.9). This model was slightly overdispersed (d =
1.79), indicating a possible spatial effect. The residuals
of Model 1b showed a weak spatial effect (p = 0.055)
and explained approximately 21% of the variation. The
AIC of Models 3 and 4 was different enough
(> 3) to be used as criterion for best model, being
STEP
1
MODEL
MODEL 1A
MODEL 1B
2
MODEL 2
3
MODEL 3
4
5
MODEL 4
VARIOGRAM
MODEL 5
PARAMETER
AIC
AIC
Intercept
Period
Dispersion
SS FishPassage
Multiple r2
AIC
Intercept
Period
Dispersion
AIC
Spatial correlation
Nr of iterations
Intercept
Period
Size
Sigma site
DIC
Value
22129
695
-0.735 ± 0.422
-0.030± 0.560
1.35
50.31
0.218
690
-1.934 ± 0.599
0.009 ± 0.615
0.71
694
In conclusion, the densities of Herring/Sprat were
highest at the ship locks of Lauwersoog (LO2) and
declined gradually to the lowest densities as observed
at the discharge sluices of the Ems canal (DZ2) (Fig.
3.4.4; Table 3.4.4.2). The densities similarly distributed
during both periods and could not be attributed
to larger spatial patterns (Fig. 3.4.4; Table 3.4.4.2).
Locations were significantly different with regard to
density of Herring/Sprat and could be divided into 7
groups with large overlaps between groups of locations
(Table 3.4.4.2).
P-VALUE
REMARKS
Best model
0.082
0.957
< 0.001
Best model
0.001
0.989
Weakly visible
50 000
-1.576 ± 0.680
0.013 ± 0.651
0.170 ± 0.030
1.720 ± 0.534
690
Table 3.4.4.1 Summary of results of stepwise statistical
analyses of Atlantic Herring & Sprat. Almost 50% of
this data set consisted of zero counts.
64
Model 3. This implies that the Period effect on fish
densities at sampling locations did not change over
time, i.e. between the two periods. The variogram
showed no presence of spatial correlation in the data
set. The parameter values for Intercept and Period
of the fit of Model 5 were more or less comparable
to those of Model 3 and the DIC was relatively low,
indicating a reasonably good predictive power of this
model for the differences in densities of Herring/Sprat
between locations.
RANK
NAME
1
07_LO2
A
I
2
14_TEZ
AB
II
3
17_NSZ
ABC
III
4
05_ZWH
ABCD
IV
5
01_DH1
ABCD
IV
6
15_DEF
ABCDD
IV
7
13_DZ4
ABCD
IV
8
09_SPP
ABCD
IV
9
10_DZI
BCD
V
10
04_ROZ
BCD
V
11
06_LOI
BCD
V
12
12_DZ3
CD
VI
13
08_NPZ
CD
VI
14
02_DH2
CD
VI
15
11_DZ2
D
VII
12
A B C D CODES
GROUPS
Table 3.4.4.2 Grouping of sampling
locations for Atlantic Herring & Sprat
as derived from a Bayesian posthoc test. Locations are ranked from
high to low fish densities as derived
from Model 2. Green blocks indicate
similarity (non-significant differences)
within a column.
01_DH1
02_DH2
04_ROZ
05_ZWH
06_LO1
07_LO2
08_NPZ
09_SPP
10_DZ1
11_DZ2
12_DZ3
13_DZ4
14_TEZ
15_FIE
17_NSZ
9
6
3
0
12
CPUE Atlantic Herring & Sprat
9
6
3
0
12
9
6
3
0
12
9
6
3
0
PeriodI
PeriodII
PeriodI
PeriodII
Year
PeriodI
PeriodII
Fig. 3.4.4 Boxplots of densities (CPUE) of Atlantic Herring (Clupea
harengus) and Sprat (Sprattus sprattus) as caught by lift nets at various
freshwater sluices and fish passages along the mainland of the Dutch Wadden
Sea during 2001-2003 (Period I) and 2012-2015 (Period II).
65
Smelt (Osmerus eperlanus)
Summary results stepwise analysis
• No significant effect of Period
• Weak effect of Location (21% of variation explained)
• Weak spatial correlation
• Densities gradually declining from NPZ to DEF
• Locations fall into 6 (all partly overlapping) density
groups
The best model for Smelt with regard to testing the
effect of Period was the model that assumed a negative
binomial distribution of the data (lowest AIC for
Model 1b compared to Model 1a; see Table 3.4.5.1),
with no significant effect of Period on fish densities
near freshwater sluices and fish passages (p > 0.5).
This model was overdispersed (d = 2.36), indicating
a possible spatial effect. The residuals of Model 1b
showed a very weak spatial effect (p = 0.067) and
explained approximately 21% of the variation. The
AIC of Models 3 and 4 was different enough (> 10)
STEP
1
MODEL
MODEL 1A
MODEL 1B
2
MODEL 2
3
MODEL 3
4
5
MODEL 4
VARIOGRAM
MODEL 5
PARAMETER
AIC
AIC
Intercept
Period
Dispersion
SS FishPassage
Multiple r2
AIC
Intercept
Period
Dispersion
AIC
Spatial correlation
Nr of iterations
Intercept
Period
Size
Sigma site
DIC
Value
28464
1135
-0.118 ± 0.216
-0.162 ± 0.286
2.36
50.31
0.211
1110
-0.517 ± 0.335
0.073 ± 0.269
1.16
1124
P-VALUE
REMARKS
Best model
0.586
0.571
0.0069
Best model
0.122
0.786
Weakly visible
50 000
-0.414 ± 0.376
-0.074 ± 0.275
0.719 ± 0.095
1.214 ± 0.307
1098
Table 3.4.5.1 Summary of results of stepwise statistical
analyses of European Smelt. Less than 5% of this data
set consisted of zero counts.
66
to be used as criterion for best model, being Model
3. This implies that the Period effect (which was
not significant in the first place) on fish densities at
sampling locations did not change over time, i.e.
between the two periods. The variogram showed a
weak presence of spatial correlation in the data set.
The parameter values for Intercept and Period of the
fit of Model 5 were more or less comparable to those
of Model 3 and the DIC was relatively low, indicating
a reasonably good predictive power of this model for
the differences in densities of smelt between locations.
In conclusion, the densities of Smelt were highest at
the pumps of Noordpolderzijl (NPZ) and declined
gradually to the lowest densities as observed at
Fiemel (FIE) (Fig. 3.4.5; Table 3.4.5.2). The densities
similarly distributed during both periods and could not
be attributed to larger spatial patterns (Fig. 3.4.5; Table
3.4.5.2). Locations were significantly different with
regard to density of Smelt and could be divided into
6 groups with overlaps between groups of locations
(Table 3.4.5.2).
RANK
NAME
1
08_NPZ
A
I
2
07_LO2
AB
II
3
06_LO1
AB
II
4
04_ROZ
AB
II
5
17_NSZ
AB
II
6
09_SPP
B
III
7
01_DH1
B
III
8
02_DH1
B
III
9
14_TEZ
BC
VI
10
13_DZ4
BC
VI
11
10_DZ1
BC
VI
12
11_DZ2
BC
VI
13
05_ZWH
BC
VI
14
12_DZ3
CD
VII
15
15_DEF
D
VIII
20
A B C D CODES
GROUPS
Table 3.4.5.2 Grouping of sampling
locations for European Smelt as
derived from a Bayesian post-hoc test.
Locations are ranked from high to low
fish densities as derived from Model
2. Green blocks indicate similarity
(non-significant differences) within a
column.
01_DH1
02_DH2
04_ROZ
05_ZWH
06_LO1
07_LO2
08_NPZ
09_SPP
10_DZ1
11_DZ2
12_DZ3
13_DZ4
14_TEZ
15_FIE
17_NSZ
15
10
5
0
20
15
CPUE European Smelt
10
5
0
20
15
10
5
0
20
15
10
5
0
PeriodI
PeriodII
PeriodI
PeriodII
PeriodI
PeriodII
Fig. 3.4.5 Boxplots of densities (CPUE) of Smelt (Osmerus eperlanus)
as caught by lift nets at various freshwater sluices and fish passages along
the mainland of the Dutch Wadden Sea during 2001-2003 (Period I)
and 2012-2015 (Period II).
67
Three-Spined Stickleback (Gasterosteus
aculeatus)
Summary results stepwise analyses
• No significant effect of Period
• Significant effect of Location (71% of variation
explained)
• No spatial correlation
• Densities high at ZWH, ROZ & NPZ, and lower at
other stations
• Locations fall into 9 density groups
The best model for Three-Spined Stickleback with
regard to testing the effect of Period was the model
that assumed a negative binomial distribution of the
data (lowest AIC for Model 1b compared to Model 1a;
Table 3.4.6.1), with no significant effect of Period on
fish densities near freshwater sluices and fish passages
(p > 0.6). This model was slightly overdispersed (d =
1.50), indicating a possible spatial effect. The residuals
of Model 1b showed a strong spatial effect (p < 0.001)
and explained more than 70% of the variation. The
AIC of Models 3 and 4 was not different enough (<
3) to be used as criterion for the best model, therefore
Model 3 (the most simple model) was considered to
be the best model. This implies that the Period effect
STEP
1
MODEL
MODEL 1A
MODEL 1B
2
MODEL 2
3
MODEL 3
4
5
MODEL 4
VARIOGRAM
MODEL 5
PARAMETER
AIC
AIC
Intercept
Period
Dispersion
SS FishPassage
Multiple r2
AIC
Intercept
Period
Dispersion
AIC
Spatial correlation
Nr of iterations
Intercept
Period
Size
Sigma site
DIC
(which was not significant in the first place) on fish
densities at sampling locations did not change over
time, i.e. between the two periods. The variogram
showed no presence of spatial correlation in the data
set. The parameter values for Intercept and Period of
the fit of Model 5 were comparable to those of Model
3 and the DIC was relatively low, indicating a good
predictive power of this model for the differences
in densities of Three-Spined Stickleback between
locations.
In conclusion, the densities of Three-Spined
Stickleback were highest at Zwarte Haan (ZWH),
Roptazijl (ROZ) and Noordpolderzijl (NPZ) and
lowest densities were observed at as observed at the
ship locks of the Ems canal (DZ4) and Duurswold
(DZ3) (Fig. 3.4.6; Table 3.4.6.2). The densities
similarly distributed during both periods and could
not be attributed to larger spatial patterns (Fig. 3.4.6;
Table 3.4.6.2). Locations were significantly different
with regard to density of Three-Spined Stickleback
and could be divided into 9 groups with no overlap
between the first group (that included the three
locations with highest densities of Three-Spined
Stickleback) and the remaining twelve locations where
densities were significantly lower (Table 3.4.6.2).
Value
515624
1662
2.612 ± 0.234
0.142 ± 0.311
1.50
108.22
0.715
1555
1.534 ± 0.414
-0.045 ± 0.252
0.86
1556
REMARKS
Best model
< 0.001
0.647
< 0.001
Best model
0.236
0.859
Not visible
50 000
1.588 ± 0.467
-0.073 ± 0.200
1.207 ± 0.162
1.723 ± 0.388
1524
Table 3.4.6.1 Summary of results of stepwise statistical analyses of
Three-Spined Stickleback. For this species, there were no zero counts
in the data set (i.e., these fish were caught in every haul).
68
P-VALUE
RANK
NAME
1
05_ZWH
A B C D E
F
G CODES
A
GROUPS
I
2
04_ROZ
A
I
3
08_NPZ
A
I
4
14_TEZ
B
II
5
01_DH1
BC
III
6
10_DZ1
BCD
IV
7
06_LO1
BCD
IV
8
11_DZ2
CDE
V
9
17_NSZ
CDE
V
10
15_DEF
CDE
V
11
09_SPP
CDE
V
12
07_LO2
DE
VI
13
02_DH2
EF
VII
14
12_DZ3
FG
VIII
15
13_DZ4
G
IX
Table 3.4.6.2 Grouping of locations
for Three-Spined Stickleback as
derived from a Bayesian post-hoc test.
Locations are ranked from high to low
fish densities as derived from Model
2. Green blocks indicate similarity
(non-significant differences) within a
column.
01_DH1
02_DH2
04_ROZ
05_ZWH
06_LO1
07_LO2
08_NPZ
09_SPP
10_DZ1
11_DZ2
12_DZ3
13_DZ4
14_TEZ
15_FIE
17_NSZ
100
50
0
CPUE Three−Spined Stickleback
100
50
0
100
50
0
100
50
0
PeriodI
PeriodII
PeriodI
PeriodII
PeriodI
PeriodII
Fig. 3.4.6 Boxplots of densities (CPUE) of Three-Spined Stickleback
(Gasterosteus aculeatus) as caught by lift nets at various freshwater sluices
and fish passages along the mainland of the Dutch Wadden Sea during
2001-2003 (Period I) and 2012-2015 (Period II).
69
3.4.4. Discussion
Period
European Eel (Yellow Eel) was the only group for
which a significant effect of Period on densities
(Period II < Period I) was observed (Table 3.4.3.2; Fig.
3.4.3). This observation is in line with the observed
decline in densities of European Eel at tidal-basin
scale between 1970 and 2013 in general (Fig. 3.3.1)
and for the periods of lift-netting (2001-2003 and
2012-2015) in particular (Fig. 3.4.8). This decline is
observed for Eels at a large-scale, and attributed to
atmospherically driven dispersal by ocean currents and
change in hydrology and exploitation (see Chapter 2).
For Glass Eel and the other fish species, the effect of
period was not significant, implying that densities at
the freshwater sampling points did not show an overall
increase or decrease from Period I (2001-2003) to
Period II (2012-2015).
Location
The Location effect was significant for all species,
explaining 21% (Herring/Sprat and Smelt), 47%
(Yellow Eel), 60% (Glass Eel) and 71% (Three-Spined
Stickleback) of the variation in local densities. This
implies that densities, in particularly of Glass Eel and
Three-Spined Sticklebacks, were consistently low or
high at particular sampling points.
Tidal Basin MD VL BD ZL Spatial correlation
For most species, the spatial correlation was not
significant. Only for Smelt, a weak spatial correlation
was found, implying that there might be a larger (e.g.,
a gradual shift from west to east) than local (freshwater
discharge point) spatial pattern in densities for this
species. Results of the lift-net surveys (Table 3.4.5.2)
and bottom trawl surveys during Periods I and II (Fig.
3.4.8) revealed, however, no obvious spatial pattern.
Stations
Stations differed significantly in species-specific
densities of fish, with highest densities found
at Roptazijl and Zwarte Haan (4 out of 5),
Noordpolderzijl and Nieuwe Statenzijl (3 out of 5),
Lauwersoog shiplock (2 out of 5) and Den Helder
Helsdeur, Lauwersoog discharge, Spijksterpompen,
Ems channel shiplock, Termunterzijl and Fiemel (1
out of 5) (Fig. 3.4.8). Annually averaged freshwater
discharge rates vary strongly between these stations,
i.e. with more than 40 m 3 s-1 at the discharge sluices
of Lauwersoog, between 2 and 11 m 3 s-1 at Nieuwe
Statenzijl, Den Helder Helsdeur, Den Helder
Oostoever and Termunterzijl, around 1 m 3 s-1 at
Roptazijl and Zwarte Haan, and less than 1 m3 s-1 at
the shiplocks near Lauwersoog and the Ems channel
(Table 3.1).
LS ED Fish Passage DH1 DH2 ROP ZWH LO1 LO2 NPZ SPP DZ1 DZ2 DZ3 DZ4 TEZ Glass eel Yellow eel Herring Smelt SBckleback Fig. 3.4.8 Locations at (potential) fish passages with highest densities of migratory fish
species as determined by means of surveys in 2001-2003 and 2011-2015.
70
FIE NSZ roptazijl
At Roptazijl (Gemaal Ropta), highest densities of
Glass Eel, Yellow Eel, Smelt and Three-Spined
Stickleback were found (Fig. 3.4.8). Roptazijl is
positioned in the Vlie basin and discharges directly
into the relative deep water of the Kimstergat.
A location near deep water is advantageous for
the attraction of migratory fish. Moreover, the
configuration of the breakwaters near Roptazijl is such
that there is a deep and wave-sheltered basin in which
fish can wait for a suitable moment to pass through,
possibly increasing local densities (Fig. 3.4.9). In 2001,
a siphon spillway (“vishevel”) has been constructed
at Gemaal Ropta (George Wintermans, WEB, pers.
comm.).
At Roptazijl, fish passage efficiency was measured
from 2002 to 2014 by means of a comparison on
density of fish at the seaward-side measured in spring
with a 1x1 m lift net and the fish that passed the fish
passage at the inside of Roptazijl in the same period
using a fyke net (Brenninkmeijer et al., in prep.).
As the result of the species-specific differences in
catchability of the fishing gear, this type of data is very
difficult to interpret. The authors have been looking
for a multiplication factor for the CPUE with the lift
net (the numbers of fish caught in 5 lift-net hauls) as
a proxy for the numbers of fish inside (the numbers
caught in the fyke net). They found a multiplication
factor of 7 for the CPUE of Glass Eel and a factor 17
for Three-Spined Stickleback to calculate the numbers
of fish inside. Furthermore they also found large
interannual differences and recommended determining
the efficiency of fish passages with mark-recapture
experiments with fish tags (3S-Sticklebacks) or fish
dyeing (Glass Eel).
zwarte haan
At Zwarte Haan (Gemaal Miedema), highest densities
of Glass Eel, Yellow Eel, Herring and Three-Spined
Stickleback were found. In contrast to the conditions
at Roptazijl, the freshwater at Zwarte Haan is not
discharged in deep water, but on the tidal flat. Because
the end of the discharge channel has silted up, there
is no connection with the sea during low tide (Fig.
3.4.9). The sluice discharges freshwater every 1 to 2
days, so there is locally only an intermittent freshwater
discharge. Local conditions can, therefore, not easily
explain the consistently high densities of fish caught
by the lift nets at this point. Zwarte Haan is, however,
under the influence of freshwater discharges from
Lake IJssel at Kornwerderzand (Duran-Matute et al.
2014; Fig. 3.4.10). The large-scale gradient in salinity
towards the mainland coast may attract fish that will
eventually detect small-scale salinity gradients near
local freshwater sluices.
Fig. 3.4.9 Configuration of the freshwater discharge sluices
at Roptazijl (left) and Zwarte Haan (right). Images from
Google Satellite.
71
Recently (2015) a fish passage has been realised at
Zwarte Haan. Fish are lured with freshwater into a
collection basin of 100 m3. This water then flows
into the polder by gravitational current under a mild
slope, so the fish do not swim against the flow. The
efficiency of this mechanism has to be validated. It
now seems that small Eels are trapped within the
culvert since they always swim against the current
(Allix Brenninkmeijer, Altenburg & Wymenga, pers.
comm.).
The regional water authority Hunze en Aa’s is also
studying the fate of fish further upstream in the water
system (Peter Paul Schollema, Hunze en Aa’s, pers.
comm.). A number of fish passage facilities is, and
will be, equipped with PIT antennas. This enables the
tracking of tagged fish that use Nieuwe Statenzijl and
provides quantitative information on the fish passage
effectivity in the water system of the Westerwolde
river basin. This project receives funding from the
Waddenfonds.
noordpolderzijl
den helder helsdeur
Noordpolderzijl is located more or less at the
tidal divide between the Lauwers and Ems tidal
basins. The configuration of the freshwater sluice
at Noordpolderzijl is comparable to Zwarte Haan
and there is a high density in fish. The freshwater is
discharged into a long gully that runs through the tidal
flats. In 2013 a fish passage with a cat-flap has been
constructed here. Due to regular dredging activities
carried out in the Noordpolderzijl channel, the lift-net
sampling data might underestimate the attractiveness
of this location without this disturbance (George
Wintermans, WEB, pers. comm.).
Both freshwater sluices in Den Helder, i.e. Helsdeur
and Oostoever, do not seem to attract that many
fish. Helsdeur is located in the back of Den Helder
Harbour. This layback position might explain a low
attractiveness. The sluice Oostoever discharges directly
into the Wadden Sea, in a shallow subtidal gully. Two
of the four sluice openings are equipped with boosters
with a combined pump capacity of 28 m 3 s-1. One of
the four sluice openings has been equipped with a
fish passage, but due to malfunctioning this has been
removed. At present the Regional Water Authority
HHNK operates with fish-friendly sluice management
in all four openings. It has breakwaters and a basin
that make a good resting place. The problem seems to
be the low frequency of discharge, so there is a lack
of freshwater to lure and retain fish. Wintermans &
Dankers (2003) studied possibilities to optimise this
functionality.
nieuwe statenzijl
Eight freshwater sluices have been studied in the
Ems Dollard tidal basin. The one that has the highest
density of fish is Nieuwe Statenzijl. In 2013, a catflap as well as an eel-gutter have been constructed.
In spring 2014 and 2015 extensive research has been
committed by the regional water authority Hunze en
Aa’s (Peter-Paul Schollema) and Van Hall Larenstein
University of Applied Sciences ( Jeroen Huisman).
Besides standard sampling with lift nets and fykes, a
large-scale experiment with dyed glass eel was carried
out. A large number of dyed glass eels were released
on the seaside. Glass Eel was subsequently caught day
and night at the seaside, in the eel-gutter and at the
cat-flaps during a one-week period. The results of this
experiment will become available in the beginning of
2016 and will shed light on the fish passage effectivity
of Nieuwe Statenzijl.
72
& shiplock
The Lauwersoog sluice and ship locks discharge
directly into deep water of the Wadden Sea and
in large flows. These are large-scale barriers so
the sampling strategy at one spot with a 1x1 m lift
net is likely to underestimate the quantity of fish.
Furthermore, with a discharge rate of more than 40 m 3
s-1, it is not unlikely that the fish abundance and supply
at the freshwater discharge sluices of Lauwersoog
is relatively high compared to stations with similar
densities but lower discharge rates.
lauwersoog discharge sluice
Fig. 3.4.10 Numerical model results for the average concentration
of a passive tracer in the upper layer from Den Oever (left) and
Kornwerderzand (middle) and average salinity in the upper layer
(Duran-Matute et al. 2014).
3.5 Conclusions
Due to the lack of data, a quantitative assessment of
the effectivity of existing fish passages in the Wadden
Sea was not possible. With regard to the costs of
building these structures, it is therefore strongly
recommended to invest not only in the strengthening
of fish migration (a.o. by construction of fish passing
facilities) but also in appropriate monitoring before
and after strengthening. Not only to quantitatively
evaluate the activity such as the functioning of the
facilities, but also for modifications to optimize the
investments made. Such programs should include
monitoring of (1) the numbers and characteristics (e.g.,
species, life phases, condition of fish passing, (2) the
efficiency of the attraction flow, (3) the efficiency of
the fish passage and, for larger fish passages, (4) the
habitat use of the local area (e.g., for acclimatisation to
freshwater conditions).
Large-scale patterns in fish densities as determined by
means of demersal fish surveys for tidal basins do not
show a strong overlap with local densities of migratory
fish near small freshwater sluices and fish passages.
With the exception of Smelt for which a spatial
correlation was present but weak, the densities of the
species near the freshwater sluices showed no spatial
pattern. These results indicate that local densities
in migratory fish at freshwater discharge points are
mainly driven by local circumstances. High migratory
fish densities near freshwater discharge points appears
to be locally facilitated by (1) the presence of sufficient
flow (amount and frequency) of freshwater to attract
fish to the entrance location, (2) the closeness of the
freshwater sluice to deeper waters that have a salinity
gradient, and (3) the presence of a deep and wavesheltered basin to allow fish to safely wait before
passing, and (4) good possibilities for survival, growth
and migration in the freshwater systems. Present
and future fish passage facilities should, therefore,
pay attention to selecting and/or creating such local
conditions, including optimal habitats before and after
passing.
The relatively high densities at locations that are under
the influence of freshwater discharges at a larger than
local scale, underlines the necessity of the presence of
large-scale salinity patterns to guide migratory fish
through the Wadden Sea. A variety of studies suggest
that migratory fish commonly use olfactory cues to
locate streams from oceans or lakes (Creutzberg 1959;
Dodson & Leggett 1974; Doving et al. 1985). Small
inorganic ions such as calcium (Bodznick 1978),
larger organic compounds associated with microbial
decay (Tosi & Sola 1993), and pheromones (Nordeng
1977; Li et al. 1995; Vrieze et al. 2011) have all been
suggested to serve as innately, or by imprinting during
certain life stages, recognized cues. Salinity patterns
in the Wadden Sea might, therefore, reveal preferred
pathways for migration of anadromous fish and aid
in identifying optimal areas and locations for fish
migration from the Wadden Sea to adjacent freshwater
systems.
73
4. N
UMERICAL BUDGETS OF LOCAL
STOCKS
4.1 Introduction
In order to explore the most effective measures for
the strengthening of fish stocks in the Wadden Sea,
quantitative information on major inputs and outputs
of fish are compared. This exercise was performed
for the Marsdiep tidal basin, Lake IJssel and the main
connections between these basins to identify the added
value of the planned Fish Migration River as projected
by the Provincie Fryslan (2015) under the present
conditions.
Stock dynamics describes the ways in which a given
stock grows and shrinks over time, as controlled by
birth, death, and migration. The basic accounting
relation for stock dynamics is the BIDE (Birth,
Immigration, Death, Emigration) model, according to:
N1 = N0 + B − D + I − E
where N1 is the number of individuals at time 1, N0 is
the number of individuals at time 0, B is the number
of individuals born (recruitment), D the number that
died (mortality), I the number that immigrated, and E
the number that emigrated between time 0 and time 1
(Pulliam 1988). Within a stable stock, N1 equals N0.
Numerical fish budgets require actual and local
information on standing stocks, and on rates of
recruitment, mortality and migration. Within our
study area, this implies information is needed on (1)
local stocks in the Wadden Sea and Lake IJssel, (2)
recruitment and mortality (a.o., fisheries, predation)
rates, and (3) exchange by transport (or migration) of
fish between the study area and adjacent waters and
exchange within the study area between these basins
via the Afsluitdijk.
There is no quantitative information on the annual
exchange of fish between the Wadden Sea, other
tidal basins and the North Sea, nor on the exchange
of fish between Lake IJssel and its surrounding rivers
and streams. With regard to passing the Afsluitdijk,
various pathways are considered, i.e. (i) via freshwater
discharge sluices under regular discharge management,
(ii) via freshwater discharge sluices and shiplocks under
fish-friendly management, and (iii) via the projected
Fish Migration River (Fig. 4.1). Our annual budgets
are fully based upon values as derived from existing
information as supplied in cited reports, articles and
websites.
Fig. 4.1 Overview of inputs, outputs and local population dynamics (recruitment
and mortality) that jointly define the dynamics in abundance of the local fish stocks
in the Marsdiep tidal basin of the Wadden Sea and Lake IJssel.
74
4.2 Factors
4.2.1 Standing stocks
Marsdiep tidal basin
The stock estimates were based upon beam trawl
survey data (Demersal Fish Survey; WS_T610 in
Table 2.2.1) and taken as average densities (number of
fish per ha) for the period from 2000 to 2014 (autumn
surveys). These densities were subsequently converted
to stocks (number of fish per tidal basin Marsdiep) by
multiplication of the densities with the total sublittoral
area (assumed to be 66% of the total Marsdiep area of
675 km 2 ).
Lake IJssel
The stock estimates were calculated as average
densities (number of fish per ha) as determined by the
bottom trawl surveys (WFD; LIJ_TLIJ in Table 2.2.1)
multiplied by the total surface area of Lake IJssel (1100
km 2, i.e. excluding Markermeer). Because the beam
trawl surveys were generally performed in autumn,
which is outside the migration window in spring,
and sampling methods are not geared to many of the
target species in this report, stock values have to be
generally considered as an underestimate of actual
stocks.
4.2.2 Recruitment & Mortality
Recruitment and natural mortality
No data are available for local recruitment and
mortality rates of fish species. For Lake IJssel, the
productivity of Smelt in Lake IJssel was estimated to
be 3.1 kg kg -1 in the late 1990s (Mous 2000), but this
value is based upon biomass and not upon numbers.
For the North Sea, natural mortalities of the target
species range between 0.05 (European Sturgeon) and
2.96 (Three-Spined Stickleback) (Table 1.1). These
estimates refer to late juvenile and adult phases and are
based upon maximum length and temperature (www.
fishbase.se). Natural mortality includes non-human
predation, disease and old age. Therefore, these values
are probably not representative for natural mortality of
fish stocks in the Wadden Sea and Lake IJssel because
of the different environmental conditions (e.g.,
temperature, predation) and life-phases of the fish in
these areas.
Fishing mortality
In the Wadden Sea, commercial fisheries target none
of the migratory fish species under consideration.
However, Flounder, Twaite Shad, River Lamprey and
Smelt are reported as by-catch from shrimp fisheries
(Glorius et al. 2015). Data on annually averaged
fishing mortality within the Wadden Sea of Flounder
(3.78 fish per ha fished), Twaite Shad (0.95 fish per
ha fished) and Smelt (6.05 fish per ha fished) were
derived from Glorius et al. (2015). Tessa van der
Hammen (IMARES) supplied shrimp-fishing induced
mortality for River Lamprey (0.02 fish per ha fished).
Total area fished per year (ha y-1) was calculated by
multiplying the area fished by one fishing vessel per
hour (ha h-1) with total fishing time per year (h y-1)
of shrimp vessels as supplied by Van Overzee et al.
(2008). The number of fish that die annually as the
result of shrimp fisheries (fish y-1) is subsequently
calculated as the area-related fishing mortality (fish
ha-1) times the area fished per year (ha y-1).
In Lake IJssel, both European Eel and Smelt are
fished. For both species, average landings (tonnes)
for the period from 2000 to 2014 were multiplied by
average weight of an individual Eel or Smelt (33.3
and 2.9 gram fresh-weight, respectively) as could be
derived from the Lake IJssel surveys (WFD; LIJ_TLIJ
in Table 2.2.1) by dividing the fresh-weight per ha by
the number of fish per ha.
Predation mortality
Mammals - Predation by mammals was assumed only
to take place in the Wadden Sea (although it is known
from tagging studies that these species undertake
extensive feeding trips to the North Sea) by Harbour
Seals (Phoca vitulina) and Grey Seals (Halichoerus grypus)
with daily intakes of 4 kg (Härkönen et al. 1991) and
4.8 kg fish (Hammill & Stenson 2000), respectively.
It was assumed that the diet of Grey Seals, which
includes Flounder and Herring, was similar as that
described for Harbour Seals by Brasseur et al. (2004).
Total numbers of seals in the Marsdiep tidal basin
were taken from data as available 2002-2013 via
IMARES websites (www.wageningenur.nl/nl/
Expertises-Dienstverlening/Onderzoeksinstituten/
imares/show/..) for Harbour seal (…/PopulatieGewone-Zeehonden-in-de-Nederlandse-Waddenzee.
htm) and Grey Seal (…/Populatie-Grijze-Zeehondenin-de-Nederlandse-Waddenzee.htm) and the
WaLTER dataportal (www.walterwaddenmonitor.
org/tools/dataportaal/). Total predation rate (numbers
per year) was subsequently calculated as the speciesspecific daily uptake multiplied by the number of seals
and the number of days per year.
75
Since seals not only feed in the Wadden Sea but also in
the North Sea (Brasseur et al. 2004), predation of fish
by seals is most probably overestimated.
Birds - Predation by birds in the Marsdiep tidal basin
and Lake IJssel was estimated for Great Cormorants
(Phalacrocorax carbo) only. For the Marsdiep tidal basin,
annual abundance indices for Cormorants (www.
compendiumofdeleefomgeving.nl) were backcalculated to actual numbers using the information of
the year 1992 where both types of data were available
(Leopold et al. 1998). Total numbers of Cormorants
in the Marsdiep tidal basin were estimated as an
area-weighted proportion (675 km 2 of 2743 km 2 )
of the average number of Cormorants in the Dutch
Wadden Sea from 2000 to 2012. Information on daily
predation pressure on fish from the Wadden Sea (460
gram per cormorant) and diet (including Flounder,
Smelt and Twaite Shad) was derived from Leopold et
al. (1998). For Lake IJssel, annual abundance indices
for Cormorants in a wider area around Lake IJssel
(www.compendiumofdeleefomgeving.nl) for the
period from 2000 to 2011 were back-calculated to
actual numbers in the actual Lake IJssel area using the
information during the period 1995-2001 where both
76
types of data were available (RIZA 2002). Information
on daily predation pressure on fish from Lake IJssel
(460 gram per bird; Leopold et al. 1998) and diet
(including European Eel and Smelt) was derived from
Van Rijn & Van Eerden (2001).
Predation by birds is probably underestimated, because
also other birds than Cormorants feed on fish in the
Wadden Sea and in Lake IJssel (e.g. terns, Dänhardt &
Becker 2011).
Fish - Predation by fish was only available for Lake
IJssel, where mainly Pike and Pikeperch were
consuming 49% of the annual production (3.1 kg
kg -1) of Smelt between 1976 and 1994 (Mous 2000),
resulting in a fish-induced mortality of 1.5 kg kg -1.
For the stock budget of Smelt, it was assumed that the
present proportion consumed by fish is similar to that
period. In the Wadden Sea, predation of fish by fish is
assumed to be moderate, because of the relatively low
densities of top-predator fish in this area (being one
of the reasons that the Wadden Sea is thought to be a
good nursery) (Zwanette Jager, ZiltwaterAdvies, pers.
com.).
4.3 Stock Budgets
The calculated budgets first of all illustrate the
enormous gaps in data and knowledge that do not
allow us to come up with confident results for most of
the considered species (Fig. 4.2- Fig. 4.11).
There are no data on basic information, such as
recruitment, mortality and rates of exchange between
the study area and adjacent waters. But even for those
budgets that are more or less filled in with regard to
the other factors, comparison is not fully possible as
the result of different sampling methods (e.g., beam
trawls versus diet studies) and different times of the
year in which the information is gathered (e.g., import
via FFM to Lake IJssel in spring, and densities in the
Marsdiep tidal inlet in autumn). Furthermore, some
estimates are overestimated (e.g., predation by seals)
whilst others are most likely underestimated (e.g.,
predation by birds). Interpretation of the budgets to
derive the added value of the FMR and other possible
measures (e.g., reduction of fisheries) to strengthen
local fish stocks should therefore be done with all these
limitations in mind.
For River Lamprey, the projected FMR migration
is higher than observed under regular discharge
management conditions (REG), but the summed
value for import from the Wadden Sea to Lake IJssel
of REG and FMR is of the same order of magnitude
as the export via the discharge sluices and to the
fishing mortality in the Wadden Sea (Fig. 4.2). Stock
estimates appear comparable for the Marsdiep tidal
basin and the Wadden Sea, but probably much too
low as the result of the low catchability of this species
with bottom trawls (see Chapter 2). Assuming that
River Lamprey shows more or less similar migratory
behaviour as Sea Lamprey (Griffioen et al. 2014b),
then its present passing efficiency would be 15%.
For Sea Lamprey, the projected FMR migration
is higher than observed under regular discharge
management conditions (Fig. 4.3). But, as for River
Lamprey, no information is available on import rates if
fish-friendly discharge management (FFM) is applied.
Of the 25 individuals that were tagged and released in
the Wadden Sea, however, 4 found their way to Lake
IJssel under regular discharge management conditions,
implying a passing efficiency of at least 15% under
these conditions (Griffioen et al. 2014b).
Fig. 4.2 Budget for River Lamprey.
Fig. 4.3 Budget for Sea Lamprey.
77
For European Eel, migration by fish-friendly
discharge management (FFM) appears to be much
more effective than projected for the Fish Migration
River (Fig. 4.4). It must be noted, however, that the
FFM surveys caught relatively more juveniles than
were caught by fykes (i.e., on which the estimates of
passing via the projected FMR are based). In Lake
IJssel, fisheries and predation by birds appear to be an
important source of mortality for Eel.
Fig. 4.4 Budget for European Eel.
Fig. 4.5 Budget for Atlantic Herring.
78
For Atlantic Herring, the projected FMR migration
is higher than observed for regular and fish-friendly
discharge management (Fig. 4.5). The FMR estimate
that is based upon densities at the seaside of the
discharge sluices, however, might be much too
high because Herring does not favour freshwater
systems. This might also explain the relatively high
numbers observed to migrate back from Lake IJssel
to the Wadden Sea, which might be washed in
unintentionally as the result of FFM and tried to get
back to the sea. Herring is on the diet of seals.
For Twaite Shad, the projected FMR migration
is higher than presently measured during regular
discharge management (Fig. 4.6). Again, the added
value of fish-friendly discharge management is
not known here. Twaite Shad is one of the species
accidentally caught during shrimp fishing, and on
the menu of Cormorants. For this species, however,
facilitation of migration to Lake IJssel is not considered
to be of added value to its stock size (Griffioen et al.
2014b).
For European Smelt, the most complete budget
could be made (Fig. 4.7). Fish-friendly management
strengthened upstream migration compared to regular
discharge management and is in the same order of
magnitude as projected for the FMR. During regular
discharge management, the export of Smelt to the
Wadden Sea is comparable to the import into Lake
IJssel, and relatively high compared to the stock.
This is in line with observations by Phungh et al.
(2015) that almost half of the Smelt stock in the
western Wadden Sea originated from Lake IJssel and
Markermeer. In Lake IJssel, fisheries and predation
by fish and birds appear to be an important source of
mortality. In the Wadden Sea, Smelts are caught by
shrimp fishers and eaten by cormorants.
Fig. 4.6 Budget for Twaite Shad.
Fig. 4.7 Budget for European Smelt.
79
For, the projected FMR migration is in the same order
of magnitude as measured during regular discharge
management (Fig. 4.8). Export seems, however, larger
than the import (Fig. 4.8). This is in line with the
results of tagged Sea Trout, which were released in
the Wadden Sea in 1996-1999 and found their way to
Lake IJssel: five out of nine (59%) that were tagged in
Den Oever and 28 out of 61 (46%) that were tagged
in Kornwerderzand migrated into Lake IJssel (Bij de
Vaate et al. 2003). In a recent study by Griffioen et al.
(2014b), the one and only tagged Sea Trout found its
way to Lake IJssel through the discharge sluices.
Fig. 4.8 Budget for Sea Trout.
Fig. 4.9 Budget for Houting.
80
For Houting, the projected FMR migration is in the
same order of magnitude as measured during regular
discharge management (Fig. 4.8). This is in line with
the result of tagged Houting, of which 3 out of 5
(60%) of the individuals released in the Wadden Sea
found their way to Lake IJssel through the discharge
sluices (Griffioen et al. 2014b). Export seems in the
same order of magnitude as the import (Fig. 4.9).
For Three-Spined Stickleback, the projected FMR
migration is higher than the migration as determined
under regular and fish-friendly discharge management
(Fig. 4.10). During regular discharge management, the
export of Three-Spined Stickleback to the Wadden
Sea is comparable to the import into Lake IJssel
(Fig. 4.10).
For European Flounder, the projected FMR
migration is in the same order of magnitude as
measured during regular and fish-friendly discharge
management (Fig. 4.11). During regular discharge
management, however, the export of Flounder to the
Wadden Sea is higher than the import into Lake IJssel
(Fig. 4.11). Flounder is eaten by cormorants and seals,
and induced to fishing mortality.
Fig. 4.10 Budget for Three-Spined
Stickleback.
Fig. 4.11 Budget for European Flounder.
81
4.4 Discussion & Conclusions
Compared to regular discharge management, fishfriendly discharge management appeared to enhance
the migration or transport of at least European Eel,
Smelt and Three-Spined Stickleback (Fig. 4.12)
within one order of magnitude. This is underlined
by observations during previous testing of discharge
management to enhance fish migration in the early
1990s, which resulted in an increase of Flounder
(Winter 2009) and, possibly, Herring (Chapter 2) in
Lake IJssel.
Migration as projected for the Fish Migration River
is comparable as observed for Sea Trout and Flounder
during regular discharge management (Fig. 4.12).
Compared to fish-friendly discharge management,
projected migration via the FMR is higher for
Herring, Smelt and Three-Spined Stickleback, but
lower for Eel (Fig. 4.12).
It is not clear if and, if so, how much the additional
import of fish into Lake IJssel will strengthen local
stocks, because of the yet unknown survival rate of the
fish after entering. Fykenet programs show that Sea
Lamprey and Sea Trout use Lake IJssel as a corridor
to more upstream rivers and tributaries during part of
the year (Winter et al. 2014). Tagged and subsequently
released Sea Trouts have been recorded back in the
river IJssel, indicating that they do survive (Peter Paul
Schollema, Waterschap Hunze en Aa’s, pers. comm.).
The number of fish that migrate yearly from Lake
IJssel via the discharge sluices to the Wadden Sea is
comparable (European Eel, Flounder, Houting) or an
order of magnitude higher (Herring, River Lamprey,
Sea Lamprey, Sea Trout, Smelt) than the estimates of
the local stocks in the Marsdiep tidal basin (Fig. 4.2Fig.4.11). It cannot be excluded that this fish migrates
back to Lake IJssel. The high proportion of Smelt in
the Wadden Sea that originated from Lake IJssel and
Markermeer indicates, however, that these freshwater
lakes are an important source for the Wadden Sea
stocks of this fish species.
Fig. 4.12 Indication of relative effectiveness of measures to strengthen migratory
fish populations in the Marsdiep tidal basin (Wadden Sea) and Lake IJssel by
means of reduction of fisheries (red) or sluice management (blue) as derived from
local stock budgets. The number of stars indicates relative order of magnitude,
e.g. within a row, a measure of 3 stars is 1000 (103) times as much effective as
another with 1 star (101). REG = regular discharge management, FFM = fishfriendly sluice management; FMR = projected fish migration river.
82
Fisheries (catch and bycatch) appear to be an
important source of mortality, both in the Wadden
Sea (Flounder, River Lamprey, Smelt, Twaite Shad)
and in Lake IJssel (European Eel, Smelt) (Fig. 4.2-Fig.
4.11). For these species, the mortality as the result
of commercial fishing (catch, bycatch) appears to
be in the same order of magnitude as the projected
migration via the Fish Migration River. Furthermore,
although it is not allowed at present to land Eel by
recreational fishers in Lake IJssel, illegal fishing
might still occur (Van der Hammen et al. 2015). In
the Marsdiep tidal basin, the by-catch from shrimp
fishing on diadromous fish is estimated to be almost
2.5 million fish per year, comprising of Twaite Shad
(213 000 fish per year), Smelt (1 400 000 fish per year),
Flounder (840 000 fish per year) and River Lamprey
(4500 per year). Reduction of fishing activities in Lake
IJssel and the Wadden Sea might, therefore, be an
alternative or additional measure to strengthen local
fish stocks within the study area as a whole.
For the estimations in this chapter, a total passing
efficiency of 50% (see 4.2.3) for the proposed Fish
Migration River complex was assumed in accordance
with Provincie Fryslan (2015). Based on a very limited
small-scaled number of tagging experiments and
several assumptions, however, the passing efficiency
at Kornwerderzand under present regular discharge
conditions appears already to be around 15% for
River Lamprey, 25% for Sea Lamprey and 50% or
more for Sea Trout, Atlantic Salmon and Houting
(Bij de Vaate et al. 2003, Griffioen et al. 2014b, Ben
Griffioen, IMARES, pers. com.). And, compared to
regular discharge management, fish-friendly discharge
management appeared to at least double the passing
efficiency for European Eel, European Smelt, ThreeSpined Stickleback and Flounder (Vriese et al., 2015,
this chapter). Taking such species-specific variation
at present into account, an average value of 50% most
probably does not correctly addresses future passing
efficiencies under FMR conditions for all target
species.
Winter et al. (2014) give an extensive overview of
the complex and multifactorial configuration of the
FMR that affects the passing efficiency. They indicate
that migratory fish need to make many choices, and
need to be very motivated to pass through. Fish has
to be able to find the entrance of a fishway such as
the FMR (i.e. attraction efficiency), and subsequently
to successfully pass the fishway itself (i.e. passing
efficiency) (Bunt et al. 2012). With regard to finding
the entrance, a choice has to be made to migrate either
towards the FMR, the ship locks or the discharge
sluices (Winter et al. 2014). Once within the FMR
complex, fish can decide to just stay there (e.g., to
forage) or pass the FMR, including several potential
barriers (e.g., the breach in the Afsluitdijk and the
sluice doors between the FMR and Lake IJssel). Once
the fish has successfully passed the FMR, it runs the
risk of being flushed back to the Wadden Sea again
via the discharge sluices (Winter et al. 2014). Because
the FMR is designed as one river consisting of several
parts that are aligned after each other, a bottleneck for
fish in one part will strongly limit the overall passing
efficiency. In addition, when taking species-specific
needs and behaviour into account, an optimal situation
for one fish species might be limiting to another.
An overview of efficiencies of riverine fishways
indicated that, for nature-like fishways (with 21
evaluations), both the attraction and passing efficiency
varied between 0 and 100% (Bunt et al. 2012). On
average, the passing efficiency was higher (mean=70%,
median=86%) than attraction efficiency (mean=48%,
median=50%) (Bunt et al. 2012). Taking both aspects
into account, the mean of the total passing efficiency
of nature-like fishways would then be 50% x 70%
= 35%. When considering all types of fishways
(technical and nature-like), variations in attraction
efficiency appeared to be mostly related to variation in
entrance location, near-field hydraulic conditions and
entrance configuration. Although passing efficiency
was positively correlated to naturalness and negatively
to slope, it could not be excluded the performance
of nature-like types was largely attributable to slope
rather than to any other intrinsic benefit of their
design (Bunt et al. 2012). Bunt and co-authors (2012)
conclude that more work is needed on the design
of nature-like fishways before they can be reliably
prescribed, as they often are, for passing a broad
variety of species.
Although the Fish Migration River has the potential
to be of added value, present data and knowledge do
clearly not justify the projected passing efficiency of
50% for all fish species. Species-specific questions
that remain include (1) the conditions of fish species
(e.g. life phase, size, physiology, condition) under
which they are motivated to migrate from the sea to
freshwater, (2) species-specific aspects of attraction
efficiency (e.g. entrance location, near-field hydraulic
83
conditions and entrance configuration), (3) speciesspecific aspects of passing efficiency (e.g. currents,
salinity gradient, bottom structure), and (4) habitat
requirements of fish when passing (e.g. for feeding and
acclimatisation to freshwater conditions). To gather
such knowledge, a dedicated Fish Migration Testing
Facility would be needed with access to natural
seawater and freshwater. To our knowledge, such
a testing facility does not presently exist, but these
conditions could be created at Kornwerderzand.
Summarized, the size of fish stocks in the Wadden
Sea area is generally driven by a combination of
natural factors (e.g. predation by fish, birds and seals)
and by human-induced drivers (e.g. fisheries and
supplied options to pass via the Afsluitdijk). Reduction
84
of fishing activities in the Wadden Sea and Lake
IJssel is most probably beneficial for stocks of River
Lamprey, European Eel, Twaite Shad, European
Smelt and European Flounder. Fish-friendly discharge
management (FFM) at a structural base would be
beneficial for European Eel, European Smelt, ThreeSpined Stickleback and Flounder. The Fish Migration
River could have added value to FFM, but only when
optimally designed. Existing data and information are
presently, however, not sufficient to support design
recommendations. By making the design adequate for
testing facilities, however, this necessary knowledge
could be gained, which is not only relevant for
strengthening migratory fish in the Wadden Sea area
but also in other coastal systems with comparable manmade barriers world-wide.
5. POTENTIAL MEASURES AND COSTS
5.1 Introduction
From a historical perspective, large-scale and longterm anthropogenic pressures such as fisheries,
pollution and habitat destruction have impacted fish
stocks in coastal ecosystems including the Wadden
Sea to historic lows (Wolff 2000; Jackson et al. 2001;
Lotze et al. 2005; Limburg & Waldman 2009).
Although improvement of local conditions might
result in strengthening of local stocks, this can only
occur within the boundaries set by the large-scale
long-term drivers. Large-scale regulations are also
taken and prove to be successful, as shown for the
recovery of Herring in the North Sea following a
fishing ban (Dickey-Collas et al. 2010). Acting locally
while fish stocks are still hampered by large-scale
anthropogenic pressures might still be worthwhile, in
anticipating (and possibly even stimulating) further
international measures to strengthen fish populations
globally.
For all species addressed in this study, the Wadden
Sea is (or was) an essential area to fulfil a particular
phase in their life cycle (Zijlstra 1978; Van der
Veer et al. 2000, 2015). For the diadromous species
Atlantic salmon, Allis shad, Sea Lamprey, and
European Eel, this shallow coastal sea functions as a
corridor between freshwater systems and the open
sea. Flounder, Herring and Anchovy use this area
as a nursery, and as spawning grounds (Anchovy).
Sea Trout and Twaite Shad use it for feeding, whilst
juvenile European Smelt, Houting and River Lamprey
migrate from freshwater to the Wadden Sea to grow
and mature. Improving environmental conditions for
fish recruitment, growth, survival and migration in
the Wadden Sea area might counteract the observed
declines in local fish stocks.
Furthermore, fish abundance in the Wadden Sea area
is considered to be of main importance for fisheating birds such as Great Cormorants (Phalacrocorax
carbo), Common Terns (Sterna hirundo) and Eurasian
Spoonbill (Platalea leucorodia) (Dänhardt & Becker
2011; El-Hacen et al. 2014) and for seals (Brasseur et
al. 2004). The importance of particularly Herring,
Twaite Shad, Smelt and Flounder as a food source
for local birds and mammals (Chapter 4) underlines
the importance of strengthening local fish stocks to
enhance these natural values of the Wadden Sea as
well.
In this Chapter, an overview is given of potential
measures (including indications of efforts and costs, if
available) to strengthen local stocks of migratory fish
in the Wadden Sea, based upon the information from
the previous chapters and within the boundaries of the
Wadden Sea area covered by the Waddenfonds, i.e. the
Wadden Sea, the islands, the tidal inlets, the North Sea
coastal zone (up to 3 nautical miles from the coastline)
and the municipalities at the mainland bordering the
Wadden Sea (including a part of Lake IJssel). Measures
addressed include the reduction of fisheries, the
provisioning of suitable habitats (such as brackish zones
and freshwater spawning grounds) and the facilitation
of fish migration.
85
5.2 Fisheries
5.2.1 Wadden Sea
Annual fishing mortality in the Marsdiep tidal basin
as derived from reported by-catch of shrimp fisheries
(individuals per ha fished; Glorius et al. 2015; Tessa
van der Hammen, IMARES, pers. comm.) and fishing
pressure (ha y-1; Van Overzee et al. 2008), adds up
to almost 2.5 million fish per year (see Chapter 4).
This by-catch includes Twaite Shad (213 000 fish y-1),
Smelt (1 400 000 fish y-1), Flounder (840 000 fish
y-1) and River Lamprey (4500 y-1). Total numbers of
diadromous fish caught in the entire Dutch Wadden
Sea would add up to more than 4 million fish per year.
If this fishing mortality limits local fish stocks, then a
reduction of by-catch could be strengthening these fish
species and their predators (birds, seals). Reduction of
the fishing efforts is considered to increase additional
natural values of the Wadden Sea (e.g. benthic
organisms and biogenic structures such as mussel beds
(Ramsay et al. 1998; Jongbloed et al. 2014), whilst the
yield might remain stable (Temming & Hufnagl 2015).
Turenhout et al. (2016) recently published an overview
on economic aspects of shrimp fisheries. They
calculated that, on average between 2012 and 2014,
23% of landed shrimp by Dutch shrimping vessels was
caught in the Wadden Sea. In 2014, 3446 tonnes of
Wadden Sea shrimp was landed. The direct revenues
were 19.6 million Euro in 2013 and 11.2 million Euro
in 2014. In 2013, the subsequent processing of the
shrimps created an added value of 9.2 million Euro.
Additionally, the feasibility of possible alternative
sources of income for fishermen are explored in case
shrimp fishing in the Wadden Sea would be no longer
possible.
86
5.2.2 Lake IJssel
In this freshwater lake, there is direct fishing on
European Eel (but not for fish smaller than 28 cm,
and not allowed between September and November;
www.sportvisserijnederland.nl/vispas/visserijweten-regels/binnenwater/paling.html) and a recently
(possibly temporarily) abandoned fishery on Smelt.
Landings of European Eel were, on average, almost 6
000 000 fish and on Smelt almost 200 000 000 fish per
year during the last 15 years (De Boois et al., 2014; De
Graaf & Deerenberg 2015; see Chapter 1).
A proportion of the stocks of these fish species in Lake
IJssel migrate (or are being transported) to the Wadden
Sea via the sluices, i.e. approximately 16 000 Eels and
22 600 000 Smelt per year (see Chapter 4). The Eels
are probably silver eels on their way to the open ocean
for reproduction, but Smelt might add to the local
stocks of the Wadden Sea. For Smelt, for example,
almost half of the population (45%) in the Marsdiep
tidal inlet originated from Lake IJssel and the adjacent
Markermeer (Phung et al. 2015). Strengthening of
the fish population in Lake IJssel, e.g. by means of
structural reduction of fishing efforts, can therefore
be expected to also to strengthen fish stocks in the
Wadden Sea.
For European Eel, the annual market value of
fish caught in Lake IJssel en Markermeer was
approximately 1.5 million Euro (10% silver eel; 90%
larger eel) between 2003 and 2012 (Kees Taal &
Wim Zaalmink, LEI Wageningen UR, presentation
2/12/2013). For Smelt, the annual landings of Smelt
amounted 2 to 3 million kg in the 1980s and 1990s,
but this was considered an unsustainable yield.
According to fishermen the maximum sustainable
yield is 0.8 million kg per year. Assuming a market
price for Smelt of 0.50 Euro per kg, the economic
revenue for Smelt fishery would then be 400 thousand
Euro per year.
5.2.3 Coastal zone
For several migratory fish species, long-term variation
in the coastal zone north of the Wadden Sea islands
appears to be different than that in the adjacent North
Sea and tidal basins of the Wadden Sea (Chapter 2).
This might be due to natural circumstances typical for
this area, but impacts of fisheries or coastal protection
cannot be excluded (Van der Veer et al. 2015).
Sand nourishments, for example, usually take place at
the foreshore in water depths of -5 to -8 m NAP (De
Ronde 2008), an area that has special significance as
nursery habitat for fish (Teal & Van Keeken 2011).
The recovery time for benthic communities, a
potential food source for fish, is 4 to 6 years (Baptist
et al. 2008). Shoreface nourishments from Texel
to Ameland occurred on average and at different
subareas, however, once every 3 yrs between 2004 and
2014 (Rijkswaterstaat 2014). Impacts might be reduced
when natural sediments are used, and when foreshore
sand is nourished outside the seasonal windows with
high fish abundances in the coastal zone (Annelies De
Backer, ILVO, presentation 28/1/2016). Foreshore
nourishment started only in 1993, sand was supplied
onto beaches before that time. Measures could,
therefore, include adaptations of the timing (outside
nursery and migration windows) and location (beach)
of sand nourishment.
5.3 Brackish water zones
5.3.1 Introduction
Within the Wadden Sea area (within the Waddenfonds
boundaries), most estuarine gradients are presently
characterised by an abrupt salinity gradient. In former
days, Anchovy, Herring, Smelt and Flounder used the
large open brackish water bodies with tidal influence
in this area to spawn, to grow and to mature (Redeke
1939). According to Redeke (1939), Anchovy spawned
in the northern funnel-shaped part of the area (with
salinities of 10 ppt). After hatching of the eggs, larvae
moved (by drift and active migration) to the central
part of the basin. Herring deposited its eggs spawned
on the stems and branches of hydroids, especially
Laomedea gelatinosa, which grew abundantly on
the sand banks near the western and southern coasts
of the former Zuiderzee (with salinities of 5 ppt).
Herring larvae and juveniles subsequently fed on the
diatoms and copepods that occurred in this brackish
environment, until seaward migration in autumn.
Smelt was generally found in the less brackish parts,
and this fish species migrated upstream to the IJssel
and other, smaller, rivers to spawn. Juvenile Flounder
entered the former Zuiderzee in early spring, feeding
mainly on benthic fauna, i.e. small polychaetes during
the first weeks after arrival, and then switching to
Corophium volutator and Nereis diversicolor. In de
Zuiderzee, Flounder stayed approximately 3 to 4 years
until they were mature.
In general, strongly developed salinity gradients appear
to favour the estuarine recruitment of marine fish
species (Schlacher & Wooldridge 1996). In addition to
protection and improvement of such areas at present
(e.g. small creeks and the Ems estuary), restoration
of former gradients may aid in strengthening fish
stocks in the Wadden Sea area. Basically, there are
two alternative ways to create such large habitats
with estuarine gradients, i.e. input of seawater into
freshwater systems or (continuous) input of freshwater
into marine waters. On the mainland bordering the
Dutch Wadden Sea, several large freshwater bodies
exist that potentially could be turned into brackish
ecosystems, i.e. Lauwersmeer, Amstelmeer and Lake
IJssel. Alternatively, the southern part of the Marsdiep
tidal basin could become more permanently under the
influence of freshwater discharges.
87
5.3.2 Lauwersmeer
This freshwater lake was formed when a tidal inlet
to the former Lauwerszee was closed off in 1969.
Full restoration of tidal brackish water dynamics
is considered not feasible due to ecological and
technological constraints (Raad voor de Wadden
2008). Because of subsidence of the mainland due to
peat settling, the (formerly natural) brooks in the area
can no longer drain by gravity into the Wadden Sea.
In addition, the Lauwersmeer is considered crucial for
the freshwater management of the northern provinces.
The migration of fish from Lauwersmeer to streams in
Drenthe is dependent on water level management and
future plans might hinder migration. Furthermore, the
natural spawning habitat for Zuiderzee-Herring and
Anchovy is coarse sand whilst at present the bottom
of Lauwersmeer consists of soft muddy sediments.
Restoration and maintenance of the bottom of this
lake would require another major operation unless full
tidal dynamics can be restored.
The Marnewaard located in the northeastern part
of the Lauwersmeer area, however, appears to be
suitable to be further developed as a brackish zone
(Raad voor de Wadden 2008). This would require a
newly built sluice in the sea defence at the (former)
Vierhuizergat connecting with the Nieuwe Robbengat
in the Wadden Sea (Raad voor de Wadden 2008). A
rough cost estimate can be obtained by comparing
with a similar project near Delfzijl (Oterdum),
where building a new sluice including fish passage
facility and brackish transfer zone was estimated to be
approximately 3 million Euro (Verhoogt et al., 2014).
5.3.3 Amstelmeer
In this freshwater lake, the restoration to full tidal
estuarine conditions with brackish gradients might
be possible but only with large adaptations to existing
infrastructure. There is, however, an opportunity
to create a salinity gradient including a brackish
zone in the Amstelmeer and Balgzandkanaal, the
channel that connects the lake with the Wadden Sea
(Wintermans & Dankers 2003). The bottom layer
of water of the Amstelmeer and Balgzandkanaal is
already brackish due to saltwater seepage from the
neighbouring Wadden Sea (Rijkswaterstaat 1979). The
Balgzandkanaal has a length of 8 km and the borders
of the first 3 km of this channel do not have any users
that are dependent on freshwater. The sluices might be
88
operated such that a brackish zone persists along this 3
km length that extends to the brackish water into the
deep parts of the Amstelmeer, which creates favourable
conditions to, for example, Three-spined Sticklebacks
in this area. This species spawns in inland waters with
aquatic vegetation, and overwinter in large freshwater
bodies or at sea. A wintering population can be found
in the Amstelmeer (George Wintermans, WEB, pers.
comm.).
A fish passage from the Wadden Sea towards
Amstelmeer via the Balgzandkanaal appears feasible at
the freshwater discharge sluice at Oostoever near Den
Helder (Wintermans & Dankers 2003). Although a
fish passage facility has been realised in the discharge
sluices Oostoever, this is not working properly at the
moment (George Wintermans, WEB, pers. comm.).
The Amstelmeer is already connected with Lake
IJssel via the water system south of Wieringen and
the discharge sluice Stontelerkeer near Den Oever.
Since the water level of Lake IJssel is higher than
the Amstelmeer, the flow direction is towards
the Amstelmeer. The water system OostoeverBalgzandkanaal-Amstelmeer-Stontelerkeer offers
good opportunities for a small fish migration river.
When the sluices at Oostoever will be opened
at flood tide, this can bring in salt water and fish
into the Balgzandkanaal. Brackish conditions with
reduced tidal amplitude can thus be created in the
Balgzandkanaal. An existing sill at the end of the canal
prevents further transport of salt into the Amstelmeer.
The Amstelmeer has rather brackish water at the
bottom, but its waters are still used for irrigation.
Moreover, by opening the sluices at Stontelerkeer,
fresh water from the IJsselmeer can flush out the salt
water in the Amstelmeer. An additional advantage
is that a larger freshwater current can be maintained
over a longer period at Oostoever, thereby attracting
migratory fish to this sluice.
Flushing with fresh water from Lake IJssel is currently
not part of the operational water management by
the responsible water board. Realisation is thought
to be feasible for only several Euros per day (Sas &
Wanningen 2014). Measures to increase the capacity of
the fish passage at Stontelerkeer are necessary as well.
No estimate for the construction costs is available at
this moment.
5.3.4 Lake IJssel
5.3.5 Wadden Sea
Because Lake IJssel is a main source of drinking
water for The Netherlands, salt intrusion is kept to a
minimum. In 2015, a fish passage facility was created
near the discharge sluices of Den Oever. This facility
will be operational together with fish-friendly sluice
management at the sluices and locks of Den Oever
and Kornwerderzand. To overcome potential drinking
water problems from the saline Wadden Sea water
coming into the lake, a salt-water return system
will be installed at both locations (Den Oever and
Kornwerderzand). Here the relatively heavy seawater
is taken in at the bottom near the sluice and pumped
back into the Wadden Sea (Staatscourant 2014;
www.rijkswaterstaat.nl/water/projectenoverzicht/
verbeteren-vismigratie-afsluitdijk/index.aspx).
Alternatively to creating brackish environments in
these freshwater systems, it might be considered
to establish a large and permanently brackish zone
at the seaside of the Afsluitdijk. At present, this
area is already brackish during and after discharge
of freshwater from Lake IJssel (Fig. 3.4.10). The
discovery of eggs of Anchovy north of the sluices of
Kornwerderzand in 1994 (Boddeke & Vingerhoed
1996) and the observed increase in Anchovy stocks in
NW European waters since 1995 (Beare et al. 2004),
underlines the potential of this area for development
into a brackish water zone. In addition, the coarse
sediments in this area might provide ample spawning
grounds.
This new system may even allow for an extension
of the brackish zone in Lake IJssel, in particular if
this area is semi-enclosed by means of a wall. First
calculations of such a design suggest that even without
additional pumping, salinity in Lake IJssel can be
controlled by management of freshwater discharges
(Marcel Klinge & Coen Kuiper, Witteveen+Bos,
presentation 5/3/2015). The advantages of having a
brackish water zone in Lake IJssel are threefold, i.e. (i)
migratory fish such as Flounder ( Jager 1998) that enter
Lake IJssel via the discharge sluices will get more time
to adapt to freshwater conditions than at the present
situation, (ii) unintentional transport of marine fish
in Lake IJssel can be corrected by return flushing, and
(iii) unintentionally flushing of freshwater fish might
be reduced as the increase of salinity towards the
sluices might act for them as an early warning system.
Total costs were estimated to be between around 40
million Euro, excluding VAT (Marcel Klinge & Coen
Kuiper, Witteveen+Bos, presentation 5/3/2015).
Because The Netherlands is highly depending on
Lake IJssel as a source for drinking water, such
measurements can only be considered if the restriction
of intrusion of salt water into this freshwater lake can
be guaranteed.
However, when discharges are limited (e.g., during
high tide and in periods of drought) this area becomes
saline again. Due to expected accelerated sea level rise
it will become more difficult to discharge freshwater
using gravity at low tides. To compensate for sea
level rise, Rijkswaterstaat plans to install pumps at
Den Oever in order to maintain the present drainage
capacity of Lake IJssel (mirt2016.mirtoverzicht.nl/
mirtgebieden/project_en_programmabladen/527.
aspx). Such discharge pumps could potentially
offer a continuous outflow of freshwater into the
Wadden Sea. Average discharges of Den Oever and
Kornwerderzand add presently up to 530 m 3 s-1,
implying that high and continuous pumping capacity
is required in order to create a continuous brackish
zone. In particular during dry periods, such amounts
of freshwater might not be available for discharging
into the Wadden Sea (Theo van der Kaaij, Deltares,
pers. comm.). Limiting the additional supply of
freshwater to the Wadden Sea during the periods with
surplus of freshwater, however, is in line with natural
salinity variations in estuaries and might be sufficient
to aid in strengthening local fish stocks by supplying
brackish conditions during their main migration and
spawning periods.
89
5.4 Freshwater habitats
Once fish has passed from the sea to the freshwater
system, it should be able to reach its final destination.
In general, this implies that connectivity between all
tributaries, polder ditches and brooks are important
to population sizes of migratory fish. In addition,
fish might require particular conditions in freshwater
habitats, e.g. to spawn.
Twaite Shad is a species that spawns on gravel beds
or coarse sand of tidal rivers near freshwater tidal
interfaces. It is presently unknown if and, if so,
where Twaite Shad spawns in the Dutch Wadden Sea
regio. Suited conditions for spawning can, however,
probably be found in (side channels of ) the upstream
Ems, a river presently characterized by high fluid mud
concentrations and low oxygen levels during summer.
Since fine sediment, either in suspension or when
deposited, can have multiple negative impacts on fish
(Kemp et al. 2011), the water and habitat quality of
the river Ems needs to be improved drastically before
becoming an important spawning ground.
Sea Trout, Atlantic Salmon and Allis Shad spawn
in upstream parts of river basins, mainly in the
Rhine and accessible via the IJssel (De Groot 2002).
It is questionable if sufficient habitat is available
for a spawning population of Sea Trout in small
Dutch streams such as the Drentsche Aa (Peter Paul
Schollema, Waterschap Hunze en Aa’s, pers. comm.).
Sea Lamprey spawns on gravelly substrates in upstream
parts of large rivers and might find suitable habitat in
(side-channels of ) larger rivers such as the Ems and
IJssel. Such local habitat improvement in small streams
can relatively easily be achieved by re-meandering
without the need for introducing gravel in these
systems (Peter Paul Schollema, Waterschap Hunze en
Aa’s, pers. comm.).
90
River Lamprey is known to spawn in the Gasterensche
Diep, a tributary to the Drentsche Aa (Riemersma &
Kroes 2004, Brouwer et al. 2008; Winter et al. 2013,
Hofstra 2014; Schollema 2015) and can find suitable
habitat in the IJssel and Vecht tributaries. Studies
into River Lamprey in the Drentsche Aa show that
not the availability of spawning and nursery habitat
is currently limiting, but migration is. The biggest
problem is in the long stretches of almost stagnant
water in the canals between the sea sluices and the
Drentsche Aa. In years with wet winters and high
freshwater discharges towards sea, the resulting
increased flow velocities attract and accommodate
the landward migration of River Lampreys (Peter
Paul Schollema, Waterschap Hunze en Aa’s, pers.
comm). For River Lamprey, both safeguarding of the
habitat quality and the connectivity between various
tributaries in the northeastern part of The Netherlands
and the sea are important for strengthening their
populations in the Wadden Sea region.
5.5 Fish passages
More expensive solutions are pumps. Pumps can
be used for fish bypasses (usually with low capacity
pumps) or the pumps themselves are fish-friendly
(usually with a high pump capacity). The construction
costs for pumps are highly dependent on the capacity
and the ‘hydraulic head’, i.e. the difference in surface
elevation between the two water bodies. The costs for
pumps range between 50 and 900 thousand Euro.
Fish passages can, in some cases, also be realised
without pumps in the form of fish ladders, sliding
doors or siphon spillways (“vishevel” in Dutch). The
construction costs are dependent on the design and
scale and fall in a similar range as for pumps, i.e. 45 to
657 thousand Euro.
5.5.1 Introduction
Over the past years, many studies have listed
bottlenecks for fish migration in The Netherlands (e.g.
Jager 1999, Wanningen & Van Herk 2007, Wymenga
& Brenninkmeijer 2007, Kroes et al. 2008). These
studies stress the importance of unhindered fish
migration for sustainable populations of migratory
species. To facilitate migration via potential barriers,
a suite of technical solutions is available. The
construction costs for the realisation of fish passages
differ widely and are dependent upon type and scale
(Table 5.1 and references herein).
The most expensive are fish passage constructions that
include collection and return flow for the inflow of salt
water with the incoming flood tide, of which the costs
range between 2.9 and 5 million Euro. The high costs
(52 million Euro) of the Fish Migration River are
related to its size and its complexity.
Relatively simple and low cost solutions for tidal
barriers are tide gates, tidal flaps or valves in smallscale barriers, or small openings or slots made in
medium to large-scale locks. These can be realised for
5 to 27.5 thousand Euro. Another low-cost solution is
formed by fish-friendly sluice management, which can
be realised in medium to large-scale barriers for 5 to
50 thousand Euro.
Ref.
Type of passage
Barrier
Scale
Costs (Euro)
Remarks
Gates, valves, slots
1
Adjustable tidal flap
tidal
small
1
Self regulating tide gate
tidal
small
tidal
medium to
large
5,000
27,500 Range of 25 to 30 thousand Euro
Fish-friendly lock management
1
Fish-friendly lock management
5,000 Software costs
2
Fish-friendly lock management
inland
small
10,000 For defining a protocol
2
Fish-friendly lock management
tidal
medium to
large
37,500 Software costs; 25 to 50 thousand Euro
Fish ladders, siphon spill ways, sliding doors
2
DeWit fish ladder
inland
small
2
Manshanden siphon spillway fish
passage
tidal
small
155,000 Range of 130 to 180 thousand Euro
45,000
3
Fish bypass at pumping station
tidal
medium to
large
500,000
2
Siphon spillway fish passage
tidal
small
510,000
1
100 m long culvert with two sliding
doors
tidal
small
657,000
Fish-friendly by-pass pumps
1
Fish bypass at pumping station
tidal
small
140,000 Pump capacity 7.25 m 3/min
1
Fish-friendly screw pump
inland
medium
130,000 Pump capacity 2x 25 m 3/min, Head 1.0 m
1
Fish-friendly screw pump
inland
medium
200,000 Pump capacity 2x 32 m 3/min, Head 0.8 m
1
Fish-friendly pumping stations by
Venturi system
inland
small
250,000 Pump capacity 18 m 3/min, Head 1.05 m
1
Archimedes screw pump with return
flow and lights
inland
small
318,000 Pump capacity 8.3 m 3/min, Head 2,68 m
335,000 Pump capacity 2x 60 m 3/min, Head 1.5 m
1
Fish-friendly screw pump
inland
medium
1
Fish-friendly screw pump
inland
medium
677,000 Pump capacity 2x 40 m 3/min, Head 1.95 m
1
Fish bypass at pumping station
inland
small
900,000 Pump capacity 1.2 m 3/min, Head 4.35 m
Fish passage with salt water return flow system
4
Fish bypass at pumping station with
brackish basin
tidal
medium to
large
2,900,000
5
Siphon spillway fish passage with
brackish basin
tidal
large
4,500,000 Range of 4 to 5 million Euro
tidal
large
Fish Migration River
6
Fish migration river
52,000,000
Table 5.1 Estimated costs of
various types of fish passages.
Sources:
1 Heemstra & Veneberg
(2012);
2 w ww.scheldestromen.nl/
actueel/nieuwsberichten
/@215422/innovatieve;
3 Verhoogt et al. (2014);
4 Heuer et al. (2011);
5 Brenninkmeijer &
Wymenga (2007);
6 Ecorys (2015)
91
5.5.2 Small passages
5.5.3 Passing at the Afsluitdijk
Based on the comparison of fish densities near
freshwater outlets at the mainland into the Wadden
Sea, it appears that high densities near freshwater
discharge points are locally facilitated by (1) the
presence of sufficient flow (amount and frequency) of
freshwater to attract fish to the entrance location, (2)
the closeness of the freshwater sluice to deeper waters
that have a salinity gradient, and (3) the presence
of a deep and wave-sheltered basin to allow fish to
safely wait before passing, and (4) good possibilities
for survival, growth and migration in the freshwater
systems (see Chapter 3).
A study by Arcadis (2014) on preferred alternatives
of fish-friendly lock and sluice management (FFM)
at the Afsluitdijk gives a qualitative estimate of
its costs (Table 5.2). The implementation of fishfriendly management mainly requires adjustments
in the working protocols because of the number and
complexity of additional actions controlling the sluices
and locks. Some alternatives require large changes in
control actions and additional staff might be necessary.
For some alternatives technical adjustments in the
control panel for the lock and sluice operation are
desired (Table 5.2).
The relatively high densities at locations that are under
the influence of freshwater discharges at a larger than
local scale, underlines the necessity of the presence
of large-scale salinity patterns to guide migratory
fish through the Wadden Sea. Present and future fish
passages should, therefore, pay attention to creating
such conditions at basin-wide and local scale, including
optimal habitats for migration through the Wadden
Sea and for retention areas before passing.
To estimate the potential added value of a Fish
Migration River (FMR), the prospected numbers
of fish passing the Afsluitdijk were compared with
those being observed during fish-friendly discharge
management (FFM) of sluices and shiplocks (Table
5.3). It must be noted that this comparison is based on
a relatively small data set (FFM) and on abundances
on fish in front of the Afsluitdijk of which 50% of all
fish species was assumed to pass the FMR, and that
no distinction was made between fish of different size
and/or age classes (see Chapter 4).
Afsluitdijk discharge sluices
Alternative
Open both doors
at large water level
difference (10 – 20
cm)
Open both doors
Use sluice doors
at small water level
separately as fish
difference (< 10 cm) lock
Costs
Minor adjustment to Minor adjustment to New action; extra
sluice control
sluice control
personnel needed,
control panel
adjustments desired
Afsluitdijk ship locks
Alternative
Use small openings
in both lock doors
Use one open door
and small openings
in the other
Use two open doors
Costs
Few extra personnel
needed, control
panel adjustments
desired
Some extra
personnel needed,
control panel
adjustments desired
Many extra
personnel needed,
no adjustments
Table 5.2. Qualitative assessment of costs for fish-friendly lock
and sluice management at the Afsluitdijk (Arcadis 2014).
92
Use sluice doors
with variable
opening
New action; extra
personnel needed,
control panel
adjustments desired
Under these assumptions, the FMR appears to be
more effective than FFM for Herring and ThreeSpined Stickleback (Table 5.3). For Smelt, Sea Trout
and Flounder, the numbers of fish passing as the result
of fish-friendly sluice management (FFM) appear to
be comparable to that as projected for the FMR (Table
5.1). Because Smelt has a migrating and a landlocked
population (Tulp et al. 2013, Phung et al. 2015),
improving connectivity between both populations by
either method might increase population size of Smelt
in the Wadden Sea. For European Eel, fish-friendly
sluice and shiplock management (FFM) would bring in
much more fish in than the FMR. Since the facilities
are already in place, fish-friendly management via
discharge sluices and shiplocks appears to be a highly
cost-effective measure that can contribute to the
restoration of the Eel population on Lake IJssel.
REG (x103 )
The total initial one-time investment cost of the
FMR is 52 million Euro, i.e. 18 million Euro
(including VAT) for the passage in the Afsluitdijk,
and approximately 34 million Euro (excluding VAT)
for the facilities at the northern and southern side of
the Afsluitdijk including 1 million for maintenance
and management. If annual costs for maintenance
and management range between 1% (FMR specific;
Meinard Bos, DNA, pers. comm.) and 2% of the
investment costs (general investments Afsluitdijk;
Ecorys 2015), then this would be approximately 500
000 thousand to 1 million Euro per year. These costs
will be annually budgeted.
FFM (x103 )
FMR (x103 )
Ratio
REG:FFM:FMR
Ratio
FFM:FMR
River Lamprey
0.12
no data
2.4
1: nd : 20
no data
Sea Lamprey
0.06
no data
0.9
1 : nd : 14
no data
European Eel
0.6
105094
608
1 : 17516 : 1015
17 : 1
Herring
800
200
111800
4 : 1 : 559
1 : 559
no data
no data
1203
no data
no data
Smelt
400
9700
19400
1 : 24 : 49
1:2
Sea Trout
0.06
no data
0.09
1 : nd : 2
1:2
2
no data
no data
no data
no data
400
1100
30100
1 : 3 : 75
1 : 27
20
50
60
1:3:3
1:1
1623
116114
163174
1 : 72 : 101
1:1
Twaite Shad
Houting
Stickleback
Flounder
Total
Table 5.3 Numbers of fish potentially passing annually through the
Afsluitdijk with regular discharge management (REG), fish-friendly
discharge management (FFM) and as projected for the fish migration
river (FMR; assuming 50% total passing efficiency). Ratios are given
in rounded numbers (see Chapter 4 for details on estimates).
93
5.6 Concluding remarks
Construction of brackish zones within freshwater
systems should be preceded by feasibility studies,
including the behaviour of salt water and possible
consequences for other use of the water (e.g.
agricultural, drinking). To come up with an optimal
design, additional measurements on flows, tides,
salinity gradients and fish passage rates under present
conditions might be required. If a construction of a
more permanently brackish zone at the northern side
of the Afsluitdijk is considered, potential impacts of
pumps on migrating fish from freshwater to the sea
should be kept to a minimum (e.g. by means of using
fish-friendly pumps) and an inventory of possible
effects of additional freshwater discharges on local
species and habitats in this zone should be made (c.f.
Smit et al. 2003).
Unfortunately, the efficiency of existing fish passages
could not be quantified due to lack of data. For present
and future passages, measurements on fish abundances
should be performed in a comparable way before
and after such facilities, e.g. by means of comparable
fishing gear and/or mark-recapture experiments.
To evaluate efficiency and to develop knowledge on
optimal designs, monitoring programs should include
monitoring of (1) the numbers and characteristics
(e.g., species, life phases, condition) of fish passing, (2)
the efficiency of the attraction flow, (3) the efficiency
of the fish passage and, for larger fish passages, (4)
the habitat use of the local area (e.g., for feeding or
acclimatisation to freshwater conditions).
All potential actions to strengthen fish stocks in
the Wadden Sea area, in particular the ones that
are long-term and costly, should be considered and
ranked in the light of climate change. The supply of
Anchovy, European Eel and Herring to our waters
appears to be driven by large-scale climate-induced
currents and increasing water temperature. At a
more local scale, Smelt migrates to deeper and cooler
places (such as old gullies or sand mining pits in Lake
IJssel, or to the open North Sea) when confronted
with water temperatures higher than 20°C (De
Leeuw et al. 2008). The strength and locations of
estuarine gradients will be determined by the supply
of freshwater and by wind-driven currents (Duran
Matute et al. 2014). Sea level rise will hamper the
future natural flows of freshwater by gravity into the
sea.
94
To estimate the added value of a new fish passage,
monitoring should be performed at all other
surrounding fish passages before and after a new
passage is constructed. Otherwise, it will not be
possible to distinguish between additional fish passing
or fish passing through the new facility (and no longer
through the old facilities). This would require an
integrated monitoring scheme for migratory fish in the
Wadden Sea. With standard techniques, equipment
and protocols, and addressing the preferred pathways
of migratory fish species at various spatial (e.g.,
freshwater discharge points, tidal basins) and temporal
(tide, season, year-to-year) scales. Incorporating the
migratory behavior of fish into hydrodynamic models
could produce biophysical models of fish movements
through the Wadden Sea. Such models would allow
for building scenarios for optimizing investments to
strengthen (migratory) fish stocks in the Wadden Sea
under various environmental conditions such as the
creation of large brackish zones, climate change and
sea level rise.
ACKNOWLEDGEMENTS
We acknowledge:
Loes Bolle (IMARES)
Meinard Bos (Provincie Fryslân)
Allix Brenninkmeijer (Altenburg & Wymenga)
Erik Bruins Slot (Provincie Fryslân)
Ingeborg de Boois (IMARES)
Wilco de Bruine (LINKit Consult)
Martin de Graaf (IMARES)
Jan Doornbos (Provincie Fryslân)
Dennis Ensing (Agri-Food and Biosciences Institute Northern Ireland, IRL)
Eelke Folmer (Ecospace)
Ben Griffioen (IMARES)
Henk Heessen (IMARES)
Zwanette Jager (Ziltwater Advies)
Guus Kruitwagen (Witteveen+Bos)
Romuald Lipcius (Virginia Institute of Marine Science, USA)
Roef Mulder (Provincie Fryslân)
Gualbert Oude Essink (Universiteit Utrecht)
Jaap Quak (Sportvisserij Nederland)
Peter Paul Schollema (Waterschap Hunze en Aa’s)
Ingrid Tulp (IMARES)
Tessa van der Hammen (IMARES)
Theo van der Kaaij (Deltares)
Henk van der Veer (NIOZ)
Hans van Hilten (Waddenfonds)
Herman Wanningen (PRW)
Erwin Winter (IMARES)
George Wintermans (Wintermans Ecologen Buro)
Alain Zuur (Highland Statistics)
Joey Zydlewski (University of Maine, USA)
for supplying data, supplying information, data analyses, reviewing or a
combination of these much appreciated contributions to this report.
95
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