Download seals and fish stocks in the north-east atlantic note

DIRECTORATE GENERAL FOR INTERNAL POLICIES
POLICY DEPARTMENT B: STRUCTURAL AND COHESION POLICIES
FISHERIES
SEALS AND FISH STOCKS IN THE
NORTH-EAST ATLANTIC
NOTE
Abstract:
Industrial and offshore fisheries using active gear have minor or insignificant
interactions with seals, whereas some coastal fisheries based on passive
gear can experience severe losses in catches and damages to gear. Seals
also have impacts on fish farms, which can be protected by a combination of
mitigation measures such as hunting, predator nets and acoustic deterrents.
Seals spread a parasitic worm, which is seldom a problem in oceanic
systems, but may be locally abundant close to main seal colonies.
IP/B/PECH/NT/2010-118
October 2010
PE 438.615
EN
This document was requested by the European Parliament's Committee on Fisheries.
AUTHOR
Tero HÄRKÖNEN
Swedish Museum of Natural History, Department of Contaminant Research, Stockholm
RESPONSIBLE ADMINISTRATOR
Irina POPESCU
Policy Department Structural and Cohesion Policies
European Parliament
E-mail: [email protected]
EDITORIAL ASSISTANTS:
Virginija KELMELYTE
Lea POLJANCIC
LINGUISTIC VERSIONS
Original: EN
ABOUT THE EDITOR
To contact the Policy Department or to subscribe to its monthly newsletter please write to:
[email protected]
Manuscript completed in October 2010.
Brussels, © European Parliament, 2010.
This document is available on the Internet at:
http://www.europarl.europa.eu/studies
DISCLAIMER
The opinions expressed in this document are the sole responsibility of the author and do
not necessarily represent the official position of the European Parliament.
Reproduction and translation for non-commercial purposes are authorized, provided the
source is acknowledged and the publisher is given prior notice and sent a copy.
Seals and fish stocks in the North-East Atlantic
CONTENTS
LIST OF ABBREVIATIONS
4
LIST OF FIGURES
5
LIST OF TABLES
5
General information
9
1. Effects of seals on the dynamics of fish stocks
1.1. Impacts of seals in oceanic ecosystems
11
11
1.1.1.
Insignificant or unlikely impacts on sizes of fish stocks in
oceanic ecosystems
12
1.1.2.
14
Potential or likely impacts on fish stocks in oceanic ecosystems
1.2. Impacts on fish stocks in coastal and estuarine environments
15
2. Impacts on catches and fishing gear
17
2.1. Impacts in oceanic ecosystems
18
2.2. Impacts in coastal and estuarine environments
18
2.2.1.
Angling
18
2.2.2.
Encircling gill nets
18
2.2.3.
Set gillnets
19
2.2.4.
Bag nets and fish traps
19
2.3. Mitigation measures
20
2.3.1.
Protective hunting
20
2.3.2.
Modification of fishing gear
20
2.3.3.
Acoustic deterrants
21
3. Spread of parasites where seals act as final hosts
23
3.1. Seal worm in fish in oceanic ecosystems
25
3.2. Seal worm in coastal ecosystems
26
3.3. Seal worm in estuarine and brackish water ecosystems
26
4. Aquaculture
27
4.1. Interactions with seals and mitigation measures
28
4.2. Considerations
28
References
31
3
Policy Department B: Structural and Cohesion Policies
LIST OF ABBREVIATIONS
ADD Acoustic Deterrent Device
AHD Acoustic Harassment Device
HELCOM The Helsinki Commission
ICES International Council for Exploration of the Sea
OSPAR Convention for the Protection of the Marine Environment of the
North-East Atlantic
4
Seals and fish stocks in the North-East Atlantic
LIST OF FIGURES
Figure 1
Interactions between seals and fisheries are evaluated in OSPAR areas I and
II and the Baltic Sea
8
Figure 2
Harbour seals tagged with satellite transmitters in the Danish Wadden Sea
can forage over extensive areas, but also in the vicinity of their haul-out
sites
15
Figure 3
Fisheries using passive gear operating vessels <12m contribute by 13%,
and larger vessels by 2% to the economic value of the Swedish fisheries
18
Figure 4
Encircling gillnets have been used as a main fishing method for thousands
of years
19
Figure 5
A grey seal raiding a herring gillnet in the Baltic
20
Figure 6
Whale worm (Anisakis simplex) in the
it is also common in herring and cod.
whale worm, toothed whales such as
indicated to be more important for the
24
abdominal cavity of a mackerel, but
Although seals act as final hosts for
harbour porpoises and dolphins are
occurrence of whale worm in fish
Figure 7
Location of cod samples taken for analyses of whale worm and seal worm
25
LIST OF TABLES
Table 1
Abundance (counts) of harbour seals in the North-East Atlantic
13
Table 2
Abundance of grey seals in the North-East Atlantic. *Numbers of pups x 4.5
14
Table 3
Frequency (%) and mean numbers (brackets) of whale worm (Anisakis
simplex) and seal worm (Pseudoterranova decipiens) in east Atlantic cod
26
5
Policy Department B: Structural and Cohesion Policies
Table 4
Production of farmed salmon and trout in the North-East Atlantic. Losses to
seals and mitigation methods
27
Table 5
Effectiveness of anti-predator controls in Scottish fish farms
28
6
Seals and fish stocks in the North-East Atlantic
EXECUTIVE SUMMARY
Background
Man and seals have a long and turbulent common history in the North-East Atlantic. A
thousand years ago the grey seal was the dominating seal species in the coastal regions
of the European continent including the Baltic, whereas harbour seals mainly occurred in
tidal areas at the English east coast and the Norwegian fiords (Härkönen et al., 2007).
Since grey seal pups need to be nursed on land during their first two to three weeks,
they are very vulnerable to hunting in areas where man has access to their breeding
grounds. Decreasing numbers of grey seals were noted along the Dutch coasts already in
the early Middle Ages, after which they became extirpated from the continental North
Sea coasts in the late Middle Ages (Härkönen et al., 2007). Grey seals were here
replaced by harbour seals since their pups can swim directly after being born, and are
thus less vulnerable to hunting. A similar change occurred later in the Skagerrak, where
grey seals were extirpated in the 1750s and replaced by harbour seals (Härkönen et al.,
2005).
Most countries in the North-East Atlantic introduced bounty systems in the late 19th
century and the early 20th century with the explicit objective to exterminate the seals
(Harding and Härkönen, 1999). Seal numbers were substantially reduced around the
British Isles, the North Sea coast and the Baltic Sea in the 1920s and 1930s (Harding and
Härkönen, 1999, Härkönen et al., 2007). In this process grey seals were extirpated from
the Kattegat and the Southern Baltic coast, and populations of harbour seals decreased
typically by 90-95% Härkönen et al., 2007).
Some protection for British seals was introduced in the 1930s, but hunting wasn’t
prohibited until the 1960’s in the North Sea area and the late 1970s and 1980s in the
Baltic. After protective measures were taken, the very depleted seal populations started
to recover and most seal populations have been growing exponentially until very
recently, where North Sea populations in Scotland have showed slow or negative growth
rates during the last decade.
Seal populations have been regulated by hunting in Iceland, the Faroes and Norway
during the 20th century, which still is the current management strategy in those
countries.
The growing seal populations have led to increasing interactions with especially the
coastal small scale fisheries, where fishing techniques developed under periods with very
low abundances of seals are vulnerable to different types of damages.
Aim
The aim of the present note is to provide a comprehensive qualitative analysis of
interactions between seals and fisheries in the North-East Atlantic, which is defined as
the OSPAR areas I and II and the Baltic Sea (Figure 1).
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Policy Department B: Structural and Cohesion Policies
The approach is focused on compiling and evaluating available information from
published papers, published and unpublished reports and databases. Impacts of seals on
fisheries are studied by categorizing effects on:
Figure 1: Interactions between seals and fisheries are evaluated in OSPAR
areas I and II and the Baltic Sea
Source: ICES (2004)

Size and dynamics of fish stocks

Seal predation on catches and impacts on fishing gear

Spread of parasites where seals act as final hosts

Aquaculture
8
Seals and fish stocks in the North-East Atlantic
GENERAL INFORMATION
KEY FINDINGS

The consumption of seals on commercially valuable fish stocks is typically less
than 1 % of catches taken by fisheries or spawning stock sizes in offshore areas.
Effects of seals on sizes of fish populations are therefore insignificant in oceanic
systems.

Local high densities of seals in coastal areas can under some circumstances have
measurable effects on catches of salmonids, although this only has been shown in
few cases.

Approximately 90% of all fish are taken by active gear off the coasts of the NorthEast Atlantic and these fisheries are to a minor or insignificant extent affected by
seal predation on catches and damages on gear.

Static gear such as set gillnets in coastal areas and the Baltic are very vulnerable
to impacts from seals, and catch losses may vary between 20% and more than
50% in cod gillnets and herring gillnets. Gillnets are difficult to protect against
seal damages since seals get accustomed to acoustic deterrents.

Fish traps, bagnets and fykenets are also vulnerable to impacts from seals, which
have caused substantial damages to fisheries in the Baltic. Modifications of fish
traps have successfully reduced damages both to catches and gear.

Seal worms are found in fillets of cod, hake and other cod fishes. The prevalence
of seal worms depends on the proximity to main seal colonies, availability of
intermediate hosts, and fish species which accumulate worm larvae.

Offshore fisheries are much less affected by seal worms as compared with coastal
fisheries.

Seal worms are often confused with whale worms, which occur in greater
abundances and also in oceanic environments. Whale worms are predominantly
spread by toothed whales and minke whales.

Fish farms are affected by seals but losses seldom exceed 1% in Norway, Iceland
and the Faroes. Losses in Scotland range between 1-5% depending on location
and which protective measures are taken.

Allocation of farms far away from main seal colonies reduces losses. Shooting of
seals is an effective mitigation method, but hunting pressure needs to be intense
enough to deplete local seal populations. Harbour seal populations in Iceland,
Norway and Scotland are decreasing rapidly, and the Faroese population is
extinct.

Reductions of seal populations are in conflict with the EU-Habitats Directive and
the 2006 HELCOM seal recommendation, which state the long-term objectives for
management of marine mammals are to attain “natural abundances and
distributions”.
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Policy Department B: Structural and Cohesion Policies
One often disregarded aspect in discussions on seal fisheries interactions is that fisheries
using active gear offshore, accounting for approximately 90% of all landings, have minor
or insignificant impacts from seals (Kashner et al., 2006). Consequently, interactions with
seals do not constitute a general problem for fisheries, but affect segments of the
industry that account for about 10% of all landings in the North-Eastern Atlantic.
More specifically, small scale coastal fisheries using passive gear are in many cases
affected by seals damaging catches and gear. It is also the small scale fisheries in some
regions with large seal colonies that to a greater extent than the offshore fisheries are
affected by seal worms. Recent advances in modifying fish traps set for salmon, whitefish
and cod have significantly reduced damages to both catches and gear, whereas problems
with gillnets remain since acoustic deterrents are inefficient and also difficult to deploy in
practice.
10
Seals and fish stocks in the North-East Atlantic
1. EFFECTS
STOCKS
OF
SEALS
ON
THE
DYNAMICS
OF
FISH
KEY FINDINGS

The ecological complexity, multi-species interactions and a high degree of
interannual variation make it difficult to assess impacts of seals on the species
they prey upon. However:

Seals are unlikely to have significant impacts on sizes of fish stocks in complex
oceanic ecosystems.

Seal predation on commercially important fish species is insignificant for the
offshore fisheries.

In ecosystems such as the Barents Sea, where seals are very abundant (millions),
they can have measurable impacts on the abundances of their prey species.

Although seals generally have minor or insignificant impacts on the size of fish
stocks in coastal and estuarine ecosystems, seal predation on salmonids in some
river mouths reduce catches in the fishery.
Studies attempting to evaluate effects of seals on the dynamics of fish stocks face the
challenge to model species interactions at the ecosystem level. Multispecies models
aiming at predicting outcomes of different scenarios encounter severe problems since
each of the included parameters contain a measurement error (variance). Adding the
variances for all parameters will lead to model outputs where the predictive values are
close to zero. This is also reflected in the ongoing debate whether or not specific
ecosystems are controlled “top-down” or “bottom up” (e.g. Frederiksen et al., 2006;
Kashner et al., 2006; Baum et al., 2009). However, such models can be useful for
studying specific features such as interactions between parameters under specific
scenarios where a number of other parameters are kept constant (Pinnegar et al., 2008).
A more constructive approach is to evaluate potential impacts by studying how large a
proportion of the total mortality of commercially important fish species is caused by
seals, or to relate the consumption of seals to the size of the fishery. However, sizes of
commercial catches are not directly comparable with removals caused by seals, since
fisheries and seals in most cases target different size classes of fish.
1.1.
Impacts of seals in oceanic ecosystems
The role of top predators such as seals for the dynamics of fish stocks is very complex
since seals prey upon a large variety of fish species and also coexist with a number of
other predators such as whales, piscivorous sea birds, predatory fish, and invertebrate
predators. This top-down effect caused by single predator species appears to be weak in
most oceanic ecosystems (Kaschner et al., 2006; Baum et al., 2009), but may be
important in specific cases, where a top predator is very abundant (Haug et al., 1995).
Seals may also have measurable effects on local abundances of fish in some coastal and
estuarine environments (Fjalling et al., 1995; Kauppinen et al., 2005; Butler et al.,
2006). These situations will be evaluated separately in the following.
11
Policy Department B: Structural and Cohesion Policies
1.1.1.
Insignificant or unlikely impacts on sizes of fish stocks in oceanic ecosystems
The decreasing numbers of grey seals (5,000) and harbour seals (8,000) in Iceland
(Hauksson and Bogason, 1997; ICES, 2004; Tables 1 and 2) are unlikely to have
significant effects on the fish stocks they prey upon. The summer diet of grey seals in
Icelandic waters is constituted of cod (24%), sandeel (23 %), catfish (15%), lumpsucker
(7%), whereas other species made up less than 1% of the diet (Olafsdottir and
Hauksson, 1997).
Both grey seals and harbour seals occur along the Norwegian coast, but the about 6,000
grey seals and 12,000 harbour seals (Tables 1 and 2) are too few to have measurable
effects in off-shore areas. Total fish consumption by seals in Norway is approximately
30,000 tonnes as compared with 2,5 million tonnes caught by the fisheries. Seal
predation on species targeted by the fisheries is less than 1% of of catches.
A similar situation in found in Faroese waters, where the less than 500 grey seals,
preying predominantly on sandeels, catfish and cod (Mikkelsen et al., 2002), are too few
to have significant impacts on fish stocks. Catches in the Faroese fishery vary around
500,000 tonnes.
Increasing numbers of harbour seals, which are the most abundant seal species in the
Kattegat and the Skagerrak, have led to concerns from the fisheries about possible
impacts on commercially important fish stocks. However, since harbour seals are
generalists feeding on a large variety of fish species, where the main prey size is beneath
30cm, the competition with fisheries in oceanic ecosystems is insignificant (Härkönen,
1987; Härkönen and Heide-Jörgensen, 1991; Hansen and Harding, 2006). Potential
landings of cod would not be affected even if all harbour seals were removed from the
ecosystem, since their predation on cod is minor compared with the mortality caused by
the fisheries (Hansen and Harding, 2006).
Grey seals are the most abundant seal species around the British Isles, and the North
Sea stock is close to 100,000 animals (Anon., 2008, Table 2). They range over large
areas in the western North Sea (McConnell et al., 1999), where different species of
sandeels constitute the most important prey (Prime and Hammond, 1990). Sandeels are
also targeted by industrial fisheries, but a combination of factors such as grey seals also
preying upon other main predators of sandeels, and the occurrence of other highly
abundant predatory fish species such as mackerel and whiting, make direct competition
with the fisheries unlikely (Prime and Hammond, 1990; Furness, 2002).
Grey seals in the North Sea also prey upon many other commercially important fish
species (Hammond and Grellier, 2005). Comparing consumptions of grey seals in 1985
and 2002, it was found that the estimated prey consumption was less than 1% of
spawning stock sizes of all species targeted by fisheries. As a consequence of increasing
grey seal populations and decreasing fish stocks the prey consumption of grey seals was
higher in 2002, but exceeded 1% of stock sizes only for cod (3.7%), sandeel (2.7%) and
plaice (1.5%). Consequently, grey seal predation on commercially important fish species
is insignificant or minor in the North Sea area.
There is not enough data and information to assess potential impacts of seals on fisheries
in the Wadden Sea.
12
Seals and fish stocks in the North-East Atlantic
Table 1: Abundance (counts) of harbour seals in the North-East Atlantic
LOCATION
COUNT
YEAR
CHANGE OVER PAST 5 YEARS
Svalbard
1000
2005
Unknown
Iceland
8023
2006
Decreasing
Russia (Murman coast)
<500
1998
Unknown
Norway (East Finnmark)
207
2003-2005
Decreasing
Norway (West Finnmark to 62°N)
5485
2003-2005
Decreasing
Norway south of 62°N
685
2003-2006
-40%
Norwegian Skagerrak
291
2005-2009
+20%
Swedish Skagerrak
2689
2005-2009
+40%
Kattegat
6182
2005-2009
+15 %
Baltic (Kalmarsund)
592
2005-2009
+ 30%
Southern Baltic
527
2005-2009
+ 20%
Limfjord
879
2005-2009
- 4%
Wadden Sea Denmark
3063
2005-2009
+10%
Wadden Sea Germany
11680
2005-2009
+10%
Wadden Sea Netherlands
6339
2005-2009
+10%
Faroes
0
UK Shetland
3021
2001-2006
-40%
UK Orkney
4256
2001-2006
-45%
UK Scotland east
1819
1997-2006
-26%
UK England east
3617
2001-2006
-34%
France
239
2005-2006
Slight increase
Extinct
Source: Diverse
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Policy Department B: Structural and Cohesion Policies
Table 2: Abundance of grey seals in the North-East Atlantic. *Numbers of pups
x 4.5
LOCATION
COUNT
YEAR
CHANGE OVER PAST 5 YEARS
Iceland
4950
1997-2007
-30%
Russia (Murman coast)
2427*
1991-1992
Unknown
Norway (East Finnmark)
670*
2001-2006
+30%
Norway (West Finnmark-62°N) 5000*
2001-2006
Unknown
Norway (South of -62°N)
158
2001-2006
Stable
Skagerrak
5
2006-2009
Stable
Kattegat
21
2006-2009
Stable
Baltic
22050
2006-2009
Stable
Wadden Sea
2194
2006-2009
+100%
France
90
2005
Stable
Faroes
<500
2005
Decreasing
UK Shetland
3047*
2006
Stable
UK Orkney
86994*
2002-2006
+1.5%
UK North Sea colonies
5322*
2002-2006
+20%
TOTAL
133428
Source: Author; ICES (2004); Anon. (2008)
1.1.2.
Potential or likely impacts on fish stocks in oceanic ecosystems
Impacts of seals on fish stocks can be significant in ecosystems where seals are highly
abundant. It is estimated that the 2.06 million harp seals in the Barents Sea consume
3.5-5 million tonnes of krill and fish (Lindstrøm et al., 2006). The composition of the diet
varies among years and is linked to large scale ecosystem dynamics. The most important
fish prey species are polar cod, cod and capelin (Nilssen et al., 2000), and seal predation
can under some modelled scenarios have effects on prey species (Bogstad et al., 1997).
However, the model was more sensitive to changes in food preferences of cod than
changes in diet or abundances of marine mammals (Bogstad et al., 1997).
The Norwegian coast was invaded by large numbers of harp seals in 1987 (crash in
capelin stocks) and dietary analyses showed a dominance of cod, capelin, saithe and
herring (Ugland et al., 1993). It was estimated that 110 million individuals of cod and
saithe of year classes 1985 and 1986 were consumed by harp seals. These year classes
were later found to be severely depleted (Ugland et al., 1993).
Consequently, very abundant stocks of seals (millions) may have measurable impacts on
fish stocks they prey upon also in oceanic ecosystems.
14
Seals and fish stocks in the North-East Atlantic
1.2.
Impacts on
environments
fish
stocks
in
coastal
and
estuarine
Both grey and harbour seals are confined to coastal areas during reproduction and moult.
They also show explicit site fidelity to specific haul-out sites, from where they make
foraging trips. Whereas harbour seals seldom travel more than 100 km (Figure 2), grey
seals may travel more than 500km (McConnell et al., 1999). However, both species also
forage in the vicinity of their haul-out sites, and high local densities of seals can in some
cases lead to competition with specific fisheries, although a general impact on coastal
stocks have not been demonstrated. Some investigations have shown that foraging areas
of harbour seals and grey seals often overlap with areas used by coastal fisheries (e.g.
Bjørge et al., 2002). Seal predation on fish species, such as cod, also targeted by the
fisheries are assumed to reduce catches (Bjørge et al., 2002), which may be true, but
there is no information available on extent to which seals reduce local fish stocks.
Figure 2: Harbour seals tagged with satellite transmitters in the Danish
Wadden Sea can forage over extensive areas, but also in the vicinity
of their haul-out sites
Source: http://www.hornsrev.dk/
Investigations in the Limfjord (Denmark) and the Southern Baltic showed that seals
mainly targeted non-commercial fish species apart from Atlantic herring which was the
most important species. The competition with the fisheries was regarded minor or
insignificant (Andersen et al., 2007).
The impact of seal predation on local salmon or trout stocks in river mouths varies with
the size of local seal stocks, fish abundance, and fishing mortality caused by the fishery
(Carter et al., 2001). Although seal predation on salmonids in many Scottish estuarine
environments is ten-fold less than that caused by the fishery (Carter et al., 2001), it was
shown in one case that removal of some seals increased the catches by 30% (Butler et
al., 2006). Tornijoki river in the Northernmost part of the Baltic is one of the most
important spawning rivers for salmon in the Baltic and it is indicated that seal predation
in the river mouth could reduce numbers of salmons entering into the river by 20%
(Juonela et al., 2006).
15
Policy Department B: Structural and Cohesion Policies
16
Seals and fish stocks in the North-East Atlantic
2. IMPACTS ON CATCHES AND FISHING GEAR
KEY FINDINGS

Fisheries using active gear in offshore areas, contributing to about 90% of
landings in the North-East Atlantic, have minor or insignificant impacts from seals
on gear and catches.

Some fisheries using active gear close to the coast experience losses to seals that
can exceed 10% of catches.

Artisanal fisheries based on angling and encircling gillnets generally show low
impacts of seals.

Set gillnets and drift nets are very vulnerable to seals. Catch losses can exceed
50% in gillnets set for cod, herring and white fish in the Baltic Sea, whereas lower
loss rates are reported in other parts of the North-East Atlantic. Gillnets are
difficult to protect against seals since acoustic deterrents, which may have effects
initially, eventually become ineffective and might even attract seals.

Bagnets, fish traps and fykenets are vulnerable to seals, and in the Baltic where
these types of static gear are commonly used, seal-fisheries interactions are
severe and losses exceed 6 million € only in the Swedish fishery. Modifications of
fish traps have successfully reduced damages to catch and gear.

Shooting of seals has not significantly reduced damages to catches or gear in the
Baltic.
The impact of seals on catches and fishing gear is highly variable and is affected by a
number of factors, of which type of used gear and geographical location of fishing
operations in relation to densities of seals are the most important parameters. Here the
structure of the fishery is a critical factor since the offshore fishery using active gear is
affected to a minor or insignificant extent by seals, whereas fisheries based on specific
types of passive gear in coastal areas are influenced by seal predation and damage to
fishing gear to a much greater extent.
Many types of static gear such as bag nets and fish traps or fyke nets (eel) used in
regions such as the Baltic Sea and the Limfjord (Denmark), are not commonly used in
other parts of the North-East Atlantic. These types of gear are less suitable for fishing in
tidal areas with strong currents and large fluctuations in water levels.
The offshore fisheries using active gear such as pelagic or bottom trawls dominate the
fisheries in the North-East Atlantic and take about 90% of the total catches. However,
catches in the coastal fisheries are to a greater extent focussed on providing fish for
human consumption, which is why the economic value of these catches are greater than
the 10% of the total landings. This can be illustrated by the situation in the Swedish
fisheries, where coastal or Baltic fisheries using passive gear contribute by 15% to the
total economic value of the fisheries (Figure 3). Much of this fishery is severely affected
by seals.
17
Policy Department B: Structural and Cohesion Policies
Figure 3:
Fisheries using passive gear operating vessels < 12m contribute by
13%, and larger vessels by 2% to the economic value of the
Swedish fisheries.
Relative economic value of
different Swedish fisheries
Passive
gear<12m
Passive
gear>12m
Nephrops trawl
Shrimp trawl
Other bottom
trawls
Pelagic trawls
< 40 m
Pelagic trawls
> 40 m
Source: Swedish Board of Fisheries (2009)
2.1.
Impacts in oceanic ecosystems
Most reports on interactions between seals and fisheries operating offshore mention that
seals sometimes get caught in pelagic trawls or bottom trawls, which appears to be the
main negative effect for the offshore fisheries (Morizura et al., 1999). Information on
impacts of seals on catches and gear is very scarce and appears to be minor or
insignificant.
2.2.
Impacts in coastal and estuarine environments
Many different fishing methods have been developed in coastal regions of the North-East
Atlantic of which a majority use some type of static gear. However, in some regions
Nephrops and shrimp trawls are towed close to the shore, and some of these fisheries
report interactions with seals that damage by-catches of cod and hake although gear
damage was minor (Moore, 2003).
2.2.1.
Angling
Angling is basically not used by professional fishermen, but is important for the
recreational fishery. Anglers occasionally report interactions with seals, but their main
complain is that seals and anglers compete for the same resource, especially for salmon,
trout and some white fish.
2.2.2.
Encircling gill nets
Encircling gillnets have been used for thousands of years and much of the fishery in the
North-East Atlantic, e.g. during the herring periods was based on this method. It was the
18
Seals and fish stocks in the North-East Atlantic
most common fishing method in the North-East Atlantic until the Middle Ages, but was
still extensively practised in Scandinavia until the 18th century (Figure 4). Diminishing
fish stocks made this method unprofitable and the fishing effort was gradually allocated
further offshore. However, encircling gill nets are still common in artisanal fisheries
worldwide, and is practised in river mouths in Scotland to catch salmon. This type of
fishery seldom report impacts by seals on gear or catches.
Figure 4:
Encircling gillnets have been used as a main fishing method for
thousands of years
Source: Laksefiskaren (Salmon fisherman) painted by Elif Peterssen (1889)
2.2.3.
Set gillnets
Small scale fisheries using gillnets are generally not very common in the North-East
Atlantic, whereas fisheries along the Scandinavian coasts and the Baltic to a great extent
use gillnets to target specific species of fish such as cod, herring and white fish (Fjälling,
2006). This fishery reports substantial damage to gear and catches and losses to seals
may exceed 50% of the catches under some circumstances. A dedicated study on a cod
gillnet fishery in the Baltic showed that such losses could vary from 67% of the landed
catches in the gill net fishery in 2005 to 19% in 2006 (Königson et al., 2009). A similar
situation is seen in the gillnet fisheries focussing on Baltic herring, experiencing
substantial losses to grey seals, and where damages to nets are very common (Figure.
5).
2.2.4.
Bag nets and fish traps
Also bag nets, fish traps and fykenets are less commonly used in tidal areas, which is
why problems with seals in this type of fishery are not significant in e.g. the Wadden Sea
and the incidents are very limited compared with the Skagerrak, Kattegat and the Baltic
where such gear are commonly used. There have been increasing interactions between
grey seals and the coastal trapnet fisheries for salmon and white fish in the Northern part
of the Baltic (the Bothnian Bay), where catch losses have ranged between 3% and 29%
(Juonela et al., 2006), whereas other studies found that losses could be as high as 61%
(Fjälling, 2005).
19
Policy Department B: Structural and Cohesion Policies
Figure 5: A grey seal raiding a herring gillnet in the Baltic
Source: Fjälling (2005)
2.3.
2.3.1.
Mitigation measures
Protective hunting
Slightly more than 800 grey seals and 50 harbour seals were shot in the Baltic, the
Skagerrak, the Kattegat in 2009 (Helcom website). Effects of this protective hunting have
not been evaluated recently, but earlier investigations indicate that this type of hunting
has limited or no effects on damages caused by seals on catches and gear (Sand and
Westerberg, 1997).
2.3.2.
Modification of fishing gear
Simple modifications of salmon traps have proved to efficiently reduce losses to seals and
reduced gear damages considerably. A wire grid in the funnel of the trap in combination
with stronger net in the bag reduced losses by 70% as compared with traditional traps
(Lehtonen and Suuronen, 2004). Other modifications such as increasing the mesh size of
side panels of salmon traps substantially reduced damages to catch and gear (Lunneryd
et al., 2003). Using stronger nets in fyke nets set for eel and preventing seals from
entering into traps have considerably reduced interactions with seals (Fjälling, 2006).
Modification of fishing gear has thus proved to be the most efficient way to reduce
damages caused by seals.
20
Seals and fish stocks in the North-East Atlantic
2.3.3.
Acoustic deterrants
Acoustic harassment devices are extensively used in Scottish fish farms, but proper
evaluations on their effectiveness are not available (Quick et al., 2004). Such evaluations
have been done in the Baltic, where it was found that acoustic harassment devices
reduced interactions with seals initially, but that positive effects diminished with time
(Fjälling et al., 2006). Other studies even suggest that acoustic deterrents may attract
grey seals to gillnets (Stridh, 2008), which is why such devices are unlikely to
substantially contribute to mitigations between seals and fisheries in the future.
21
Policy Department B: Structural and Cohesion Policies
22
Seals and fish stocks in the North-East Atlantic
3. SPREAD OF PARASITES WHERE SEALS ACT AS FINAL
HOSTS
KEY FINDINGS

Whale worms (Anisakis simplex) and seal worms (Pseudoterranova decipiens) are
commonly confused in the general understanding of the impact of seals on the
prevalence of parasites in commercially important fish species. Whale worms are
predominantly spread by harbour porpoises, dolphins and minke whales, whereas
seal worms are spread by grey seals, bearded seals, harp seals and harbour seals.

Larval stages of both whale worms and seal worms occur in the fillets of
commercially important fish species, and removal of the parasites cause
considerable costs to the fish processing industry. Since reports from the fisheries
do not distinguish between whale worms and seal worms, it is not possible to
evaluate the contribution of seal worms in this context.

The frequency of whale worms exceeds 90% in cod from Iceland, Barents Sea and
the Celtic sea. The contribution of seals to this situation is minor or insignificant.

Burdens of seal worms are very low in fish caught offshore, whereas fish caught
inshore close to seal colonies can be heavily infected.

The occurrence of seal worms in coastal areas is limited by the availability of
intermediate invertebrate hosts, fish species such as sea scorpion accumulating
seal worm larvae, and sometimes, but not always the size of the seal population.
Both grey seals and harbour seals are final hosts for a number of parasites such as tape
worms (Cestoda), hook worms (Acanthocephala) and Anisakid nematodes. Tape worms
and hook worms infest the seals, but seldom cause any problems for the marine
fisheries, which is why this review will focus on Anisakid nematodes. The most common
nematodes found in seals are Pseudoterranova decipiens (seal worm, cod worm),
Contracaecum osculatum, Phocascaris cystophorae and Anisakis simplex (whale worm,
Figure 6). Larval stages of C. osculatum and Ph. cystophorae appear in the gut or
abdominal cavities of many fish species, and can in some cases penetrate the skin in
deteriorating fish. However, those parasites are normally removed when the fish is
rinsed, and losses for the fishery caused by these two species are minor or insignificant.
Inactive larval stages of whale worm are often found as incapsulated spirals on the liver
or intestines of mackerel, herring and other species of fish (Figure 6). Larvae can also be
abundant in the muscle of cod where prevalence of 96% has been reported from the
Barents Sea (Aspholm, 1995). Removal of whale worms from fillets constitutes significant
costs for the fish processing industry. However, toothed whales are the main final hosts
of the whale worm, which appears in most oceans, and densities of this parasite seem to
be predominantly affected by the occurrence of harbour porpoises and dolphins. Although
whale worms develop in seals to the adult stage, seals are indicated to be suboptimal
final hosts since the size of the adult stage of A. simplex is considerably smaller in seals
as compared with toothed whales. Whale worms are often confused with seal worms and
the confusion is accentuated by the Norwegian word “kveis” being used both for whale
worm and seal worm.
23
Policy Department B: Structural and Cohesion Policies
Figure 6: Whale worm (Anisakis simplex) in the abdominal cavity of a
mackerel, but it is also common in herring and cod. Although seals act
as final hosts for whale worm, toothed whales such as harbour
porpoises and dolphins are indicated to be more important for the
occurrence of whale worm in fish.
Source: Author
The seal worm is by far the most important parasite spread by seals and causes
considerable economical costs for some fisheries, although there are substantial
differences among regions as well as among fish species.
Grey seals, harbour seals, bearded seals and harp seals are final hosts for the seal worm
in the Barents Sea and the North-East Atlantic. The seal worm represents a species
complex and genetic analyses have identified three sibling species: Pseudoterranova
decipiens A infecting grey seals, P. decipiens B infecting harbour seals, and P. decipiens C
infecting bearded seals (Paggi et al., 1991). In all sibling species the adult worm shed
eggs in the intestine of their host, and the eggs are shed to the sea with the faeces.
When the eggs are ingested by invertebrate intermediate hosts such as benthic small
crustaceans and polychaetes, the egg starts developing and passes a number of larval
stages. If the intermediate host is ingested by a fish, the larvae transform to the third
stage and remain so even if the fish is ingested by a predatory fish. The larvae are
mostly found in the muscle and in greatest abundances in sea scorpion, bullroute, and
bullheads, which act as important reservoirs for seal worm larvae. Commercially
important fish species feeding on these benthic species can therefore be heavily infected,
whereas others such as mackerel or salmon very rarely get infected. When infected fish
is consumed by a seal, larvae transform to the adult stage. It is the third stage larvae
that cause problems to the fishery, since they are found in the muscle and must be
removed manually.
Consequently, the prevalence or abundance of seal worms in commercially important fish
species is affected partly by seal density and abundance, but also by the
presence/absence of intermediate invertebrate hosts, and by species such as sea
24
Seals and fish stocks in the North-East Atlantic
scorpion, which accumulate the larvae. A general pattern is that the occurrence of seal
worms in fish is greatest in fish caught close to seal colonies, whereas fish caught further
away often are uninfected (Jensen and Idaas, 1992).
3.1.
Seal worm in fish in oceanic ecosystems
The frequency of seal worm in commercially important species caught in offshore
fisheries in the North-East Atlantic is generally low (Hemmingsen and MacKenzie, 2001)
and constitutes a minor problem to the fisheries. The situation is somewhat different in
the Barents Sea and adjacent areas where harp seals are very abundant. However, the
main problems with seal worm are found in the coastal fisheries.
Figure 7: Location of cod samples taken for analyses of whale worm and seal
worm
Source: Alonso (2008)
25
Policy Department B: Structural and Cohesion Policies
Table 3: Frequency (%) and mean numbers (brackets) of whale worm
(Anisakis simplex) and seal worm (Pseudoterranova decipiens) in east
Atlantic cod
LOCATION
A. simplex
P. decipiens
Sample size
Baltic
15 (2.3)
3.9 (0.07)
180
North Sea and Skagerrak
50.3 (34.3)
12.9 (1.4)
147
Celtic Sea
92 (74.9)
0.8 (2.2)
138
Irish Sea
36 (4.6)
22.1 (0.9)
136
Trondheimsfjord
5 (0.05)
1.7 (0.02)
60
Iceland
99.4 (327)
0.7 (3.1)
165
Barents Sea
96
0
Not given
Data source: Alonso (2008) and Aspholm (1995).
3.2.
Seal worm in coastal ecosystems
The occurrence of seal worms shows very large variations along the coasts of Iceland and
Norway (e.g. Andersen et al., 1995; Hemmingsen and MacKenzie, 2001; Figure 7, Table
3). In some areas with large colonies of seals the frequency of infestation is high
especially in cod. In those regions processing plants scan each filet for seal worm, which
increases the processing cost, whereas in other Icelandic and Norwegian regions the
problems with seal worm are insignificant or minor. Although more seal worms in fish
occur close to haul-out site of seals, the problems for the coastal fisheries vary
substantially depending on which type of gear is used and migration patterns of targeted
fish species (Andersen et al., 1995; Alonso, 2008, Table 3). However, decreasing
numbers of seals in some areas have resulted in lower infestation rates (des Clers and
Andersen, 1995; Hauksson, 2002), suggesting that local high densities of seals is linked
to infestation rates in demersal relatively stationary fish species.
The frequency of seal worm in cod from the Skagerrak and the Kattegat is generally low
(Andersen et al., 1995, des Clers and Andersen, 1995)
Consequently, seal worm is a minor problem for fisheries using active gear, but the
parasite can be locally abundant especially in cod caught in passive gear in areas with
high densities of seals.
3.3.
Seal worm in estuarine and brackish water ecosystems
Larval stages of the seal worm develop also in sub zero temperatures and are present in
the Arctic as well as in the Antarctic (Palm, 1999). However, its distribution seems to be
severely restricted in brackish water systems as the Baltic Sea. It has been found in cod
caught in the southern part of the Baltic (Perdiguero-Alonso et al., 2008), but only in
small numbers. A possible reason for its low occurrence in brackish water systems may
be lack of suitable intermediate hosts. Another reason could be that species accumulating
larvae, such as sea scorpion, are not present in these ecosystems. Consequently, seal
worm may occur in brackish water systems, but its distribution and prevalence is
predicted to be low in such environments.
26
Seals and fish stocks in the North-East Atlantic
4. AQUACULTURE
KEY FINDINGS

The first-hand value of fish farmed in the North East Atlantic exceeds 3 Billion €,
and aquaculture is a rapidly increasing industry.

Losses to seals in fish farms in Norway, Iceland and the Faroes are less than 1%,
whereas losses in Scotland are estimated at 1-5%.

Shooting and predator nets are the most common mitigation methods, but
acoustic deterrents are also used in Scotland.

Shooting of seals is an effective mitigation method if the hunt is intense enough to
deplete local seal populations.

Scottish, Icelandic, and Norwegian populations of harbour seals are decreasing as
a consequence of excessive hunting. The Faroese harbour seal population is
extirpated.
Finfish farming is a rapidly growing (10-15% per year) industry and the current firsthand value of produced fish, predominantly salmon and trout, exceeds 3 billion € in the
North-East Atlantic (Table 4). Most fish farms are found in Norway and Scotland, where
they are situated in fiords which give them protection against high waves and strong
currents. Most fish farms are visited by seals, but the loss and damage caused by seals
vary greatly among farms (Quick et al. 2004).
Table 4: Production of farmed salmon and trout in the North-East Atlantic.
Losses to seals and mitigation method
*Inferred from 8.5 million smolts delivered to fish farms. Shooting is the most common
mitigation method in all countries, whereas Acoustic Harassment Devices (AHD) or
Acoustic Deterrent Devices (ADD) are predominantly used in Scotland.
PRODUCTION
(tonnes)
FIRST HAND
VALUE
YEAR
LOSS TO
SEALS
MITIGATION
METHOD
Norway
980,000
2,400 million €
2009
<1%
Shooting
Predator net
Iceland
No info
No info
-
<1%
Shooting
Predator net
Denmark
40,415
92 million €
2008
<1%
Shooting
Predator net
Faroes*
25,000
68 million €
2009
< 1%
Shooting
Scotland
135,000
500 million €
2009
1-5%
Shooting
Predator net
AHD, ADD
COUNTRY
Source: Author; Fiskeridirektoratet (2010); Quick et al. (2004)
27
Policy Department B: Structural and Cohesion Policies
4.1.
Interactions with seals and mitigation measures
The impact of seals on fish farms is comparatively low in Norway, Iceland and the Faroes,
where a combination of shooting of seals and predator nets are practised. However, to be
effective, lethal removal of seals must be large enough to reduce seal populations in the
vicinity of the farms. There are no colonies of seals in fiords used for fish farms in
Norway and the Faroes and occasional visiting seals are shot. As a consequence of this
type of antipredator strategy, in the Faroes the harbour seals have been extirpated and
the total population of grey seals is below 500 animals. About 200-250 grey seals, of
which a majority is believed to come from Scottish seal colonies, are shot annually as a
part of a protection programme for the aquaculture (ICES, 2004). Also the Icelandic seal
hunt has resulted in decreasing seal populations, whereas the hunting in Norway
balances the recruitment in both harbour seals and grey seals in some areas, but
decreasing numbers are seen in others (Table 2).
Shooting of seals is also practised in Scotland and the annual 2-5% reduction in harbour
seal numbers in The Moray Firth over the last decade would be explained by the annual
removal of 66–327 harbour seals at salmon fisheries and fish farm sites (Thompson et
al., 2007).
A Scottish study (Quick et al., 2004) investigated how fish farmers experienced different
types of protective measures against seal predation and 190 out of 195 used some kind
of anti predator controls (Table 5). Seals were reported to have visited 81% of the farms,
but losses caused by seals have been constant over the period 1987-2001. Shooting was
practised at 73 sites and was considered to be effective by a majority, whereas acoustic
seal scarers were considered as less effective. Predator nets were experienced as
effective at some farms but not at others.
Table 5: Effectiveness of anti-predator controls in Scottish fish farms
No OF SITE
MANAGERS
CONSIDERED
EFFECTIVE
Shooting
73
62%
Seal scarers
92
23%
180
varying
METHOD
Predator nets
Source: Quick et al. (2004).
4.2.
Considerations
The most effective mitigating measure is to place fish farms far away from larger colonies
of seals, since there is a strong correlation between distance to seal haul-out sites and
intensity of damage caused by seals (Kashner et al., 2006). This feature should be
considered in detail before establishing fish farms in the future.
Another effective way to protect fish farms is to shoot seals, but hunting has to be
carried out to an extent where seal populations get depleted. This is not always a
desirable option especially since the EU-habitats Directive states that the long-term
28
Seals and fish stocks in the North-East Atlantic
objectives for the management of marine mammals are “Natural abundance, natural
distribution, and a health status that ensures their future persistence”. It is also stated
that these long-term objectives should not be affected by socioeconomic considerations,
although such considerations could be taken in the implementation of management
plans. These basic components are also the basis for the 2006 HELCOM recommendation,
which has been ratified by all Baltic countries.
29
Policy Department B: Structural and Cohesion Policies
30
Seals and fish stocks in the North-East Atlantic
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33
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34
Seals and fish stocks in the North-East Atlantic
NOTES
35