Download Plaice in the North Sea (ICES subarea IV), Demersal otter trawls

Plaice in the North Sea (ICES subarea IV), Demersal otter trawls
Plaice in the North Sea (ICES subarea IV),
Demersal otter trawls
Content last updated
8th Nov 2016
Stock:
Plaice in the North Sea (ICES subarea IV)
Management:
EU
Overview
European plaice (Pleuronectes platessa) is a widely distributed bottom-dwelling flatfish that inhabits the temperate waters of the
Northeast Atlantic from the White Sea to the Bay of Cadiz most frequently at depths ranging from 0 to 50 m. Adults are usually solitary,
burrowing into sandy and muddy habitat, and migrating seasonally between feeding areas and spawning grounds. In Subarea IV (North
Sea), most females reach maturity by age 4 and trawl surveys during the spawning period indicate a maximum age of around 10.
Spawning occurs in the central and southern North Sea as well as the Eastern English Channel generally between December and March
usually at depths ranging from 20 to 40 m. Juveniles in the first two years of life mainly inhabit nursery grounds in estuaries and inshore
coastal areas in the southern North Sea before migrating to deeper offshore waters. An offshore shift in the spatial distribution of juvenile
plaice has been observed in recent years most likely resulting from higher water temperatures, and juveniles are becoming mature at
younger ages and smaller sizes than in the past which may be linked due to fishing activities selectively removing larger individuals and
preventing reproduction. Stock size has increased and mortality rates have decreased over the last decade. Plaice feed on worms,
molluscs and small crustaceans mainly at night; and they are preyed on by other fish, marine mammals and sea birds.
Landings of plaice in Subarea IV (North Sea) averaged around 100 000 tonnes per year between 1957 and 2014, and discarding of mostly
undersized fish has ranged from 30 to 40% of total international catch weight over the last three years. Plaice are mainly taken by beam
trawlers, demersal otter trawlers and demersal seine netters targeting mixed species in the central and southern North Sea. A substantial
proportion of plaice caught during the first quarter of the year in the North Sea (50%) consist of spawning migrants from Division VIId
(Eastern English Channel). The status of the plaice stock in Subarea IV has improved over the last decade, with spawning stock biomass
increasing since 2007 to a record high and fishing mortality decreasing since 2003 to a record low (ICES, 2015).
References
ICES. 2015. Plaice (Pleuronectes platessa) in Subarea IV (North Sea) and Division IIIa (Skagerrak) (Update). ICES Advice on fishing
opportunities, catch, and effort. Greater North Sea and Celtic Seas Ecoregions. ICES Advice 2015, Book 6, Section 6.3.31.
Stock Status
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The status of plaice in Subarea IV (North Sea) has been scored a very low risk. This is because spawning stock biomass is at a record
high and fishing mortality is at a record low, below the MSY reference point.
Management
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The management of plaice in Subarea IV (North Sea) has been scored a very low risk. This is because management decisions are
informed by an annual stock assessment, there is a management plan in place, catches of North Sea plaice have been below the advised
catch over the last decade, and there is a comprehensive regulatory framework in place.
Bycatch
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The bycatch risk in this fishery has been scored a high risk. This is because otter trawls have the potential to take relatively high
quantities of bycatch (> 40% of catch weight). However, the cod recovery plan has been implemented in this area and this is expected to
have had an influence on the discarding levels in North Sea demersal fisheries and the incoming EU landings obligation is also intended to
reduce discarding.
There are also catches of demersal elasmobranchs and protected, endangered and threatened (e.g. sharks and rays) species in certain
circumstances.
Habitat
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The habitat risk of this fishery has been scored a high-moderate risk. This is because, although otter trawls are considered to have a
potential to cause significant habitat damage, damage to vulnerable and sensitive marine habitats is likely to be minimised given that the
footprint of the fishery is within core areas, typically historically fished ground.
Spatial management to reduce potential interactions with vulnerable habitats are being developed, as there remains uncertainties about
the location of some sensitive seabed habitats so these remain at risk.
Outlook
Type
Current Risk Status
Outlook
Reason
Stock
Very low
Stable
The status of the stock is likely to continue to improve in the future given
the recent increase in spawning stock biomass to a record high and the
decrease in fishing mortality to a record low. Catches have been below
the recommended TAC over the last decade and are in in-line with the
MSY approach.
Management
Very low
Stable
The CFP is going through reform and there is some uncertainty on how
this will impact fisheries management in the North Sea, particularly due to
the introduction of a landings obligation. However, management is likely
to remain stable.
Bycatch
High
Improving
Bycatch of non-target species in this fishery is relatively high with poor
selectivity. However, with technical and spatial management measures
continuously under development and the incoming EU landings
obligation intended to reduce discarding of managed species, the bycatch
risk is likely to reduce in the future.
Habitat
Moderate
Improving
Otter trawls have the potential to affect seabed habitats, but spatial
management measures are continuously being developed and will likely
reduce the risk. As planned networks of Marine Protected Areas become
established, larger areas of sensitive habitat will become protected from
trawling.
Stock Status Details
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Time-trends
Landings of plaice in Subarea IV (North Sea) gradually increased to a peak around 171 000 tonnes in the late 1980s, followed by a marked
decline to 72 500 tonnes in the late 1990s and then steadily rose from 2008 onwards (Figure 1). The current age distribution of North Sea
plaice landings is similar to that of the 1960s due to a substantial reduction in exploitation rates. Plaice are mainly taken in a targeted
beam trawl fishery in the central North Sea and a mixed flatfish beam trawl fishery for sole (Solea solea) in the southern North Sea. A large
number of undersized plaice are discarded in the mixed flatfish beam trawl fishery targeting sole in the southern North Sea due to a
mismatch between the minimum mesh size of 80 mm and the minimum landings size of 27 cm for plaice. Absolute discard levels for all
gears peaked in the late-1980s followed by a gradual reduction since the early-1990s. Discard levels were relatively high in 1997 and 1998
as well as in 2001 and 2003 due to strong incoming year classes increasing catches of undersized plaice. A demersal otter trawl fishery
and a seine net fishery for mixed species also catches North Sea plaice (ICES, 2015ab).
North Sea plaice was harvested well above the fishing mortality giving Maximum Sustainable Yield (FMSY) throughout the majority of the
time-series (ICES, 2015a). Fishing mortality gradually increased until late-1990s and then rapidly decreased thereafter, falling below FMSY
from 2013. The rapid reduction in fishing mortality during the late-1990s resulted from a decrease in the capacity of the North Sea Beam
trawl fleet and the implementation of TAC reductions as outlined in the EU multi-annual management plan. Trawl fleets that have
traditionally taken North Sea plaice have decreased in size over the last twenty years, with substantial reductions in the capacity of the
beam trawl fleet since 1995 including the decommissioning of 25 vessels in 2008. Reductions in the size of beam trawl fleet, however, may
have been partially compensated by increases in the technical efficiency of the vessels. In the Dutch beam trawl fleet, for example, an
increase in technical efficiency of approximately 1.65% was reported between 1990 and 2004 (Rijnsdorp et al., 2006).
Fishing patterns have changed over the last decade, with Dutch beam trawl vessels increasingly using more innovative gears such as
electronic “pulse trawls”. At the end of 2014, there were 42 derogation licenses for flatfish vessels to use pulse trawls. Debate is ongoing in
the EU about possible amendments to the regulations to permanently legalise the use of pulse trawls for the entire fleet. Fishing effort for
the beam trawl fleet has also shifted towards the southern North Sea to target sole over the past decade due to an increase in the
profitability of the sole fishery.
Stock size has increased substantially since 2007, with the most recent estimate of spawning stock biomass the highest on record. This
increase in spawning stock biomass has been driven by a combination of a decrease in fishing mortality since the early-2000s and an
increase in the abundance of prey species (ICES 2015b).
An offshore shift in the spatial distribution of juvenile North Sea plaice occurred during the 1990s most likely due to higher water
temperatures exceeding the maximum tolerance range or increased food requirements above the available food resources (Van Keeken et
al., 2007). Juveniles have also become mature at younger ages and smaller sizes than in the past due to fishing activities selectively
removing larger individuals and preventing reproduction (Grift et al., 2003; 2007). The overall spatial distribution of North Sea plaice stock
has shifted northwards into deeper offshore waters due to climate change increasing sea surface temperatures (Engelhard et al., 2011),
which has been accompanied by commensurate changes in the spatial distribution of the North Sea plaice fishery (ICES, 2015b).
Figure 1. Plaice in Subarea IV and Division IIIa combined. Summary of stock assessment (weights in thousand tonnes) (ICES, 2016).
Stock structure and recruitment
For assessment and management purposes, the plaice population inhabiting Subarea IV (North Sea) is considered to be a single stock.
Tagging information indicates that there is a degree of exchange of individuals (i.e. connectivity) among Subarea IV (North Sea), Division
IIIa (Skagerrak and Kattegat) and Division VIId (eastern English Channel). Population mixing occurs between western Skagerrak and the
North Sea (Ulrich et al., 2013), and a large number of plaice undertake seasonal migrations from the North Sea into the Eastern English
Channel to spawn during winter (Hunter el al., 2003). As a consequence, ICES includes 50% of the mature component of the quarter 1
catches from the Eastern English Channel in the North Sea plaice stock assessment (ICES, 2015a).
Recruitment-at-age 1 has been variable throughout the time-series, averaging in the range of 0.4 – 4.7 million recruits between 1957 and
2014. Above average year classes were evident in 1964, 1984 and 1997. More recently, recruitment has been around the long-term
average of 1 million recruits since the mid-2000s.
Data gaps and research priorities
More information on the degree of temporal and spatial mixing in the plaice population inhabiting the North Sea is required. Tagging and
genetic studies focusing on stock structure and the migration patterns of plaice among Division IIIa (Skagerrak and Kattegat), Division VIId
(Eastern English Channel) and Subarea IV (North Sea) should be undertaken to better understand metapopulation dynamics between the
North Sea and Skagerrak as well as to improve the determination of stock units. An offshore expansion in the spatial distribution of North
Sea plaice has resulted in the coverage of the standardised research surveys no long matching the current population distribution.
Consequently, the coverage of the standardised research surveys should be expanded to correspond with the current population
distribution of North Sea plaice (ICES, 2015b).
Concern has been expressed about the impacts of electronic pulse gears on the North Sea plaice population. Nevertheless, exactly how
the legalisation of the use of pulse gears across the entire fleet will impact the North Sea plaice population remains unclear. ICES has
recommended that further studies examining the catch composition of pulse gears compared to traditional beam trawls should be
undertaken. Improved estimates of catch-at-age, length-at-age and weight-at-age divided into discard and retained catch components
should be provided to enhance the accuracy of the stock assessment and the ICES management forecast. Further information on
discarding of plaice in Subarea IV should be collected to improve estimates of total mortality. Quantitative estimates of discarding from
additional sampling programmes should be provided for inclusion in the assessment. Developing a time-series of available discard
information raised to the fleet level would help guide management advice. Research into suitable stock assessment methods with updated
natural mortality, survey catchability and exploitation estimates should be ongoing (ICES, 2015b).
References
Engelhard, G.H., Pinnegar, J.K., Kell, L.T, and Rijnsdorp, A.D. 2011. Nine decades of North Sea sole and plaice distribution. ICES Journal of
Marine Science 68: 1090–1104.
Grift, R.E., Rijnsdorp, A.D., Barot, S., Heino, M., and Dieckmann, U. 2003. Fisheries-induced trends in reaction norms for maturation in
North Sea plaice. Marine Ecology Progress Series 257: 247-257.
Grift, R.E., Heino, M., Rijnsdorp, A.D., Kraak, S.B.M., and Dieckmann, U. 2007. Three-dimensional maturation reaction norms for North Sea
plaice. Marine Ecology Progress Series 334: 213-224.
Hunter, E., Metcalfe, J.D., and Reynolds, J.D. 2003. Migration route and spawning area fidelity by North Sea plaice. Proceedings of the Royal
Society Series B – Biological Sciences, 270: 2097–2103.
ICES. 2015a. Plaice (Pleuronectes platessa) in Subarea IV (North Sea) and Division IIIa (Skagerrak) (Update). ICES Advice on fishing
opportunities, catch, and effort. Greater North Sea and Celtic Seas Ecoregions. ICES Advice 2015, Book 6, Section 6.3.31.
ICES. 2015b. Report of the Working Group on the Assessment of Demersal Stocks in the North Sea and Skagerrak (WGNSSK), 28 April-7
May, ICES HQ, Copenhagen, Denmark. ICES CM 2015/ACOM:13. 1182 pp.
ICES. 2016. Plaice (Pleuronectes platessa) in Subarea IV (North Sea) and Division IIIa (Skagerrak) (Update). ICES Advice on fishing
opportunities, catch, and effort. Greater North Sea and Celtic Seas Ecoregions. ICES Advice 2015, Book 6, Section 6.3.36.
Rijnsdorp, A.D., Daan, N., and Dekker, W. (2006). Partial fishing mortality per fishing trip: a useful indicator of effective fishing effort in
mixed demersal fisheries. ICES Journal of Marine Science 63: 556-566.
Ulrich, C., Boje, J., Cardinale, M., Gatti, P., LeBras, Q., Andersen, M., Hemmer-Hansen, J., Hintzen, N.T., Jacobsen, J.B., Jonsson, P., Miller,
D.C.M., Nielsen, E.E., Rijnsdorp, A.D., Sköld, M., Svedäng, H., Wennhage, H. 2013. Variability and connectivity of plaice populations from the
Eastern North Sea to the Western Baltic Sea, and implications for assessment and management. Journal of Sea Research 84: 40–48.
Van Keeken, O.A., Van Hoppe, M., Grift, R.E., and Rijnsdorp, A.D. 2007. Changes in the spatial distribution of North Sea plaice (Pleuronectes
platessa) and implications for fisheries management. Journal of Sea Research 57: 187-197.
Management Details
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TAC Information
Catch 2015 (t)
134875
Advised Catch 2016 (t)
216,345
Agreed TAC 2016 (t)
131714
Advised Catch 2017 (t)
158,201
Advised and agreed catches
The 2015 catch for the North Sea and Skagerrak was estimated to be 134 875 tonnes, comprising 84 767 tonnes landings and 50 108
tonnes discards. The main gear was beam trawl which accounted for 65% of the landings. Trawls accounted for 30% with the remaining
5% taken by other gears.
Management of this stock is by combined Total Allowable Catch (TAC) for Divisions IIa, IIIa (excluding Skagerrak and Kattegat) and Subarea
IV. The combined TAC for 2016 was set at 131 714 tonnes. ICES advises under the MSY approach that catches in 2017 should be no more
than 158 201 tonnes in Subarea IV and Division IIIa (Skagerrak part) combined (ICES 2016).
The long-term management plan for North Sea plaice and sole, which was evaluated by ICES to be in accordance with the precautionary
approach, is not used by ICES in 2016 as the basis for the advice for plaice. The European Commission has informed ICES that agreement
has not been reached between the EU and Norway on a method to split the joint advice between the North Sea and Skagerrak. Therefore,
advice is provided based on the MSY approach (ICES 2016).
Stock Harvest strategy
Plaice in Subarea IV (North Sea) is assessed annually to estimate biomass and fishing mortality in relation to reference points for long-term
sustainability. The stock assessment is based on a time-series of international fishery catches including age and size compositions and
indices of abundance derived from the catch rates of three standardised research trawl surveys. Landings are monitored exhaustively
from EU log books and sales notes; and discard data from Denmark, the United Kingdom, Germany, Belgium and the Netherlands has
been included in the assessment since 2004 (ICES, 2015a).
A degree of uncertainty exists in the landings statistics for plaice in Subarea IV (North Sea) given the difficulties in separating catches of
spawning adults into distinct stocks by division or Subarea. Tagging studies indicate that 50% of North Sea plaice catches during the first
quarter of the year are comprised of spawning migrants originating from Division VIId. ICES, therefore, includes 50% of the mature
component of catches during the first quarter of the year from Division VIId in the North Sea plaice assessment to estimate resident
spawning stock biomass based on migration estimates from tagging studies (ICES, 2015b).
The fisheries for plaice in Subarea IV (North Sea) are managed under the EU Common Fisheries Policy (CFP) primarily through annual TACs
set to achieve FMSY targets (Council Regulation (EC) 1380/2013). Landings of North Sea plaice have been below the advised TAC set by the
European Commission since 2004. The current ICES assessment indicates that controls have been effective in increasing stock size and
decreasing exploitation rates to the value giving Maximum Sustainable Yield, with spawning stock biomass currently at a record high and
fishing mortality at a record low. Spawning stock biomass has remained above MSY Btrigger since 2007 and fishing mortality has been
below FMSY since 2013 (ICES, 2015a).
An EU multi-annual management plan for plaice and sole in Subarea IV (North Sea) was introduced in 2007 (Council Regulation (EC) No.
676/2007). The EU multi-annual management plan for these stocks has two distinct phases: (1) a recovery phase with the objective of
bringing the stocks within safe biological limits; and (2) a management phase with the objective of adjusting fishing effort (kW-days per
fleet) in relation to exploitation rates that give Maximum Sustainable Yield. ICES has evaluated the EU multi-annual management plan and
consider it to be consistent with the precautionary approach and the principle of Maximum Sustainable Yield (ICES, 2010; Miller and Poos,
2010; ICES, 2012). North Sea plaice has been within safe biological limits over the last three years (ICES, 2015a), and therefore the stock is
presently in the second stage of a multi-annual EU management plan and should be managed on the basis of Maximum Sustainable Yield
(ICES, 2015b).
In 2016, this management plan was not used as the basis for providing 2017 advice (now MSY) due to no agreement being reached
between the EU and Norway on how the TAC should be allocated.
Direct control of fishing effort for fleets catching plaice in Subarea VIId was introduced in 2008 (Council Regulation (EC) 1342/2008) and
requires Member States to allocate effort in kW-days to different groups of vessels, depending on gear, mesh size and area. The current
EU multi-annual management plan for plaice and sole in Division IV (Council Regulation (EC) No. 676/2007) also specifically alters fishing
effort for the beam trawl and trammel net fleets in relation to the fishing mortality exerted on plaice and sole each year. Plaice are mainly
taken in a targeted beam trawl fishery in the central North Sea and a mixed flatfish beam trawl fishery for sole in the southern North Sea.
A demersal otter trawl fishery and a demersal seine net fishery for mixed species also catches North Sea plaice (ICES, 2015b).
An area partially closed to fishing was established in 1989 to reduce discarding of undersized plaice in the major nursery areas and
enhance recruitment to the fishery. The area closed was named the “plaice box” and was expanded in phases until the entire area
covering ~40 000 km2 along the continental shelf was closed to fishing throughout the year in 1995. The plaice box prohibits most
categories of large beam trawlers over 300 horse power from fishing in the area. Although fishing effort from beam trawlers has
decreased by over 90% since the establishment of the plaice box, this closed area has not protected and rebuilt the North Sea plaice stock
as anticipated with landings and biomass declining since the mid-1990s (Pastoors et al., 2000; Beare et al., 2013).
Spatial management measures have resulted in a shift towards smaller 300 horse power vessels and larger 2000 horse power vessels. The
smaller 300 horse power vessels are permitted to fish within 12 nautical mile zone and in the plaice box. In contrast, larger vessels with
engines over 300 horse power have concentrated their fishing effort outside the 12 nautical mile zone given that they have been
engines over 300 horse power have concentrated their fishing effort outside the 12 nautical mile zone given that they have been
prohibited from fishing in the plaice box since 1995. The combination of days-at-sea regulations, the constrained TAC for plaice and the
relatively stable TAC for sole has increased fishing effort in the southern North Sea where sole are more abundant. High oil prices may
have contributed to this southerly shift in fishing effort given that sole is economically more valuable than plaice. The southerly shift in
fishing effort has resulted in high discarding of juvenile plaice that are mainly distributed in this area (ICES, 2015b).
Annual landings closely follow the set TACs and absolute discard rates across the entire fleet have gradually declined since 2001. Discard
rates of undersized plaice in the beam trawl fishery targeting sole in the southern North Sea, however, remain high due to a mismatch
between the minimum mesh size of 80 mm and the minimum landings size of 27 cm for plaice. The combined TAC for 2015 was set at 128
376 tonnes. ICES advises that when the second stage of the EU management plan (Council Regulation No. 676/2007) is applied, catches in
2016 should be no more than 216 345 tonnes in Subarea IV and Division IIIa (Skagerrak part) combined. If discard rates do not change
from the average (2012–2014), this implies landings of no more than 159 197 tonnes (ICES, 2015a).
A minimum landings size of 27 cm is in operation in the North Sea (Council Regulation 850/1998), and discarding has historically mainly
comprised of fish below the minimum landings size. Plaice are mainly taken as bycatch in the small mesh (80 mm) beam trawl fishery for
sole in the southern North Sea. Discard rates in the large mesh (100 to 120 mm, depending on area) beam trawl fishery for mixed species
in the central North Sea are relatively low. In 2014, 40% of the total international catch in weight is estimated to have been discarded.
Management measures applied to the small meshed beam trawl fishery affect both North Sea plaice and sole (ICES, 2015b).
Surveillance and enforcement
Fisheries on plaice in Subarea IV (North Sea) are carried out by eight countries (Belgium, Denmark, France, Germany, the Netherlands,
Norway, Sweden and the United Kingdom) and surveillance activities to record compliance with national and international fishery control
measures are primarily the responsibility of the competent fishery inspection authorities in each country. In addition, the European
Fisheries Control Agency (EFCA), established in 2005), organises operational coordination of fisheries control and inspection activities by
the Member States as well as cooperation with third countries and other Regional Fishery Management Organisations.
The requirements for surveillance and sanctions for infringements are laid down in the EU Control Regulation (EC) No 1224/2009.
Surveillance activities on fisheries for plaice in Subarea IV include the use of vessel monitoring systems (VMS) on board vessels over 12 m
overall length, direct observation by patrol vessels and aerial patrols, inspections of vessels, gear, catches at sea and on shore, verification
of EU logbook data against sales documents. The EU Control Regulation specifies that Member States should set up electronic databases
containing the inspection and surveillance reports of their officials as well as records of infringements.
Management controls are routinely enforced and independently verified through surveillance of fishing activities (e.g. VMS, logbooks,
dockside monitoring and visual inspections). Infringements happen only very occasionally and are unlikely to compromise harvest
objectives.
References
Beare, D., Rijnsdorp, A., Blaesbjerg, M., Damm, U., Egekvist, J., Fock, H., Kloppmann, M., Röckmann, C., Schroeder, A., Schulze, T., Tulp, I.,
Ulrich, C., van Hal, R., van Kooten, T. and Verweij, M. 2013. Evaluating the effect of fishery closures: Lessons learnt from the Plaice Box.
Journal of Sea Research 84: 49–60.
EFCA. [http://www.efca.europa.eu/] [Date accessed: 23-Dec-15]
ICES 2010. Request from the Netherlands on the evaluation of the long-term management plan for sole and plaice in the North Sea (part
2). In: Report of the ICES Advisory Committee, 2010. ICES Advice 2010, Book 6, Section 6.3.3.4.
ICES. 2012. Request from the Netherlands on the North Sea flatfish Management Plan. In: Report of the ICES Advisory Committee, 2012.
ICES Advice 2012, Book 6, Section 6.3.3.4.
ICES 2015a. Plaice (Pleuronectes platessa) in Subarea IV (North Sea) and Division IIIa (Skagerrak) (Update). ICES Advice on fishing
opportunities, catch, and effort. Greater North Sea and Celtic Seas Ecoregions. ICES Advice 2015, Book 6, Section 6.3.31.
ICES. 2015b. Report of the Working Group on the Assessment of Demersal Stocks in the North Sea and Skagerrak (WGNSSK), 28 April-7
May, ICES HQ, Copenhagen, Denmark. ICES CM 2015/ACOM:13. 1182 pp.
ICES. 2016. Plaice (Pleuronectes platessa) in Subarea IV (North Sea) and Division IIIa (Skagerrak) (Update). ICES Advice on fishing
opportunities, catch, and effort. Greater North Sea and Celtic Seas Ecoregions. ICES Advice 2015, Book 6, Section 6.3.36.
Miller, D.C.M. and Poos, J.J. 2010. Combined ex post and ex ante evaluation of the long-term management plan for sole and plaice in the
North Sea, including responses to ICES review. ICES Document CM 2010/ACOM: 62, 109 pp.
Pastoors, M.A., Rijnsdorp, A.D. and van Beek, F.A. 2000. Effects of a partially closed area in the North Sea (“plaice box”) on stock
development of plaice. ICES Journal of Marine Science 57: 1014-1022.
Bycatch Details
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Demersal otter trawls operating in the North Sea (Sub-area IV) and eastern English Channel (Division VIId) can be towed by a single boat as
a single or multiple rig. The trawl doors create sand clouds that herd the fish into the net. Otter trawls can be rigged with different types
of ground gear depending on seabed topography and the species targeted (Løkkeborg, 2005).
Demersal otter trawling is not a well targeted fishing activity given that a wide variety of non-target species can be caught. This fishery
catches a wide variety of mixed demersal finfish, such as sole (solea solea), whiting (Merlangius merlangus), and plaice (pleuronectus
platesa) and these mixed catches means that the minimum cod end mesh size of 80 mm and other aspects of the management regimes
and markets are not optimal for all the species caught. Fish may be discarded because they are smaller than the Minimum Conservation
Reference Size, or the size and/or species are not marketable. Discarding due to the vessel being short of quota for the managed species
also occurs.
Evidence of bycatch risk
Discard rates have been estimated from surveys at around 30-40% of total catch weight in European demersal otter trawl fisheries
(European Commission, 2011). According to the Discard Atlas for North Sea (Anon, 2014a) the main managed species discarded (2010 2012) in the North Sea are species such as: dab (97%), plaice (77%), haddock (59%), cod (56%), and whiting (51%) by weight. There are
high rates of discards for individual species, the total proportion of discards for the Greater North Sea in this fishery is around 41%.
In addition, common skate (Dipturus batis) and spurdog (Squalus acanthias) can be taken as bycatch. Currently landings of these species
is prohibited and fishermen are required to return them to the sea where they have a chance of survival.
However, catch composition and discard rates vary according to regional variations in species composition, the design of the trawl, the
cod-end mesh size and other selectivity devices used (see mitigation measures).
Mitigation measures
A wide variety of non-target species are caught in European mixed otter trawl fisheries. Optimising gear selectivity in mixed fisheries is
challenging given that different species have different selectivity requirements.
Many measures have been designed and tested over recent years, designed to increase selectivity in otter trawls. To actually be effective
in controlling bycatch levels in fisheries, these measures have to be operationally viable, enforceable and used within an incentive scheme
which encourages fishers to use them.
The cod recovery plan (EU 1342/2008) which is in force in North Sea, Eastern English Channel, Irish Sea, West of Scotland and the
Skagerrak, has encouraged a diversity of measures to incentivise the use of more selective gear by vessels targeting whitefish with
demersal otter trawls. Effort per vessel is limited to a number of days at sea per year. Additional days at sea are available for vessels using
certain specified gears and measures, tested for their efficacy at improving selectivity.
The different European Nations have implemented different measures in their fisheries, described below are measures implemented in
the UK fisheries.
Fully documented fisheries scheme
The ‘fully documented fisheries scheme’, which participating vessels (representing 27% of landings in 2012) use independent electronic
monitoring devices (video recordings which monitor fishing activities) to ensure that all caught fish are recorded, and all caught cod are
retained onboard, landed and count against quota (including undersized fish). In return, these vessels are allowed additional quota and
days at sea, though they have to stop fishing when they use up their quota. By this means the total mortality on the cod stock is capped,
wasteful discarding is reduced, and improved information on catches is available. Moreover, there is more flexibility for fishermen to use
the additional days at sea up to the point when they run out of quota, and there is an incentive to use more selective gear in order to
target more marketable fish.
Since cod grow to a relatively large size compared with other species before they are marketable, the scheme benefits the selectivity of
fishing on other fish in the catch thereby helping to reduce the overall discards. So far the scheme has not been in force long enough to
fully evaluate the results, but it an illustration of the way in which measures to reduce discards can be incorporated into the overall
management of the fishery thus increasing efficacy and incentives.
Real time closures
The cod avoidance plans also include ‘real time’ closures of areas where concentrations of cod are found. Since 2007 UK administrations
have operated these areas for UK vessels in the North Sea and English Channel in response to concentrations of juvenile cod; when a
vessel catches more than threshold number of cod per hour agreed by a boarding officer and the skipper or concentrations of adult cod
through analysis of landings and Vessel tracking data (VMS). The size and duration of the closure varies according to administrative body.
This a similar scheme to designed operated under European legislation (EU 783/2011) which is designed to protect juvenile cod, saithe,
haddock and whiting and there are also seasonal closures to protect spawning stocks (See Marine Scotland and Marine Management
organisation websites).
It has proved difficult to quantify the efficacy of each of these measures, however ‘it seems likely that cod avoidance measures have
contributed significantly to the reduction in the discard rate of cod’ in the North Sea fishery (Anon, 2014).
Landings obligation
The introduction of the landings obligation or ‘discard ban’ under the EU Common fisheries policy (EU 1380/2013) is intended to take place
over the period 2016 – 2019 in this fishery. This landings obligation will ultimately apply to all species managed by TAC; it will not apply to
non-TAC species, however many of these are likely to benefit from improved selectivity.
References
Anon. 2014. Prepared by Scheveningen group, Discard Atlas of the North Sea Fisheries, August 2014, 80 pp.
Løkkeborg, S. (2005). Impacts of trawling and scallop dredging on benthic habitats and communities. FAO Fisheries Technical Paper 472.
Food and Agriculture Organisation of the United Nations, Rome, 58 pp.
Habitat Details
less risk
more risk
Gear effects, targeting and behaviour
Fishermen use their knowledge of seasonal fish aggregations and seabed types together with information from the vessel’s electronic
mapping tools to make informed decisions on where to trawl. Gears are adapted to the substrate type and the species targeted, with a
relatively narrow range of conditions in which they can operate. Most otter trawling occurs within core areas where yields are high and it
is safe to trawl, typically historically fished grounds (Jennings and Lee, 2011).
Risk of habitat impact
Otter trawl impact risk on the seabed habitat can include modification of bottom topography and disturbing biogenic features. Biological
communities can potentially be disturbed both directly and indirectly by changes in the physical attributes of the areas being fished. Trawl
doors have the most pronounced impacts on seabed habitats by creating scouring marks and furrows up to 20 cm deep (Løkkeborg,
2005).
The habitat risks are related to the types of seabed communities and other sources of seabed disturbance such as wave and tidal action. A
number of theoretical and field studies have focused on the effects of towed gears, which show that areas outside core fished areas tend
to be more sensitive to fishing (Grey et al., 2006; Jennings et al. 2012). Consequently, habitats that have not traditionally been disturbed by
fishing activities are relatively more sensitive to the effects of otter trawling. However, TR2 gears predominate in shelf waters for plaice,
sole and dab which occupy habitats subject to relatively high levels of natural disturbance (due wave and tidal action). The impacts of TR2
gears on habitats is therefore less significant than the potential impacts of TR1 gears which operate in deep water muddy habitat
environments.
Understanding the nature of these differences is important in the management of the effects of otter trawling. Communities that inhabit
areas where there is more disturbance by wave and tidal action are less likely to be affected by trawling, whereas communities inhabiting
deeper waters unaffected by disturbance from wave and tidal action or on harder more gravely substrate are relatively more sensitive to
trawling (Bolam et al., 2014). However, there are some habitats such as ross worm (or sabellaria) which inhabit shallower areas which are
considered relatively sensitive, but such areas are traditionally avoided by TR2 fisheries.
In the North Sea, demersal otter trawlers have reduced the biomass and production of bottom-dwelling organisms (Hinz et al., 2008;
Hiddink et al., 2006). Sustained fishing has resulted in a shift from communities dominated by relatively sessile, emergent and high
biomass species to communities dominated by infaunal, smaller bodied organisms (Kaiser et al., 2000).
Within the core area of this fishery where the substrate is mostly sandy or muddy sand the risks of further ecosystem change due to
fishing are minimised because the area has been continually heavily fished so the sea bed ecosystem has adapted to the fishing pressure.
However, outside the core area of fishing there is the potential for significant habitat impact..
The fishery generally targets relatively shallow areas which are expected to be subject to wave and tidal action and hence relatively
resilient to the effects of fishing. Fishing which has occurred in a specific location over many years is likely to result in the seabed
ecosystem adapting to the fishing activity. However, there are sensitive areas, mostly outside the core areas of the fishery which may be
vulnerable and such areas have in recent years been identified and protected to some extent through the Natura 2000 network of MPAs
Mitigation measures
Under the Marine Strategy Framework Directive (MSFD) from the European Union (Council Directive 56/2008), Member States have
committed to aim towards ‘good environmental status’ (GES) for the seabed habitats by 2020. The Convention for the Protection of the
Marine Environment of the North-East Atlantic (the ‘OSPAR Convention’), which was signed up to by 15 nations plus the European Union, is
developing a coherent network of Marine Protected Areas to protect vulnerable marine habitats in the North-East Atlantic. The
development of offshore Special Areas of Conservation under the European Habitats Directive (Council Directive 43/1992) contributes to
this process as does the UK Marine Act and Marine Scotland Act designating Marine Protected Areas in UK waters.
These initiatives have resulted in improvements in habitat mapping and risk assessment of the effects of trawling on the seabed, and the
UK Marine Management Organisation (MMO) and Marine Scotland are engaging in a programme designed to assess the effects of
fisheries and implement management measures where sites are considered at risk.
References
Bolam, S.G., Coggan, R.C., Eggleton, J., Diesing, M. and Stephens, D. 2014. Sensitivity of macrobenthic secondary production to trawling in
the English sector of the Greater North Sea: A biological trait approach. Journal of Sea Research 85: 162–177.
Gray, J.S., Dayton, P., Thrush, S. and Kaiser, M.J. 2006. On effects of trawling, benthos and sampling design. Marine Pollution Bulletin 52:
840–843.
Hiddink, J.G., Jennings, S., Kaiser, M.J., Queirós, A.M., Duplisea, D.E, and Piet, G.J. 2006. Cumulative impacts of seabed trawl disturbance on
benthic biomass, production, and species richness in different habitats. Canadian Journal of Fisheries and Aquatic Sciences 63: 721–736.
Hinz, H., Hiddink, J.G., Forde, J., and Kaiser, M.J. 2008. Large-scale responses of nematode communities to chronic otter-trawl disturbance.
Canadian Journal of Fisheries and Aquatic Sciences 65: 723–732.
Jennings, S. and Lee, J. 2011. Defining fishing grounds with vessel monitoring system data. ICES Journal of Marine Science 69: 51–63.
Jennings, S., Lee, J and Hiddink, J.G. 2012. Assessing fishery footprints and the trade-offs between landings value, habitat sensitivity and
fishing impacts to inform marine spatial planning and the ecosystem approach. ICES Journal of Marine Science 69: 1053-1063.
Kaiser, M.J., Ramsay, K., Richardson, C.A., Spence, F.E., and Brand, A.R. 2000. Chronic fishing disturbance has changed shelf sea benthic
community structure. Journal of Animal Ecology 69: 494–503.
Løkkeborg, S. (2005). Impacts of trawling and scallop dredging on benthic habitats and communities. FAO Fisheries Technical Paper 472.
Food and Agriculture Organisation of the United Nations, Rome, 58 pp.
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