Download Witch sole, Northern stock, demersal trawl

Witch sole, Northern stock, demersal trawl
Witch sole, Northern stock, demersal trawl
Content last updated
16th Feb 2017
Stock:
Management:
Overview
Witch sole or witch flounder (Glyptocephalus cynoglossus), is a right-eyed flounder of the family Pleuronectidae which occurs on both
sides of the Atlantic Ocean. The species is mainly found on soft bottoms, mostly clay or clean sandy bottoms between 100–400 m depth.
The main diet consists of crustaceans, worms, brittle stars and fishes.
Witch sole is typically a bycatch species, mostly captured in bottom otter trawls (88%) targeting whitefish (such as cod and haddock), or
other flat fish species such as plaice, etc. It is also an important bycatch species in some Nephrop fisheries.
Stock Status
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The status of the witch flounder stock in Subarea IV and Divisions IIIa and VIId (North Sea, Skagerrak and Kattegat, Eastern
English Channel) has been scored a moderate risk. This is because the species has low resilience to fishing exploitation (FishBase, 2015)
and the stock abundance has shown a marked increase in the last couple of years. No biomass or fishing mortality reference points have
been defined for this data-limited stock, and therefore cannot be used to derive a risk score. Instead, a risk score was calculated using a
data-limited approach where the low resilience of lemon sole to fishing exploitation was weighted by an increasing population trend.
Management
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The management of witch flounder in Subarea IV (North Sea) and Divisions IIIa (Skagerrak–Kattegat) and VIId (Eastern Channel)
has been scored a moderate risk. This is because data-limited approaches are used for setting management controls. No specific
management controls have been established to restrict harvesting, and no stock-specific TAC has been defined by the European
Commission. A TAC combining lemon sole and witch sole may not be an appropriate management measure to control the harvest rates of
bycatch species.
Bycatch
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The bycatch risk of witch flounder in Subarea IV (North Sea) and Divisions IIIa (Skagerrak–Kattegat) and VIId (Eastern Channel)
has been scored a high risk. This is because demersal fisheries operating in these areas discard on average between 10 and 40% of the
catch in weight. Demersal trawls are an active form of fishing gear that have the potential to take relatively high quantities of bycatch in
certain circumstances.
Habitat
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The habitat risk of this fishery has been scored a high-moderate risk. Although otter trawls are considered to have 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, of historically fished ground. However, 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
Moderate
Stable
The status of the stock is likely to remain stable in the future given that no
major changes in stock status are anticipated.
Management
Moderate
Stable
The management of the stock is likely to remain stable in the future. 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.
Bycatch
High
Improving
Bycatch in this fishery is relatively high. 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
High
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 witch sole increased from the mid 1960’s reaching an historical high in the early 2000’s. Landings declined thereafter, with
evidence of an increase in the last 3 years. The abundance indices from the fishery independent International Bottom Trawl Survey (IBTS)
survey show a declining abundance trend since the peak observed in 2000, followed by an increase in the most recent years both in
landings and survey data (ICES, 2015).
Fig. 1. Witch sole in Subarea IV and Division IIIa. Official landings (in thousand tonnes). Discards data have only been included since 2012.
Biomass indices (kg.hr-1) from the IBTS survey in quarter 1 and quarter 3 estimated from the mature stock (all individuals larger than 34
cm.)
Stock structure and recruitment
Knowledge of witch sole’s stocks is based on old studies that identified two stocks in ICES divisions IIIa and IV, one in Kattegat and one in
the North Sea and Skagerrak. However, considering that catches in Kattegat are small and scattered and that there is no firm evidence of
spawning grounds in this area, witch sole is assessed by ICES as a single stock in Subarea IV, Division IIIa and VIId (ICES, 2013).
In 2009, witch sole was included as a mandatory species in the EU Data Collection Framework (DCF). Accordingly, Denmark and Sweden
started the regular sampling of biological data, i.e. length, weight, maturity status and age, in 3.a and 4 both in discards and landings.
Scotland has also been collecting biological samples since 2009 but only from the landings.
Data gaps and research priorities
In order to perform an analytical assessment, information on stock structure, biological data and catch at age information would be
needed. Survey indices are noisy because the survey is not designed for catching witch sole and catch per hour is too low for robust
estimates (ICES, 2015). While age readings techniques are now well established, the macroscopic evaluation of maturity status is still
uncertain. A project assessing gonad histology is under development and it is planned to be ready before the end of 2017 (ICES, 2016). A
reliable analytical assessment will be possible when long time-series data are available.
References
ICES. 2013. Report of the Working Group on Assessment of New MoU Species (WGNEW), 18 – 22 March 2013, ICES HQ, Copenhagen,
Denmark. ACOM.
ICES. 2015. Witch (Glyptocephalus cynoglossus) in Subarea IV and Divisions IIIa and VIId (North Sea, Skagerrak and Kattegat, Eastern
English Channel). ICES Advice Book 6.3.57
ICES. 2016. Report of the Working Group on the Assessment of Demersal Stocks in the North Sea and Skagerrak (WGNSSK), 26 April-5 May
2016, Hamburg, Germany. ICES CM 2016/ ACOM:14. 19 pp.
Management Details
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TAC Information
Catch 2014 (t)
Advised Catch 2015 (t)
3107
Agreed TAC 2015 (t)
Advised Catch 2016 (t)
3107
Advised and Agreed Catches
Management of witch sole is conducted using a combined Total Allowable Catch (TAC) with lemon sole (Microstomus kitt) set for Division
IIa and IV. The TAC for 2015 was set at 6391 tonnes. This combined species TAC might prevent effective control, as exploitation rates at
species level are masked by combined catch rates and could potentially lead to overexploitation of either species (ICES, 2016). Witch sole is
mainly a bycatch species in mixed fisheries and consequently, a TAC alone may not be appropriate as a management tool (ICES, 2016).
ICES advises that in Subarea 4 and Divisions IIIa, IV, and VIId, when the precautionary approach is applied, catches of witch sole should be
no more than 3107 tonnes for 2016 and 2017.
Stock Harvesting Strategy
Witch sole in Subarea IV (North Sea) and Divisions IIIa (Skagerrak–Kattegat) and VIId (Eastern Channel) is assessed biennially using official
landings and survey data to estimate temporal trends in mature biomass. Witch sole is classified as category 3 Data limited stock. This
category includes stocks for which survey indices (or other indicators of stock size) are available and provide indications of trends in stock
metrics. Landings are monitored from EU logbooks and sales notes.
Witch sole is managed under EU Common Fisheries Policy (CFP) through a precautionary Total Allowable Catch (TAC) together with lemon
sole (Microstomus kitt). A lack of spatial overlap exists between the ICES assessment area and the management area in the North Sea. The
assessment area covers a broader region (Subarea IV as well as Divisions IIIa and VIId) than the management area (Division IIa and
Subarea IV).
There is no official Minimum Landing Size (MLS) specified in EU waters. However, in most of the countries reporting catches the landing of
witch below 28 cm is prohibited. Recent studies have estimated the size at which 50% of the stock is mature as 34 cm in length. Therefore,
since 2015, the mature biomass indices are estimated based on specimens larger than 34 cm (ICES, 2015). In 2014, witch sole was
included into the data call for WGNSSK 2014 and in 2015 this call was extended to obtain landing and discard data for the years 2012–
2015 from all countries.
Surveillance and Enforcement
Fisheries for witch sole in Subarea IV (North Sea) and Divisions IIIa (Skagerrak–Kattegat) and VIId (Eastern Channel) are carried out by a
number of countries and surveillance activities to record compliance with national and international fishery control measures are
primarily the responsibility of the 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 that take witch sole include the use of vessel monitoring systems (VMS) on board vessels over 12 m
overall length; direct observation by patrol vessels and aerial patrols; inspection of vessels, gear, catches at sea and on shore, and
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.
References
ICES. 2015. 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. 1229 pp.
ICES. 2016. Report of the Working Group on the Assessment of Demersal Stocks in the North Sea and Skagerrak (WGNSSK), 26 April-5 May
2016, Hamburg, Germany. ICES CM 2016/ ACOM:14. 19 pp.
Bycatch Details
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Targeting and behaviour
Witch sole is typically a bycatch species, 88% of all witch sole is captured in bottom otter trawls targeting whitefish (such as cod and
haddock), or other flat fish species such as plaice etc. It is also an important bycatch species in some Nephrops fisheries. In the North Sea
the vast majority of witch sole landings in 2015 was reported by Denmark in Division IIIa using bottom otter bottom trawls targeting
crustaceans (mesh size 90-119mm, TR1 and TR2 combined) and by Scotland in Subarea IV using otter bottom trawls targeting demersal
fish (mesh size >=120 mm, TR1) (ICES, 2016). Demersal otter trawls operating in the North Sea and West of Scotland are 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. The functions of the ground gear are to ensure close
contact with the bottom and to enable fishing on rough bottoms without damage to the trawl net. When targeting flatfish, a chain may be
used as ground gear to chase the fish off the bottom. The trawl doors keep the trawl mouth spread open laterally and create the sand
clouds that herd fish into the opening of the net (Løkkeborg, 2005).
Larger mesh demersal otter trawling catches a variety of mixed demersal finfish, such at Atlantic cod (Gadus morhua), haddock
(Melanogrammus aeglefinus) and saithe (Pollachius virens). Because of the nature of the mixed fishery, management measures which suit
one stock may not be suitable for another captured in the same fishery.
Evidence of bycatch risk
Demersal fisheries operating in the North Sea discarded on average 40% of the catch in weight between 2010-2012. The average discard
ratio for the years 2010-2012 in the Skagerrak was 23% and for the Eastern Channel between 10 – 40% of catches were discarded during
the same time period (Quirijns et al, 2014).
In 2014, witch sole was included into the data call for WGNSSK 2014 and since 2015 the data call was extended to obtain landing and
discard data for the years 2012–2015. Estimated witch flounder-specific discard rates from 2012-2015 ranged between 10 and 30%.
However, it should be noted that not all metiers were sampled in every quarter (ICES, 2016).
Mitigation measures
A wide variety of non-target species are caught in mixed European 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 fisheries these measures have to be operationally viable,
enforceable and used within an incentive scheme which encourages fishermen to use them. This includes the introduction of 120 mm
minimum mesh cod end sizes in the northern North Sea (North of 56°N) from 2002, which has helped to reduce the levels of discarding
and increase selectivity in the fishery.
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 vessel. Additional days at sea are available for vessels
using certain specified gears and measures, tested for their efficacy at improving selectivity.
The ‘fully documented fisheries scheme’, in 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 counted 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. The total mortality on the cod stock is thus 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 other fish in the catch thereby helping to reduce the overall discards. The scheme has not been in force long enough to fully
evaluate the results, but it provides an illustration of the way in which measures to reduce discards can be incorporated into the overall
management of the fishery at increasing efficacy.
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 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
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
Quirijns FJ, Pastoors MA, Uhlmann SS, Verkempynck R. 2014. Discard Atlas of North Sea fisheries. IJmuiden: IMARES, prepared by the
Scheveningen group. 84 pp.
ICES. 2016. Report of the Working Group on the Assessment of Demersal Stocks in the North Sea and Skagerrak (WGNSSK), 26 April-5 May
2016, Hamburg, Germany. ICES CM 2016/ ACOM:14. 19 pp.
Habitat Details
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Gear effects, targeting and behaviours
Demersal otter trawls are designed to catch fish and shrimps that stay above the sea bed, from close to the bottom to several metres
from the bottom (Løkkeborg, 2005). 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 environmental conditions in which they can operate. Most otter trawling
therefore occurs within ‘core’ areas where yields are high and it is safe to trawl, typically historically fished grounds (Jennings and Lee,
2011).
Evidence of habitat risk
Demersal trawling causes physical disturbances on the sea bed, the extent of which will depend on the door weight and the sediment
structure (Løkkeborg, 2005). Decreased habitat complexity has been suggested through the destruction of biogenic structures such as
tubes and burrows (Schwinghamer et al, 1996). Otter trawls also remove, damage and kill seabed biota, can cause reductions in biomass,
production and species richness (Jennings and Kaiser, 1998; Hinz et al, 2009), and ultimately can lead to substantial changes in benthic
community structure (Kaiser et al., 2000). The severity of impacts are habitat and gear specific, the most severe impact occur when fishing
on biogenic reefs composed of sessile epifauna trawled with heavy fishing gears. In the North Sea, demersal otter trawlers have reduced
the biomass and production of bottom-dwelling organisms (Hinz et al., 2009; 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).
Mitigation measures
Under the Marine Strategy Framework Directive (MSFD) from the European Union (Council Directive 56/2008), the European nations have
committed to aim for ‘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 Special Areas of Conservation under the European Habitats Directive (Council Directive 43/1992) contributes to this
process as does the UK Marine 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) is
engaging in a programme designed to assess the effects of fisheries and implement management measures where sites are considered at
risk. Similar initiatives are taking place in other European countries.
References
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., Prieto, V. & Kaiser, M.J. (2009). Trawl disturbance on benthic communities: chronic e ects and experimental predictions. Ecological
Applications, 19, 761–773.
Jennings, S. & Kaiser, M.J. (1998). The e ects of fishing on marine ecosystems. Advances in Marine Biology, 34, 201–352.
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., K. Ramsay, C. A. Richardson, F. E. Spence, and A. R. Brand. 2000. Chronic fishing disturbance has changed shelf sea benthic
community structure. Journal of Animal Ecology 69:494–503.
Rijnsdorp, A. D., Bastardie, F., Bolam, S. G., Buhl-Mortensen, L., Eigaard, O. R., Hamon, K. G. Hiddink, J. G., Hintzen, N. T., Ivanovic´, A.,
Kenny, A., Laffargue, P., Nielsen, J. R., O’Neill, F. G., Piet, G. J., Polet, H., Sala, A., Smith, C., van Denderen, P. D., van Kooten, T., and Zengin,
M. (2015) Towards a framework for the quantitative assessment of trawling impacts on the seabed and benthic ecosystem. ICES J. Mar.
Sci. 72, (9), doi: 10.1093/icesjms/fsv207.
Schwinghamer, P., Guigné, J.Y. & Siu, W.C. 1996. Quantifying the impact of trawling on benthic habitat structure using high resolution
acoustics and chaos theory. Canadian Journal of Fisheries and Aquatic Science, 53: 288-296.
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