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Thornback ray in ICES sub-area IV (North Sea), Demersal otter trawl

fish

Raja clavata

Content last updated
27th Jan 2016

Stock:
Thornback ray in ICES sub-area IV (North Sea)

Management:
EU

Overview

Thornback ray (Raja clavata) is one of the commonest skates in European shelf seas and is the most commercially important skate species in this region. This species is widely distributed in the North-eastern Atlantic, from Iceland and Norway to the Azores, Northwest Africa, Mediterranean Sea and Black Sea (Stehmann and Bürkel, 1984). It is a demersal species that occurs on a variety of substrates, including mud, sand and gravel. This species is distributed on the continental shelf and upper slope, but is most common in waters less than 100 m deep (Ellis et al., 2005).

 

The largest recorded specimen is 130 cm (but specimens >105 cm are rare), and they can live for about 15 years. They spend the winter in slightly deeper water and migrate into shallower coastal areas in the late spring and summer to spawn. Spawning occurs in inshore coastal waters between February and September, peaking in March to June. Eggs are deposited on the seabed and hatch after 4 to 6 months, depending on the water temperature. Juveniles are non-migratory, inhabiting inshore nursery grounds, whilst adults display seasonal migrations and longer movements. Juveniles feed on small crustaceans particularly amphipods and bottom-living shrimps, while adults feed on crabs, shrimps and small fish.

 

Thornback ray is a target species in some inshore fisheries in the south-western North Sea that use otter trawl, tangle nets and longlines. It is also a bycatch in offshore otter trawl and beam trawl fisheries targeting plaice (Pleuronectes platessa) and sole (Solea solea). Juvenile thornback ray are often discarded (Silva et al., 2012)

 

The degree of resource competition and species interactions between skate species is poorly understood. Historically, the larger common skate (Dipturus batis) was known to predate on smaller skate individuals, and earlier, longer-term declines in larger skates may have benefited populations of smaller skate species, due to a reduction in predation or competition.

 

Insufficient information is available to present longer-term trends in species-specific landings for this stock, although the species-specific reporting of skates is improving. Catch rates in scientific trawl surveys suggest a recent increase in stock abundance in the southern parts of the range (Divisions IVc and VIId, where thornback ray is most abundant). Data are more limited further north and the status of thornback ray in Divisions IVa,b is uncertain (ICES, 2015).

References

Ellis, J. R., Cruz-Martinez, A., Rackham, B. D., and Rogers, S. I. (2005). The distribution of chondrichthyan fishes around the British Isles and implications for conservation. Journal of Northwest Atlantic Fishery Science, 35: 195–213.

 

ICES (2015). Report of the Working Group on Elasmobranch Fishes (WGEF), 17–23 June 2015, Lisbon, Portugal. ICES CM 2015/ACOM:19. 711 pp.

 

Silva, J. F., Ellis, J. R. and Catchpole, T. L. (2012). Species composition of skates (Rajidae) in commercial fisheries around the British Isles, and their discarding patterns. Journal of Fish Biology, 80: 1678–1703.

 

Stehmann, M., and Bürkel, D. L. (1984). Rajidae. In Fishes of the north-eastern Atlantic and Mediterranean. P. J. P. Whitehead, M-L. Bauchot, J-C. Hureau, J. Nielsen, and E. Tortonese (eds.). UNESCO, Paris. Vol. 1, pp. 163–196.

Stock Status

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Thornback ray ICES Subarea IV and Division VIId has been scored as moderate risk. The state of the stock is uncertain, but assumed to be increasing. Catch rates is scientific trawl surveys have increased in the last decade. The advice given by ICES in 2015 was that landings could be increased by up to 20%, and “should be no more than 2110 tonnes in each of the years 2016 and 2017”.

 

Reference points for long-term sustainability have not been defined for this stock, and therefore cannot be used to derive a risk score. The risk score was calculated using a data-limited approach where the vulnerability score for thornback ray (72/100; FishBase, 2015) was weighted with an increasing population trend.

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Management

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The management of Thornback ray ICES Subarea IV and Division VIId has been scored a moderate risk. This is because data-derived management controls are in place, albeit under the framework of a generic TAC for all skates (Rajidae) over a broader management area, and that compliance can be patchy. Data-limited assessments and scientific advice for this stock are provided on a biennial basis and a comprehensive regulatory framework is in place.

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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

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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.

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Outlook

Current risk status Outlook Reason
Stock Moderate Improving There is evidence for recent stock increase in the stock, and ICES has advised that landings could be increased Species-specific landings data are improving. Estimates of dead discards are not available, and some landings data are generic. Hence, recent trends in the catches of thornback ray cannot be fully quantified. Ongoing collection of species-specific landings data should improve understanding of stock dynamics.
Management Moderate Improving No individual TAC is set for this stock at present and only a data-limited assessment has been produced to provide fisheries advice. No reference points for long-term sustainability have been defined, and no management plans are currently in place. Increasing the quantity and quality of species-specific data will help to improve targeted management techniques
Bycatch 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.
Habitat Moderate Improving Technical and spatial management measures are under development and will potentially reduce the risk further.

Type Current Risk Status Outlook Reason

Stock

Management

Bycatch

Habitat

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