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Thornback ray, West of Scotland, Irish and Celtic Seas, demersal otter trawl

fish

Raja clavata

Content last updated
27th Jan 2016

Stock:
Thornback ray in Divisions VIIa-c, f-k and Sub-area VI

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 most fish are less than 105 cm, and this species can live for about 15 years. Maximum age is reported at 12 years (Ryland and Ajayi, 1984), maturing at age 6 and 65–71 cm length (Gallagher et al., 2005), with an estimated fecundity from 60–140 eggs per year (Holden, 1975). 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 one of the skates taken in targeted skate fisheries in the Bristol Channel and Irish Sea, where it is taken by otter trawl or set-nets.  It is also a bycatch in otter trawl and beam trawl fisheries targeting roundfish and flatfish.  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.

 

ICES currently provides advice for three stocks of thornback ray in this area, one in the western English Channel, one in the Irish Sea, Bristol Channel and Celtic Sea, and one off north-west Scotland (ICES, 2014a,b,c). The degrees of connectivity between these stocks and with stocks in adjacent management areas are unknown.

 

Insufficient information is available to present longer-term trends in species-specific landings for these stocks (ICES, 2015), although the species-specific reporting of skates is improving. Catch rates in scientific trawl surveys suggest recent increase in stock abundance in the Irish Sea/Bristol Channel and off north-west Scotland. Data are more limited in the western English Channel and the status of thornback ray in Division VIIe is uncertain.

 

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.

 

Gallagher, M. J., Nolan, C. P., and Jeal, F. (2005). Age, growth and maturity of the commercial ray species from the Irish Sea. Journal of Northwest Atlantic Fishery Science, 35: 47–66.

 

Holden, M. J. (1975). The fecundity of Raja clavata in British waters. Journal du Conseil International pour l’Exploration de la Mer, 36: 110–118.

 

ICES (2014a). Thornback ray (Raja clavata) west of Scotland (Subarea VI). In: Report of the ICES Advisory Committee, 2014. ICES Advice 2014, Book 5, Section 5.3.29.4; 4 pp.

 

ICES (2014b). Thornback ray (Raja clavata) in Divisions VIIa,f,g (Irish and Celtic seas). In: Report of the ICES Advisory Committee, 2014. ICES Advice 2014, Book 5, Section 5.3.29.5; 5 pp.

 

ICES (2014c). Thornback ray (Raja clavata) in Division VIIe (Western English Channel). In: Report of the ICES Advisory Committee, 2014. ICES Advice 2014, Book 5, Section 5.3.29.6; 4 pp.

 

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

 

Ryland, J. S., and Ajayi, T. O. (1984). Growth and population dynamics of three Raja species (Batoidea) in Carmarthen Bay, British Isles. Journal du Conseil International pour l’Exploration de la Mer, 41: 111–120.

 

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 in Divisions VIIa-c, f-k and Sub-area VI have been scored as moderate risk. This is because, whilst the state of the stock is uncertain, there is evidence for a recent increase in the stock.

 

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 spotted ray (59/100; FishBase, 2015) was weighted with an increasing population trend.

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Management

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The management of thornback ray in Divisions VIIa-c, f-k and Sub-area VI (Irish Sea, Celtic Seas and west of Scotland) 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 (e.g. misidentifications in skate species can occur).  Data-limited assessments and scientific advice for these stocks 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 of this fishery is scored as high risk. This is because otter trawls have the potential to take relatively high quantities of bycatch of non-target and vulnerable species (> 30% of catch weight), including demersal elasmobranchs and protected, endangered and threatened (e.g. sharks and rays) species in certain circumstances. However, the incoming EU landings obligation is intended to reduce discarding.

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Habitat

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The habitat risk of this fishery is scored as a 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, but there remains uncertainties about the location of some sensitive seabed habitats and therefore some risk of further impact.

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Outlook

Current risk status Outlook Reason
Stock Moderate Improving There is evidence for recent stock increase in the stock. The most recent ICES advice is that landings could be increased by no more than 20%. Species-specific reporting of landings data is 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 High 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 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 target species, the bycatch risk is likely to reduce in the future.
Habitat Moderate Improving Otter trawls disturb seabed habitats, but a range of Marine Protected Areas have been established and are under development to help minimise damage to vulnerable marine habitats.

Type Current Risk Status Outlook Reason

Stock

Management

Bycatch

Habitat

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