Raja clavata

Taxonomy

Assessment Information

Geographic Range

Population

Habitat and Ecology

Threats

Conservation Actions

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Taxonomy [top]

Kingdom	Phylum	Class	Order	Family
ANIMALIA	CHORDATA	CHONDRICHTHYES	RAJIFORMES	RAJIDAE

Scientific Name:

Raja clavata

Species Authority:

Linnaeus, 1758

Common Name/s:

English

–

Thornback Skate

Assessment Information [top]

Red List Category & Criteria:	Near Threatened ver 3.1
Year Published:	2005
Assessor/s:	Ellis, J.
Reviewer/s:	Musick, J.A. & Fowler, S.L. (Shark Red List Authority)
Contributor/s:
Justification: This assessment is based on the information published in the 2005 shark status survey (Fowler et al. 2005). This demersal batoid is one of the most abundant elasmobranchs in the Northeastern Atlantic. The Thornback Skate (Raja clavata) is widespread, although the taxonomy of specimens from South Africa requires additional study. There is some limited evidence of a decline in landings in the northern part of the East Atlantic range of this species and management of the fishery is required. However, declines have not been as serious as reported for other large rajids.

Geographic Range [top]

Range Description:	The Thornback Skate, or roker, is one of the most abundant rajids in north European coastal waters and can be the dominant rajid in commercial landings and research vessel catches (Rousset 1990a, Ellis unpubl.). It is widely distributed from Iceland and Norway (south of the Arctic Circle), to the North Sea (where it is now less abundant in south-eastern areas (Walker 1998), the Mediterranean, the western Black Sea, Madeira, the Atlantic coasts of Africa, and as far south as South Africa and the south-western Indian Ocean (Stehmann 1995). The status of this species in West and South African waters, and its relationship with Raja (Raja) cf. clavata, which is reported from the waters off Namibia and southern Africa (Macpherson 1986, Ebert et al. 1991, Smale and Cowley 1992) needs further research.
Countries:	Native: Faroe Islands; France; Iceland; Mauritania; Morocco; Norway; Portugal (Madeira, Portugal (mainland)); Senegal; Spain; United Kingdom (Great Britain, Northern Ireland); Western Sahara
FAO Marine Fishing Areas:	Native: Atlantic – eastern central; Atlantic – northeast; Atlantic – southeast; Indian Ocean – western; Mediterranean and Black Sea
Range Map:	Click here to open the map viewer and explore range.

Population [top]

Population Trend:	Decreasing

Population Trend:

Decreasing

Habitat and Ecology [top]

Habitat and Ecology:	The Thornback Skate is a demersal coastal species which inhabits a variety of substrates, including mud, sand, shingle, gravel and rocky areas, in water down to 300 m, although it is most abundant in 10? 60 m of water off coastal areas (Wheeler 1969, Stehmann and Buerkel 1984). Rousset (1990a) studied the elasmobranch assemblage off the coast of Brittany and found that R. clavata was the most abundant rajid due to the fact that it was able to inhabit this range of benthic habitats. Thornback Skate is the second most important species, after the Smallspotted Catshark (Scyliorhinus canicula), in the demersal elasmobranch assemblage in the northern Bristol Channel and constitutes between 7.4?8.8% of the elasmobranch biomass (Ellis unpubl.). The age and growth of this species have been studied by examining the vertebrae of fish caught in the Bristol Channel (Ryland and Ajayi 1984), although Brander and Palmer (1985), using length-frequency analysis, have indicated inconsistencies for the smaller sized fish used in this work. Ryland and Ajayi (1984) gave maximum age and length of 12 years and 1,047 mm (k = 0.090). Vertebral growth rings have been shown to be annual (Holden and Vince 1973). Growth has also been estimated from tagging studies (Holden 1972). Validated age studies of specimens from the southern North Sea have shown thornback skates to have a maximum length of 118 cm for females and 98cm for males (Walker 1998). The size at maturity for females and males have been estimated at 45?50 cm disc width (DW) and 38?44 cm DW (Fitzmaurice 1974); 85 cm total length (TL) (54 cm DW) and 75 cm TL (48 cm DW) (Capapé 1976); 45 cm DW and 42 cm DW (Nottage and Perkins 1983) and 59 cm TL and 60 cm TL (Ryland and Ajayi 1984). Walker (1998) estimated length at 50% maturity to be 77 cm TL for males and 68 cm TL for females. The corresponding ages at 50% maturity were eight and seven years. It has been reported that R. clavata first spawn in their fifth year (Ryland and Ajayi 1984). Eggs are laid during a protracted breeding season from February to September (Holden 1975), with a peak in May and June. However, this is for the population as a whole and the egg-laying period for individual fish may be shorter. Ellis and Shackley (1995) maintained one female in captivity and reported that egg laying lasted six weeks, with a mean egg-laying rate of 1.07 eggs per day, a pair of eggs being laid on alternate days. These data concur with the observations of Holden (1971). Development lasts 16?20.5 weeks (Ellis and Shackley 1995), although this period may vary with temperature. The young hatch at a length of 10?13 cm. The nursery areas used are coastal, estuarine and tidal flat areas (e.g., the Wash and Thames estuary in the UK). The fecundity of R. clavata in British waters has been estimated at 150 eggs per year (Holden 1971), 140 eggs per year (Holden 1975) and 100 eggs per year (minimum of 62? 74) (Ryland and Ajayi 1984). Capapé (1976, 1977a) estimated a fecundity of 70?167 eggs per year in Tunisian waters, although it may be as low as 48 (Ellis and Shackley 1995). The feeding habits have been well documented from many areas over its geographical range, including British waters (Holden and Tucker 1974, Ajayi 1982, Ellis et al. 1996), Ireland (Fitzmaurice 1974), France (Du Buit 1968, 1978?79, Quiniou and Andriamirado 1979), Portugal (Marques and Re 1978, Cunha et al. 1986), the Mediterranean (Capapé 1975, 1977; Abdel-Aziz 1986), the Southeast Atlantic waters off Namibia (Macpherson 1986) and southern Africa (Ebert et al. 1991). Young and juvenile R. clavata predominantly eat small crustaceans, such as shrimps, mysids, amphipods and small crabs. Larger specimens prey on larger crustaceans, including prawns and crabs and will also consume fish. The migratory habits have been studied by Steven (1936) who found that very little movement occurred, especially in young fish, with 71% of tagged fish moving less than five miles. Fish tagged in the southern North Sea also showed a sedentary pattern, with 80% being recaptured within 40 nautical miles of their release position (Walker et al. 1997). The recapture percentage was nearly 30%. Fitzmaurice (1974) studied the populations within two bays in Ireland and reported a sex ratio of 1:1 and, of 71 tagged, eight (11.3%) recaptures. Rousset (1990b) observed that mature females were more common in exposed areas and juveniles and mature males were more common in more sheltered areas.
Systems:	Marine

Habitat and Ecology:

The Thornback Skate is a demersal coastal species which inhabits a variety of substrates, including mud, sand, shingle, gravel and rocky areas, in water down to 300 m, although it is most abundant in 10? 60 m of water off coastal areas (Wheeler 1969, Stehmann and Buerkel 1984). Rousset (1990a) studied the elasmobranch assemblage off the coast of Brittany and found that R. clavata was the most abundant rajid due to the fact that it was able to inhabit this range of benthic habitats. Thornback Skate is the second most important species, after the Smallspotted Catshark (Scyliorhinus canicula), in the demersal elasmobranch assemblage in the northern Bristol Channel and constitutes between 7.4?8.8% of the elasmobranch biomass (Ellis unpubl.). The age and growth of this species have been studied by examining the vertebrae of fish caught in the Bristol Channel (Ryland and Ajayi 1984), although Brander and Palmer (1985), using length-frequency analysis, have indicated inconsistencies for the smaller sized fish used in this work. Ryland and Ajayi (1984) gave maximum age and length of 12 years and 1,047 mm (k = 0.090). Vertebral growth rings have been shown to be annual (Holden and Vince 1973). Growth has also been estimated from tagging studies (Holden 1972). Validated age studies of specimens from the southern North Sea have shown thornback skates to have a maximum length of 118 cm for females and 98cm for males (Walker 1998). The size at maturity for females and males have been estimated at 45?50 cm disc width (DW) and 38?44 cm DW (Fitzmaurice 1974); 85 cm total length (TL) (54 cm DW) and 75 cm TL (48 cm DW) (Capapé 1976); 45 cm DW and 42 cm DW (Nottage and Perkins 1983) and 59 cm TL and 60 cm TL (Ryland and Ajayi 1984). Walker (1998) estimated length at 50% maturity to be 77 cm TL for males and 68 cm TL for females. The corresponding ages at 50% maturity were eight and seven years. It has been reported that R. clavata first spawn in their fifth year (Ryland and Ajayi 1984). Eggs are laid during a protracted breeding season from February to September (Holden 1975), with a peak in May and June. However, this is for the population as a whole and the egg-laying period for individual fish may be shorter. Ellis and Shackley (1995) maintained one female in captivity and reported that egg laying lasted six weeks, with a mean egg-laying rate of 1.07 eggs per day, a pair of eggs being laid on alternate days. These data concur with the observations of Holden (1971). Development lasts 16?20.5 weeks (Ellis and Shackley 1995), although this period may vary with temperature. The young hatch at a length of 10?13 cm. The nursery areas used are coastal, estuarine and tidal flat areas (e.g., the Wash and Thames estuary in the UK). The fecundity of R. clavata in British waters has been estimated at 150 eggs per year (Holden 1971), 140 eggs per year (Holden 1975) and 100 eggs per year (minimum of 62? 74) (Ryland and Ajayi 1984). Capapé (1976, 1977a) estimated a fecundity of 70?167 eggs per year in Tunisian waters, although it may be as low as 48 (Ellis and Shackley 1995). The feeding habits have been well documented from many areas over its geographical range, including British waters (Holden and Tucker 1974, Ajayi 1982, Ellis et al. 1996), Ireland (Fitzmaurice 1974), France (Du Buit 1968, 1978?79, Quiniou and Andriamirado 1979), Portugal (Marques and Re 1978, Cunha et al. 1986), the Mediterranean (Capapé 1975, 1977; Abdel-Aziz 1986), the Southeast Atlantic waters off Namibia (Macpherson 1986) and southern Africa (Ebert et al. 1991). Young and juvenile R. clavata predominantly eat small crustaceans, such as shrimps, mysids, amphipods and small crabs. Larger specimens prey on larger crustaceans, including prawns and crabs and will also consume fish. The migratory habits have been studied by Steven (1936) who found that very little movement occurred, especially in young fish, with 71% of tagged fish moving less than five miles. Fish tagged in the southern North Sea also showed a sedentary pattern, with 80% being recaptured within 40 nautical miles of their release position (Walker et al. 1997). The recapture percentage was nearly 30%. Fitzmaurice (1974) studied the populations within two bays in Ireland and reported a sex ratio of 1:1 and, of 71 tagged, eight (11.3%) recaptures. Rousset (1990b) observed that mature females were more common in exposed areas and juveniles and mature males were more common in more sheltered areas.

Systems:

Marine

Threats [top]

Major Threat(s):	Thornback Skate is a very important component of demersal fisheries in most European waters and is taken by trawl and gillnet, particularly as bycatch. There is or has been limited directed longlining and netting for the species. Landings of this species are not known, as landings of all rajids are combined in the records (ICES 1958?1987). Holden (1963) looked at the species composition of rajids landed by commercial trawlers at Milford Haven and Fleetwood, UK, during 1961 and 1962 and R. clavata accounted for 34.9% and 12.72% respectively. There is no evidence of severe population depletion, as has been documented for the Common Skate (Dipturus batis), although landings are considered to be in decline and a management strategy is required. Thornback Skates are also regularly caught by recreational anglers, although mortality from this source of fishing pressure will be of little impact for the population as a whole, particularly in areas where catch and release is practised.

Major Threat(s):

Thornback Skate is a very important component of demersal fisheries in most European waters and is taken by trawl and gillnet, particularly as bycatch. There is or has been limited directed longlining and netting for the species. Landings of this species are not known, as landings of all rajids are combined in the records (ICES 1958?1987). Holden (1963) looked at the species composition of rajids landed by commercial trawlers at Milford Haven and Fleetwood, UK, during 1961 and 1962 and R. clavata accounted for 34.9% and 12.72% respectively. There is no evidence of severe population depletion, as has been documented for the Common Skate (Dipturus batis), although landings are considered to be in decline and a management strategy is required. Thornback Skates are also regularly caught by recreational anglers, although mortality from this source of fishing pressure will be of little impact for the population as a whole, particularly in areas where catch and release is practised.

Conservation Actions [top]

Conservation Actions:	Several of the UK?s local Sea Fisheries Committees have by-laws for a minimum landing size (e.g., 40 cm DW in the Southern and the Kent and Essex Sea Fisheries Districts). Such localised management initiatives will not, however, be of significant effect in conserving regional populations. Due to European rajid fisheries being a component of multispecies fisheries, which also target several species of flatfish and gadoid, gear restriction using mesh size is not a viable management measure. Minimum landing sizes have been implemented in some areas of the UK by Sea Fisheries Committees.

Conservation Actions:

Several of the UK?s local Sea Fisheries Committees have by-laws for a minimum landing size (e.g., 40 cm DW in the Southern and the Kent and Essex Sea Fisheries Districts). Such localised management initiatives will not, however, be of significant effect in conserving regional populations. Due to European rajid fisheries being a component of multispecies fisheries, which also target several species of flatfish and gadoid, gear restriction using mesh size is not a viable management measure. Minimum landing sizes have been implemented in some areas of the UK by Sea Fisheries Committees.

Citation:	Ellis, J. 2005. Raja clavata. In: IUCN 2012. IUCN Red List of Threatened Species. Version 2012.2. <www.iucnredlist.org>. Downloaded on 22 May 2013.
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