|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.