Thornback skate  (Raja clavata)

The thornback skate is probably one of the commonest skates encountered by divers (3), being among the most abundant rajids in the north-eastern Atlantic and Mediterranean (1). As with all skates, the body is flattened and disc-shaped, with the pectoral fins broadly expanded and joined to the head and body (4). The tail is distinctly demarcated from the disc-like body, relatively narrow, and about as long as body length (3) (4). The upper surface of the disc and tail are covered with numerous thorns, which become thickened with button-like bases (known as bucklers) once the skate is sexually mature, hence the species' common name (3) (5). Only the snout and margins of the disc are prickly in young, and the underside is only prickly in large, mature females, which also possess more developed ‘bucklers' on their back and tail (2) (3). The colour varies from light brown to grey on the upper surface, variegated with dark and light spots and blotches that camouflage the skate in the dappled light of the sea bed, while the underside is creamy-white (2) (3).

Found in the eastern Atlantic from Iceland, Norway, North Sea and the western Baltic southward to Morocco and Namibia, including the Mediterranean and the Black Sea (2). Also reported from South Africa eastward into the south-western Indian Ocean (4).

A bottom-dwelling species occupying continental shelf and upper slope waters to depths of 300 m, but mainly from 10 to 60 m (6). However, off South Africa this species occurs to 1000 m (4). Usually found on sediment type sea beds such as mud, sand or gravel, occasionally adjacent to rocky reefs (3) (7).

All skates are oviparous (8) (egg-laying), with large female thornbacks laying up to 140 to 160 egg capsules per year (6). The species moves into shallower, inshore waters in spring when ready to spawn, laying eggs between March and September, which take a further four to six months to hatch (6). Young feed on small crustaceans and other bottom-dwelling creatures, predating more on fish and larger crustaceans such as crabs as they develop (5) (6). Maturity is reached at around eight years of age (5), and the maximum lifespan is at least 15 years (6).

The thornback skate is an important component of mixed trawl fisheries, has been targeted by recreational anglers (1), and is also caught incidentally as bycatch in beam trawl fisheries (9). The thorniness of this species adds to their chance of being entangled in nets (9). There is some evidence of a decline in catch rates in north-west European waters, which could imply a decline in population numbers, but more data is required to confirm this (1). The impact of the fishing industry on rays and skates is the subject of increasing concern due to their slow growth rate, late maturity and low fecundity, making them particularly vulnerable to population collapse due to fishing activity (10).

Fishing of the thornback skate is controlled through the authorities limiting the Total Allowable Catch (TAC), although the quota is set for all rajids combined, covering EU waters in the North Sea and Norwegian Sea. This measure helps ensure continuity of economic activity in the fisheries concerned, while giving depleted stocks a reasonable chance of recovery. The TAC for skates and rays has been reduced by approximately 47 % between 1999 and 2005, providing greater protection to the species by reducing the quota allowed to be caught. However, records show that the actual numbers caught each year have in fact been below what is permissible through the TAC (6). Thus, these skate are not currently considered to be particularly over-fished and are not seen as a high conservation priority.

For further information on the thornback skate see:

• FishBase:
  http://www.fishbase.org/search.php
• HabitasOnline:
  http://www.habitas.org.uk/marinelife/species.asp?item=ZF1360
• International Council for the Exploration of the Sea (ICES): ICES-FishMap:
  http://www.ices.dk/marineworld/fishmap/ices/pdf/thornback.pdf

For more information on sharks and their conservation see:

• Project Aware
  http://www.projectaware.org/

Authenticated (01/03/2006) by John McEachran, Professor in the Department of Wildlife and Fisheries Sciences at Texas A and M University.
http://wfscnet.tamu.edu/