Thornback skate (Raja clavata)

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Thornback skate fact file

Thornback skate description

KingdomAnimalia
PhylumChordata
ClassChondrichthyes
OrderRajiformes
FamilyRajidae
GenusRaja (1)

The thornback skate (Raja clavata) 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).

Size
Maximum male length: 105 cm (2)
Maximum female length: 120 cm (2)
Maximum weight: 18 kg (2)
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Thornback skate biology

All skates are oviparous (8) (egg-laying), with large female thornback skates laying between 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).

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Thornback skate range

The thornback skate is 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).

See this species on Google Earth.

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Thornback skate habitat

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

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Thornback skate status

The thornback skate is classified as Near Threatened (NT) on the IUCN Red List (1).

IUCN Red List species status – Near Threatened

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Thornback skate threats

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).

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Thornback skate conservation

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 percent 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.

View information on this species at the UNEP World Conservation Monitoring Centre.
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Find out more

For further information on the thornback skate: 

 For further information on the conservation of sharks and rays: 

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Authentication

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/

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Glossary

Bycatch
In the fishing industry, the part of the catch made up of non-target species.
Crustaceans
Diverse group of arthropods (a phylum of animals with jointed limbs and a hard chitinous exoskeleton) characterised by the possession of two pairs of antennae, one pair of mandibles (mouthparts used for handling and processing food) and two pairs of maxillae (appendages used in eating, which are located behind the mandibles). Includes crabs, lobsters, shrimps, woodlice and barnacles.
Disc
In rays and skates, the expanded, flattened, and continuous section of the body that consists of the head, trunk, and pectoral fins, but excludes the tail.
Fecundity
A measure of reproductive potential, such as sperm count or egg count.
Ovipary
The method of reproduction in which eggs are laid and embryos develop outside of the female’s body.
Pectoral fins
In fish, the pair of fins that are found one on each side of the body just behind the gills. They are generally used for balancing and braking.
Spawning
The production or depositing of large quantities of eggs in water.
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References

  1. IUCN Red List (March, 2011)
    http://www.redlist.org
  2. FishBase (December, 2005)
    http://www.fishbase.org/search.php
  3. HabitasOnline (December, 2005)
    http://www.habitas.org.uk/marinelife/species.asp?item=ZF1360
  4. McEachran, J. (2006) Pers. comm.
  5. First Nature (December, 2005)
    http://www.first-nature.com/fishes/raja_clavata.htm
  6. International Council for the Exploration of the Sea (ICES): ICES-FishMap (December, 2005)
    http://www.ices.dk/marineworld/fishmap/ices/pdf/thornback.pdf
  7. Elasmodiver.com (December, 2005)
    http://www.elasmodiver.com/Thornback%20skate.htm
  8. Fisheries and Oceans – Canada (December, 2005)
    http://www.pac.dfo-mpo.gc.ca/sci/sa-mfpd/elasmobranchs/Elasmo_skates.htm
  9. Walker, P. (1995) Sensitive Skates or Resilient Rays? - a North Sea Perspective. The IUCN/SSC Shark Specialist Group: Shark News, 5: 0 - 0. Available at:
    http://www.flmnh.ufl.edu/fish/organizations/ssg/sharknews/sn5/shark5news9.htm
  10. Serra-Pereira, B., Figueiredo, I., Bordalo-Machado, P., Farias, I., Moura, T. and Gordo, L.S. (2005) Age and growth of Raja clavata Linnaeus, 1758 – evaluation of ageing precision using different types of caudal denticles. Elasmobranch Fisheries Science, 17: 1 - 10. Available at:
    http://www.ices.dk/products/CMdocs/2005/N/N1705.pdf
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Thornback skate swimming  
Thornback skate swimming

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