Big skate (Raja binoculata)

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Big skate resting on sea bottom
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Big skate fact file

Big skate description

KingdomAnimalia
PhylumChordata
ClassChondrichthyes
OrderRajiformes
FamilyRajidae
GenusRaja (1)

Aptly named, the big skate (Raja binoculata) is the largest skate in North American waters (4). As with all skates, the body is flattened and disc-shaped, with the pectoral fins broadly expanded and joined to the head and body (5). In this species, the tip of the snout and tips of the pectoral fins are acutely pointed, forming a diamond-shaped disc (4) (5). The tail is distinctly demarcated from the disc, relatively narrow, and about as long as body length (5). An irregular row of approximately 33 thorns run from the end of the back, down the tail to the first of two dorsal fins. The small eyes are positioned on the upper surface relatively far back, while the mouth appears on the underside, along with the five gill slits (4). Mottled colouration on the back includes browns, reddish-browns, dark greys and blacks, with occasional small pale spots and scattered dark blotches (4), while the underside is white (6). The species’ name ‘binoculata’ means two eyes, referring to the prominent dark ocellus (eye-like spot) on the upper surface of each pectoral fin (4). Biologists believe this illusion of eyes may confuse potential predators by making the skate appear much larger than it is (4) (7).

Synonyms
Dipturus binoculata.
Size
Average length: 0.9 – 1.8 m (2)
Maximum total length: 2.4 m (2)
Maximum weight: 91 kg (3)
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Big skate biology

Little is known about the big skate’s mating behaviour but, like other skates, it is oviparous, or egg-laying, but it has unusually large egg capsules. Among skates, only the big skate and the closely related Raja pulchra have more than one egg per egg capsule. The egg capsules of the big skate are laid in pairs and usually contain three to four eggs, although up to seven have been recorded. Hatchlings are released from the egg capsule about nine months after being laid by the female. Males reach sexual maturity at approximately seven to eight years, females at 12 to 13 (4).

The big skate feeds on marine invertebrates such as shrimps, worms and clams, as well as on crustaceans and fish (3) (4). The positioning of the mouth on the underside of the body is perfect for sucking up animals hiding in the sand (4).

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Big skate range

The big skate is found in temperate waters of the north-eastern Pacific Ocean from Alaska to central Baja, California (1) (5). This range includes the eastern Bering Sea and Aleutian Islands, west to Unalaska Island and South to Baja, California (U.S.) near Cedros Island (4).

See this species on Google Earth.

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Big skate habitat

Occurring along the coast in estuaries, bays and over the continental shelf. Commonly found on sandy and muddy bottoms to depths of 120 metres (2) (4). The big skate is usually seen lying on the bottom partially covered with bottom sediments, with eyes protruding above the remainder of the body and sediments (4) (5) (7).

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Big skate status

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

IUCN Red List species status – Near Threatened

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Big skate threats

The big skate is fished for its fins, which are marketed fresh and frozen (3), but is only of minor importance to commercial fisheries (4). However, this species is also taken incidentally as bycatch, primarily by bottom trawlers in the waters off the coast of California (U.S). Indeed, during the 1990s, the skate catch off the coast of California increased nearly ten-fold, partly targeted and partly taken as bycatch by trawl fisheries that supplement their income by marketing incidentally caught skates and rays (4). Data are currently inadequate to determine the precise impact fisheries are having on big skate populations, but as one of the larger, slow maturing species with a low reproductive rate, this skate is potentially vulnerable to population collapses caused by over-fishing (1).

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Big skate conservation

Presently, the big skate is classified as Near Threatened on the IUCN Red List, and no direct conservation measures are currently in place for the species (1). However, more population data and close monitoring of this species are required to accurately assess the impact fisheries are having on its abundance and distribution.

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

For further information on the big 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.
Dorsal fin
The unpaired fin found on the back of the body of fish, or the raised structure on the back of most cetaceans
Invertebrates
Animals with no backbone, such as insects, crustaceans, worms, molluscs, spiders, cnidarians (jellyfish, corals, sea anemones), echinoderms, and others.
Ovipary
The method of reproduction in which eggs are laid and embryos develop outside of the female’s body.
Pectoral fin
In fish, one of 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.
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References

  1. IUCN Red List (March, 2011)
    http://www.redlist.org
  2. Elasmodiver.com (December, 2005)
    http://www.elasmodiver.com/big_skate.htm
  3. FishBase (December, 2005)
    http://www.fishbase.org/search.php
  4. Florida Museum of Natural History (December, 2005)
    http://www.flmnh.ufl.edu/fish/Gallery/Descript/BigSkate/BigSkate.html
  5. McEachran, J. (2006) Pers. comm.
  6. eNature: National Wildlife Federation (December, 2005)
    http://www.enature.com/fieldguides/detail.asp?recnum=FI0235
  7. Monterey Bay Aquarium (December, 2005)
    http://www.mbayaq.org/efc/efc_smm/smm_meetBrowser.asp?tf=16
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Image credit

Big skate resting on sea bottom  
Big skate resting on sea bottom

© Andy Murch / Elasmodiver.com

Elasmodiver.com
andymurch1@gmail.com
http://www.elasmodiver.com

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