Thorny Skate: Amblyraja radiata

Family: Rajidae

Common name(s)

Thorny Skate, Starry Ray.

Identification

A large skate with a kite-shaped disc, disc shape more rounded in females, disc width 1.2-1.3 x disc length. Head broad. Eyes medium-sized. Snout length 2.8-3 x orbit length. Mouth wide. Snout short and broadly rounded with a small projecting lobe at tip. Pectoral fin anterior margins undulate; with a central concavity. Pectoral apices narrowly rounded or angular. Pelvic fin anterior lobes much shorter than posterior lobes. Anterior pectoral radials extend almost to snout tip.
Thorns on disc extremely prominent, with stellate bases. 1 pre-orbital and 2 post-orbital thorns, 1-2 thorns on nape, 2-3 on each shoulder, 13-17 extremely tall thorns on midline to first dorsal fin. A row of smaller thorns flank each side of midline thorns from back of head to base of tail. Thorn patches also present on snout and pectoral fins. Tail short and firm, tapering to tip. Two small dorsal fins, joined or narrowly separated. Caudal fin rudimentary. Claspers of adult males short and extremely wide.

Colour

Dorsum brown or greyish brown, or blotchy shades of cream, grey, and brown. Usually with a few scattered irregular dark spots and blotches. Spots on pectorals may form rosettes. Ventrum White.

Size

Maximum length varies considerably by region; 111cm in the western Atlantic, 66cm in the North Sea. Length at birth 8-12cm.

Habitat

Subarctic and cold-temperate seas. Demersal on soft bottoms of continental shelf and slope. From close inshore to at least 1400m, usually 25-440m.

Distribution

North Atlantic. Widespread in northern lattitudes. Off North America from South Carolina to Baffin Island, then eastward to southern Greenland, Iceland, British Isles, and France, and northward to Norway and into the Barents Sea in northern Russia.

Conservation Status

VULNERABLE

Until recently, the thorny skate was subject to intensive target fisheries in the Northwest Atlantic and taken as bycatch in mainly trawl, but also longline and gillnet, fisheries in the USA and Canadian waters south of the Laurentian Channel (Georges Bank and Scotian Shelf). In the USA, commercial retention of Thorny Skate was banned 2003, but it is still taken as bycatch and discarded from trawl fisheries. In Canada on the Scotian Shelf, target fisheries ended in 2002 and any bycatch is now discarded. In the Northeast Atlantic, this skate has limited value and it is generally discarded and has a low discard mortality. Population trends and management vary widely, resulting in different status and trends throughout its range. On both sides of the Atlantic, the southern-most populations of this boreal species have shifted northward in response to climate-change induced habitat shifting and alteration, which, in combination with earlier intensive fishing, resulted in very steep declines (>80% reduction in three generations) in the most southerly parts of its distribution. At more northerly latitudes, earlier levels of exploitation have ceased or greatly decreased resulting in recovery and increasing abundance. Overall, Thorny skate has undergone an estimated population reduction of 30–49% over the last three generation lengths (32–48 years) due the combination of exploitation with habitat shifting and alteration due to climate change at lower latitudes while, further north, earlier levels of exploitation that have ceased or greatly decreased and it is assessed as Vulnerable A2bcd.

Citation

Kulka, D.W., Ellis, J., Anderson, B., Cotton, C.F., Derrick, D., Pacoureau, N. & Dulvy, N.K. 2020. Amblyraja radiata. The IUCN Red List of Threatened Species 2020: e.T161542A124503504. https://dx.doi.org/10.2305/IUCN.UK.2020-3.RLTS.T161542A124503504.en. Downloaded on 10 April 2021.

Reproduction

Oviparous. 10-45 egg cases each year.

Diet

Thorny skates mostly feed on small bottom fishes, crustaceans, and polychaete worms, but they will also consume hydroids, molluscs, cephalopods and echinoderms.

Behavior

In the North Sea, migration experiments showed that 85 % of tagged individuals remained within 93 km of the release site, with longest distance travelled at 180 km (Walker, P., G. Howlett and R. Millner 1997).
Able to detect weak electric fields generated by potential prey organisms and may also generate its own weak electric fields (Fritzsch, B. and P. Moller, 1995).

Reaction to divers

Fairly easy to approach. Usually remains motionless but will move away if disturbed.

Diving logistics

The best place to see thorny skates seems to be Iceland. In particular, the shallow water hydrothermal vent Strýtur Chimney appears to be a hot spot (no pun intended).

Most reports from North America are historical i.e. sightings have declined significantly in the last two decades probably due to population depletions.