Thorny Skate

Amblyraja radiata

Description

The thorny skate has a flattened body, a lender tail, a disc about 1.4 x as broad as it is long, a snout with an obtuse anterior angle of 110 - 140° depending on the size of the fish (the angle is greater on smaller individuals), and a nearly straight mouth. Teeth - in general - number 36 to 46 rows in both the upper and lower jaws (with the number of rows varying slightly with geographic region). One of the most distinctive characteristics of the thorny skate is the row of large, conspicuous thorns running along the midline to the first dorsal fin. There is some geographic variation in the total number of median thorns, with individuals caught in more northerly regions having more spines. However, they never number more than 20. Other notable spines include 2 to 3 large spines on each shoulder; 1 in front and 1 behind each eye; and 1 near the inner margin of each spiracle. There are also many smaller spines on the dorsal side of the snout, pectoral fins, and tail, and some small spines on the underside of the snout in mature individuals. Adult male thorny skate have 2 to 3 rows of recurved spines on the outer part of the pectorals (termed alar spines). The thorny skate has two dorsal fins similar in size and shape either joined at their bases or well seperated; a tail with lateral folds beginning close behind axils of the pelvics; a short caudal membrane; and pelvic lobes separated by a deep, scalloped indentation and overlapped by the pectorals. In mature males, the claspers are very large, extending about 80% of the distance from the axils of the pelvics to the first dorsal. The thorny skate is brown dorsally - occasionally with darker brown spots - and whitish ventrally, occasionally with sooty blotches. White spots are sometimes present beside each eye, on each side opposite the nuchal region or on each side of the posterior portion of the disc.

Size

The thorny skate is a large skate, reaching a maximum size of approximately 102 to 105 cm TL. Maximum size or length attained varies substantially with geographic area. For example, maximum observed size is approximately 45.0 cm TL in the North Sea; 99.0 cm TL in Iceland; 102.0 cm (Scotian banks); and about 89.5 cm TL (Georges Bank).

Range

The thorny skate is a boreal to arctic species, broadly distributed in both the eastern and western Atlantic Ocean. In the western Atlantic, it ranges from western Greenland, Davis strait, Hudson strait, Hudson Bay, and off Labrador southward as far as South Carolina. It is one of the most common skates found on/in the Grand Bank, the Gulf of St Lawrence, the Scotian Shelf, the Bay of Fundy, and Georges Bank. In eastern Atlantic the thorny skate is found from Iceland, Norway, the Barents Sea, the North Sea and the western Baltic southward to include waters around northern British Isles.

Habitat

The thorny skate lives over hard and soft bottoms, and is found mainly offshore; however, it occurs in a wide range of depths from nearshore (18 m) to at least 1400 m, and can also tolerate a wide range of temperatures (1.4 - 14°C). It is most abundant below 110 m in waters of 2 - 10°C. Seasonal migrations of the thorny skate have been noted on the Scotian Shelf and the Grand Banks, although relatively sedentary or philopatric behaviour has also been noted. For example, a tagging study involving 722 thorny skate in the NewFoundland area indicated that most movements were of distances less than 97 km from the tagging site even up to 20 yr after tagging. The longest distances traveled were between 161 and 387 km from the tagging site over a period of 0.2 - 11 years after tagging; however, only a few individuals traveled any substantial distance.

Life History

Studies on age, growth and maturity in thorny skate have demonstrated that this species is slow-growing, late-maturing and long-lived, and is therefore vulnerable to decline due to over-exploitation. A recent study on age and growth in thorny skate on the eastern Scotian Shelf carried out here at the Bedford Institute of Oceanography indicated that this species is slower-growing and longer-lived than previously reported, reaching an absolute age of at least 28 years! This is the oldest fully-validated age reported for any species of batoid. For more information on how age and growth characteristics are determined in NW Atlantic skates, go to the Age and Growth section of this website or refer to the references listed in the table below.

Size-at-maturity in thorny skate varies greatly with geographic region, with individuals maturing at smaller sizes in European and/or more northerly waters. In North American waters, there is a potential cline in increasing size at sexual maturity from Labrador to the Gulf of St Lawrence and along the Scotian Shelf to the Gulf of Maine. Great variability also exists in thorny skate maturation within each area, with a bimodal maturity pattern being reported in a number of areas including the eastern Scotian Shelf and the Gulf of Maine. For more information on how maturity is determined in NW Atlantic skate, go to the Reproduction Research section of this website or refer to the references listed in the table below.

Diet

The thorny skate feeds mainly only polychaetes, amphipods, decapods, and fishes, with diet being ontogenetic (i.e. the proportion of each prey consumed varies with the size/age of the fish). Specifically, individuals less than 40 cm TL feed mainly on amphipods, while those over 40 cm TL feed mainly on polychaetes and decapods. Individuals reaching lengths of over 70 cm appear to eat mainly fishes and occasionally cephalopods. Twenty-eight species of fish have been found in the stomachs of large thorny skate, the most frequently encountered being sand lance, small haddock and sculpins. The thorny skate is sympatric with the smooth skate (Malacoraja senta), and thus competes for food with the later. However, some prey differentiation is apparent, with smooth skate eating mostly epifauna, such as decapod crustaceans and euphausiids.

Reproduction

Thorny skate are capable of reproduction year round, with potential peaks in reproductive activity and egg case deposition in October (in the Gulf of Maine) and in May and October (on the eastern Scotian Shelf). Spawning females have been found during every month of the year in most areas where they have been studied. Egg capsules and egg capsule characteristics (i.e. yolk weight) vary in size with the mother. Hatching occurs when the embryo is approximately 10 to 12 cm TL. For more information on how reproductive cycle is determine in NW Atlantic skate, go to the Reproduction Research section of this website. For a look at the egg capsules of the thorny skate, click on the link Identification of NW Atlantic Skate Egg Capsules.

Interaction with people

Thorny skate are one of the most common NW Atlantic, and are fished both directly and indirectly in waters off Atlantic Canada and the northeastern US for human consumption and for the manufacture of fish meal. In Canada, domestic fleets began targeting skate in inshore waters in 1994/1995 when the Department of Fisheries and Oceans established a skate fishery on the southwestern Grand Banks, southern St. Pierre Bank, and the adjacent Scotian Shelf. The fishery on the eastern Scotian Shelf has since stopped (2005), although thorny skate continue to be targeted by Canadian and foreign fisheries operating on the Grand Banks and on the Tail of the Grand Banks (outside 200 miles), respectively. For more information go to the Skate Fisheries and Skate Conservation sections of this website. In the Gulf of Maine, the thorny skate is regarded as over-fished, and commercial landing of this species is now prohibited. Finally, in Europe this is one of the many skates heavily marketed for table use.