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Winter Skate (Leucoraja Ocellata)

Biology

General

Despite increased fishing effort and a broad geographic range (Sulikowski et al., 2003), little information exists on the biology and distribution of winter skate and other skate. Data have routinely been collected during research surveys in the northwest Atlantic but there has been only limited examination of this information (Kulka et al., 1996).


Reproduction

Like all elasmobranches (sharks, skate, rays), winter skate are slow-growing, produce very few young each year, and almost certainly, as a consequence, have a slow population growth rate. Skate that are part of the Eastern Scotian Shelf and the Georges Bank-Western Scotian Shelf-Bay of Fundy populations mature at considerably larger sizes than those in the Southern Gulf population. On Georges Bank and the Gulf of Maine, individuals mature between 70 and 109 cm total length (TL). Length at 50% maturity for female winter skate on the eastern Scotian Shelf has been estimated to be 75 cm (Simon et al., 2003). Skate in the Southern Gulf of St. Lawrence population mature at a smaller size (50 cm) and do not attain as large a size as in other populations (McEachran and Martin, 1977).

Preliminary aging of winter skate on the eastern Scotian Shelf suggests that the length at 50% maturity corresponds to an age of 7-8 years, although it should be noted that these estimates are based on very limited sample sizes. Maximum age for skate in this region has been estimated to be 20 to 30 years (Simon and Frank, 2000; Sosebee and Terceiro, 2000). These studies were carried out on the eastern Scotian Shelf and have not been validated.

These estimates of age at maturity and maximum age differ from those reported from a study conducted in the Gulf of Maine (Sulikowski et al., 2003). Age at maturity and maximum age were estimated from vertebral band counts, morphological variability, and histological data as well as steroid hormone concentrations. Examination of 96 males and 88 females indicate an age at 50% maturity to occur between 769 to 776 mm for females, corresponding to an age of 12-13 years. For males, 50% maturity was estimated at 730 mm, which corresponds to an age of 11 years. Maximum age was estimated at 18 years for females and 19 years for males (Sulikowski et al., 2003). The aging study by Sulikowski et al. (2003) was undertaken only on skate from the U.S. portion of the population and may not provide accurate estimates of age at maturity for winter skate in Canadian waters. It does, however, provide the most comprehensive information on the age of winter skate at sexual maturity. Mating probably occurs throughout the year although precise details are lacking (Scott and Scott, 1988).

Generation time was estimated as Age at Maturity + 1/M, where M is the instantaneous rate of natural mortality. Based on the estimates provided by Sulikowski et al. (2003), age at maturity was set at 12 years for skate in the Southern Gulf and Georges Bank-Western Scotian Shelf-Bay of Fundy populations. Available, albeit limited, data suggest an age at maturity of 7 years for Eastern Scotian Shelf skate. Based on estimates provided by J. Simon (DFO, Bedford Inst. Oceanography), M was set at 0.1, an estimate of natural mortality that is reasonable for long-lived species such as winter skate. Using these parameter estimates, the generation time is estimated to be 17 years for the Eastern Scotian Shelf population and 22 years for the Georges Bank-Western Scotian Shelf-Bay of Fundy populations.

Winter skate are thought to deposit from six (Bigelow and Schroeder, 1953) to fifty egg cases (Holden, 1977), although the exact number of purses laid is unknown (Collette and Klein-MacPhee, 2002). One fisherman from Isle aux Morts reported 4 to 6 egg cases in winter skate he has cut open. Eggs can take as long as 22 months to develop (Simon & Frank, 1998). If 22 months is an accurate estimate, this may mean that mature females only spawn every two years, but this hypothesis has not been empirically tested.

Eggs of winter skate are deposited throughout the year off southern New England and from summer to autumn off Nova Scotia (Vladykov, 1936; Collette and Klein-MacPhee, 2002). Industry has previously noted females extruding complete purses only in the late summer/early autumn west of Sable Island and suggestions were made that this may be a spawning area (Simon and Frank, 2000). Each embryo is contained in a tough brown egg case, 5.5-8.6 cm long, and 3.5-5.2 cm wide, excluding horns (Scott and Scott, 1988). There are four long filaments, which are thought to serve as anchors (Collette and Klein-MacPhee, 2002). Specimens so recently hatched that their abdomens are still swollen with yolk range from about 112-127 mm in length (Collette and Klein-MacPhee, 2002).


Survival

Little is known about predation on winter skate, but they are eaten by many predators including sharks, other skates (such as L. erinacea), and grey seals (Scott and Scott, 1988). Winter skate are also prone to several parasites, including protozoans, myxosporidian, haematazoa, trematodes and nematodes (Scott and Scott, 1988).

Winter skate are the targets of directed fisheries in the United States and Canada. The principal commercial fishing method used to catch skate is otter trawling. They are also caught as bycatch in groundfish trawls and scallop and shrimp trawls, after which they are usually discarded (Sosebee and Terceiro, 2000). These methods of fishing have been hypothesized to disrupt bottom habitats occupied by winter skate as well as other skate species (Sosebee and Terceiro, 2000).


Movements/Dispersal

Research vessel survey data for the Scotian Shelf suggest that winter skate are concentrated in deeper warmer waters in the winter and move into shallower waters during spring and summer. Three specimens tagged in the Gulf of Maine were recaptured between 130 and 190 km east of where they were released (Whitehead et al., 1984). A recent study of a related species of skate in the North Sea, the thornback ray, indicated that the annual migration cycle, based on conventional and electronic tagging experiments, was restricted to an area 1°latitude X 1°longitude in area (i.e., approximately 110 km X 110 km) (Hunter et al. submitted). In the southern Gulf of St. Lawrence, Clay (1991) concluded that winter skate move from shallow waters to deeper, warmer water in winter (see also Darbyson and Benoît, 2003).


Nutrition and Interspecific Interactions

Although rock crab and squid are favoured by winter skate, they also prey upon annelid worms, amphipods, shrimps, and razor clams, and they eat whatever small fish are readily available (Collette and Klein-MacPhee, 2002). Historical information shows that they eat other small skate and that sandlance appear to be a favoured species of fish (Simon and Frank, 2000).

Studies of food habits of winter skate and little skate by McEachran et al. (1976) have shown that although the two species occur together over most of their range, they probably avoid serious competition with one another by consuming different proportions of the same food resources. Winter skate tend to eat infauna and little skate eat epifauna. Winter skate also overlap with thorny skate (Amblyraja radiata) in the northern portion of their Canadian range, but little is known of the relationship between the two species.


Behaviour/Adaptability

Very little is known about the behaviour of winter skate.