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Winter Skate (Leucoraja Ocellata)

Limiting Factors and Threats

Narrow latitudinal ranges and a high degree of endemism have been documented for the skate family worldwide. Among other things, this is suggestive of a relatively limited dispersal capacity. Importantly, however, it may also increase the probability of local extinctions. Of 58 marine fish extirpations documented by Dulvy et al. (2003), 13 have been experienced by members of the skate family (Rajidae), underscoring the potential utility of assessing skate at relatively fine spatial scales.

Although there have been no direct studies of the factors that cause mortality in winter skate, a primary threat to this species in Canadian waters has been the unsustainable rate at which skate have been caught as bycatch in groundfish fisheries, perhaps most notably those for flatfish, and in fisheries for invertebrates, most notably those for scallops. Otter trawls and scallop drags can account for more than 90% of the total discards (Sosebee and Terceiro, 2000). Winter skate removals have been estimated in Div. 4VsW by Simon and Frank (2000) for the period from 1989 to 1999. The highest estimated level of bycatch, based on a relatively small sample size, was 2193 t in 1990. Winter skate removals declined to 150 t in 1994 concomitant with a reduction in traditional landings by Canadian and foreign fisheries. Bycatch removals of winter skate have not been estimated for Div. 4X.

Winter skate is the target of directed fisheries. The pectoral fins of skate, called “wings”, are sold for human consumption in Canada and the U.S. with the bulk of the catch exported to markets in Europe and Asia (Simon and Frank, 2000; Sosebee and Terceiro, 2000). The directed fishery on the eastern Scotian Shelf is currently small. Since the establishment of a Total Allowable Catch (TAC) for this fishery in 1994 (2000 t), the TAC has declined to 200 t, a level that has been in place annually since 2002. The directed fishery in the U.S. began in the early 1960s.

Although bottom trawling for fish and dredging for scallops and clams may constitute a threat, its potential importance has not been evaluated for winter skate. Dredging and trawling might physically alter bottom habitat; it might also result in a re-suspension of bottom sediments that might smother spawning areas and damage gills (Messeih et al., 1991). However, given the preferred habitat of winter skate on sand and gravel, there may be a low probability that alteration of the bottom by trawling would affect adults. Although survival during the egg case stage may be affected, no studies have been undertaken to evaluate the effects of trawling on the survival of skate.

Information on skate predators is scarce, but they have been found in the stomachs of sharks, seals and other skate (Simon and Frank, 2000). Winter skate are also prone to infection by several invertebrate parasites, but no studies have been undertaken to determine whether this parasitism affects mortality.