Research Document - 2011/080
Status of Smooth Skate (Malacoraja senta) and Thorny Skate (Amblyraja radiata) in the Maritimes Region
By J.E. Simon, S. Rowe, and A. Cook
The status of smooth and thorny skate within the Maritimes Region was reviewed by examining data from Department of Fisheries and Oceans (DFO) research vessel (RV) surveys, United States (US) RV surveys, Canadian industry science surveys, observer information from directed fisheries, and fisheries landings. These data suggest that smooth skate were broadly distributed throughout the Maritimes Region in the 1970s with persistent areas of concentration. In the 1980s and 1990s, abundance fell across the Scotian Shelf and has continued to decline in Divs. 4VW. In Div. 4X, abundance has increased in the 2000s, primarily driven by an increase in the number of juveniles. The decline in abundance over the whole Scotian Shelf was 80% for mature individuals while the decline when all lengths are considered was 58%. Smooth skate are uncommon on Georges Bank with only scattered reports on the edges of the northeast peak area of the Canadian Zone. Seasonal US RV surveys suggest that these observations are simply the southern edge of a population of smooth skate that are found throughout the Gulf of Maine. These seasonal US surveys provide contradictory evidence with slight decreases or increases in abundance depending on the survey. These data also suggest that thorny skate were widespread across the Scotian Shelf and Bay of Fundy with the highest concentration in Div. 4V and the Bay of Fundy prior to 1990. Since the 1990s, there has been a dramatic reduction in the distribution of thorny skate on the central Scotian Shelf and the concentrations in the east and west are much reduced. Industry Science surveys suggests that there is still significant connectivity between these areas. An examination of the length frequencies collected during the Summer RV Survey indicates a progressive loss of the largest individuals in the population so that there are very few fish greater than 53 cm caught by the survey. The decline in abundance across the shelf of mature fish is 96%, while the decline when all lengths are considered is 82%. While these declines have occurred, recruitment of fish less than 21 cm has remained steady since 1970. Thorny skate are distributed on Georges Bank primarily on the edges of the northeast peak of the bank and in the deeper waters north of the Great Southwest Channel. Their distribution is similar to that observed for smooth skate but they are also observed in slightly shallower waters as well. As for smooth skate, the US RV surveys suggest that thorny skate are found throughout the Gulf of Maine and that the distribution observed during the Canadian survey in this area simply reflects the southern edge of this population. These two surveys also indicate that there is no separation in the distribution between Brown’s and Georges Bank. The decline in mature abundance for all three surveys is similar to that observed on the Scotian Shelf while overall abundance has declined between 67 to 92%. There are no directed fisheries for smooth or thorny skate on the Scotian Shelf: although these species are caught as bycatch in other fisheries. In Div. 4X, an examination of these other fisheries suggests that annual smooth and thorny skate bycatch was relatively stable at approximately 450 and 1,750 t respectively from 1970 to 1992. Bycatch estimates declined for both species in the 1990s as catches in the traditional cod, haddock, pollock, and flatfish fisheries were reduced. In the last decade, removals have been less than half of what had been taken previously. In Divs. 4VsW, bycatch of smooth skate prior to 1992 was approximately 450 t and declined subsequently to 20 t per year due to the closures of the cod and haddock fisheries as well as changes in the silver hake fishery. Thorny skate bycatch was in the order of 4,500 t and declined to approximately 170 t over the same period. The response by each species to these levels of bycatch is not well understood. Collectively, these data suggest that in Divs. 4VW, smooth skate abundance is currently low but threats have been reduced to near zero and recruitment is increasing. In Div. 4X, smooth skate abundance has been increasing over the last 15 years at the same time that removals have decreased by about half. Recruitment to the population has been increasing over the same period on both the eastern and western Scotian Shelf. Although the reduction in abundance in Divs. 4VW has resulted in some fragmentation in the population distribution, there is no evidence to suggest that the Scotian Shelf may comprise more than one designatable unit. These data also suggest that thorny skate abundance, especially mature abundance, is currently very low on the Scotian Shelf and Georges Bank area. These declines are continuing on the eastern Scotian Shelf despite dramatic reductions in fishing effort and steady recruitment. The increase in the local grey seal population and its possible affect on the recovery of thorny skate or smooth skate have not been examined in this paper. In Div. 4X, thorny skate abundance has continued to decline despite the reduction in removals by greater than half and steady recruitment over the last 15 years. Industry surveys provide evidence that thorny skate continue to be distributed across the Scotian Shelf likely reflecting a single designatable unit.
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