Research Document - 2012/033

Smooth skate (Malacoraja senta) in the southern Gulf of St. Lawrence: life history, and trends from 1971-2010 in abundance, distribution and potential threats

By D.P. Swain, H.P. Benoît, and É. Aubry

Abstract

An assessment by the Committee on the Status of Endangered Wildlife in Canada of the status of smooth skate (Malacoraja senta) in Canadian waters in terms of its risk of extinction is scheduled for 2012. This document presents information on life history traits, trends in the abundance and distribution of this species, and threats to its persistence in the southern Gulf of St. Lawrence. A bottom-trawl survey, conducted each September since 1971, is the main source of information on the abundance and distribution of smooth skate in this area. Smooth skate occur in relatively deep areas of the southern Gulf, with highest densities occurring between depths of 150 and 300 m. There is little geographic segregation evident between length classes of smooth skate, and little seasonal variation in their geographic distribution in this area. There has been no long term trend in the area occupied by adult smooth skate (defined here as skates ³ 48 cm in total length) over the 1971-2010 period. The area occupied by juveniles was smaller in the late 1970s and early 1980s than it has been since the late 1980s. Trends in abundance also differ between juvenile and adult skates. Survey catch rates of juveniles tended to be relatively high in the 1990s compared to the period from the mid 1970s to the early 1980s. In contrast, survey catch rates of adults declined from relatively high levels in the 1970s to very low levels in the early 2000s, with an increase in recent years. The above average abundance of juveniles throughout much of the 1990s and 2000s despite below average spawner abundance suggests that juvenile mortality was relatively low during this period. The below-average adult abundance throughout much of the 1990s and 2000s despite strong recruitment suggests that adult mortality was relatively high during this period. The two main threats to smooth skate in the southern Gulf are likely fishing and predation. There are no directed fisheries for skates in the southern Gulf but skates are incidentally captured in fisheries targeting other species. Most skates are discarded at sea. From 1991-2010, discards comprised on average 95% of total estimated smooth skate catches. Estimated annual discards of smooth skate declined from a peak of around 180 tonnes in the early 1990s to about 25 t in the late 2000s. This decline is likely due in part to the reduction in fishing effort in the southern Gulf over this period. The apparent low mortality of juveniles in the 1990s and 2000s may reflect release from predation following declines in the biomass of large demersal fish in this area beginning in the early 1990s. The apparent high mortality of adults in the 1990s and 2000s despite decreasing fishing effort may be partly due to increased predation by the growing grey seal herd in this area.

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