Research Document - 2006/003

Recovery potential assessment of 4T and 4VW winter skate (Leucoraja ocellata): biology, current status and threats

By Swain, D.P., J.E. Simon, L.E. Harris, and H.P. Benoît

Abstract

In May 2005, winter skate (Leucoraja ocellata) in the southern Gulf of St. Lawrence (sGSL, NAFO Div. 4T) and on the eastern Scotian Shelf (eSS, Div. 4VW) were designated as endangered and threatened, respectively, by the Committee on the Status of Endangered Wildlife in Canada. This report summarizes information on the biology, current status and potential threats to winter skate in these areas to aid in recovery planning.

Winter skate in the sGSL appear to be distinct from those in other areas, possibly reflecting character displacement in areas outside the Gulf where winter skate are sympatric with little skate (L. erinacea). Female winter skate are mature by 42 cm in the sGSL but 50% maturity occurs at 75 cm on the eSS. In summer, winter skate occur in shallow inshore areas in the sGSL and on the offshore banks on the eSS.

Based on catch rates in the summer/fall research vessel (RV) surveys, adult winter skate have declined by 96% since 1971 in the sGSL and by 90% since 1970 on the eSS. There is no indication from the RV survey catch rates that the declines in adult abundance have ceased. A spring industry survey of the eSS, initiated in 1995, shows declines in catch rates similar to the declines in the July RV survey of the eSS. Declines in catch rates in the September RV survey of the sGSL may overestimate population declines if optimal habitat is inshore of the survey area. However, sharp declines in abundance have also occurred in these inshore areas, based on catch rates in an inshore survey conducted since 2000.

Landings of winter skate have been negligible in the sGSL. Median estimates of discarded bycatch in groundfish and shrimp fisheries in the sGSL exceeded 1000 t in most years in the 1970s but declined to low values (<50 t) in recent years. Winter skate are also likely caught in the scallop fishery in the sGSL but the magnitude of this bycatch is unknown. Reported catches by foreign fleets on the eSS averaged 1061 t annually in 1970-1993, 77 t in 1994-1999, and have been negligible since then. Landings in the directed skate fishery on the eSS declined from 2045 t in its first year (1994) to <300 t annually since 2001. Estimated annual discards by domestic groundfish fleets on the eSS averaged 1461 t in 1970-1992 and declined from 350 t to 50 t from 1993 to 2004.

Large demersal fishes, potential predators of small fishes, declined in both the sGSL and eSS in the 1980s. Seal abundance has been increasing in both systems since the 1960s.

Estimates of catchability and availability of winter skate to the RV surveys are given in the appendices. Conclusions of the Recovery Potential Assessment meeting (21-23 November 2005), based on the material in this and a companion document on population modelling, are summarized.

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