Research Document 2016/119

Recovery potential assessment of the Gulf of St. Lawrence Designatable Unit of Winter Skate (Leucoraja ocellata Mitchill), January 2016

By Swain, D.P. and Benoît, H.P.

Abstract

Winter Skate (Leucoraja ocellata) in the southern Gulf of St. Lawrence (sGSL) are distinct from those occurring elsewhere, maturing at a much smaller size and age. In 2005, the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) identified Winter Skate in the sGSL as a Designatable Unit (DU) and assessed the population as Endangered. In its 2015 assessment, COSEWIC revised the DU to include the northern Gulf of St. Lawrence (nGSL). New analyses indicate that Winter Skate occur more rarely in the nGSL than previously thought and that the few confirmed specimens are of the late maturing type. Thus analyses presented here are restricted to the sGSL. Winter Skate in the sGSL have continued to decline since the 2005 assessment. Based on survey data, biomass and adult abundance have declined by 99% since the early 1980s. Once widely distributed in shallow inshore waters, their distribution is now restricted to a small area of the Northumberland Strait. There has been no directed fishery for Winter Skate in the sGSL and bycatch in other fisheries has declined to very low levels (< 5 t since 2010). Discard mortality appears to be low, about 10% in the scallop fishery and 25 to 35% in fisheries for groundfish (declining to <10% since 2007). Based on population models, fishing mortality appears to be negligible, declining from 3% annually in the early 1970s to 0.25% since 2011 for adults, and even lower for juveniles. Natural mortality is estimated to have increased to extremely high levels for adult Winter Skate, from 10% annually during 1971 to 1977 to 63% annually during 1992 to 1998, averaging 57% annually since 1999. No trend in the natural mortality of juveniles was evident, with the median estimate fluctuating around 61% annually. Increased predation by Grey Seal appears to be an important cause of the current high adult natural mortality. A Limit Reference Point (LRP) corresponding to 40% of BMSY (biomass at maximum sustainable yield) was estimated and proposed as a candidate abundance recovery target. The probability that the population was below the LRP was estimated to be 100% since 1995. Under current productivity conditions, the population is projected to continue to decline and is almost certain to be extinct by mid-century, even with no fishery removals. The recent very low levels of fishing mortality have negligible impact on the population trajectory at the current levels of the other components of productivity, in particular adult natural mortality. With no fishing, population biomass would be expected to increase slowly if adult natural mortality were decreased by 80% and relatively rapidly if decreased by 85%.

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