Research Document - 2006/002

Estimated discards of winter skate (Leucoraja ocellata) in the southern Gulf of St. Lawrence, 1971-2004

By Benoît, H.P.

Abstract

Winter skate (Leucoraja ocellata) in the southern Gulf of St. Lawrence form a distinct population (designatable unit) that is currently considered to be endangered by the Committee on the Status of Endangered Wildlife in Canada. The causes of the decline in population status have not been established, but bycatch in fisheries targeting other species is believed to be an important contributing factor. The present document aims to contribute to better understanding the causes of decline and assessing the recovery potential of southern Gulf winter skate by providing estimates of bycatch levels for this population in groundfish and shrimp fisheries, over the period 1971-2004. Data from at-sea observers were used to this end. However, problems with the taxonomic identification of skates by those observers necessitated the use of empirical models to predict species identity when estimating discards. The statistical error associated these predictions, as well as with two other steps involved in inferring total discards from the subset of fishing trips that are covered by observers, was propagated to the overall estimates using non-parametric bootstrapping methods. The highest estimated median discard levels occurred during the early 1970s, at about two-thousand tonnes annually. Discard estimates decreased to generally under one-hundred tonnes by the early 1990s and under fifty tonnes in the 2000s. The results of three studies (one recent and two previously published) suggest that the mortality rate of these discarded skate is likely at least 50%.

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