Science Response 2010/009

Estimated bycatch mortality of winter skate (Leucoraja ocellata) in the southern Gulf of St. Lawrence scallop fishery (2006 to 2008)

Context

Winter skate (Leucoraja ocellata) in the southern Gulf of St. Lawrence was designated as endangered by the Committee on the Status of Endangered Wildlife in Canada in May 2005. A recovery potential assessment (RPA) concluded that relatively elevated adult mortality was responsible for the previous and ongoing declines in abundance (DFO 2005; Swain et al. 2006a,b; 2009). There has been no directed fishery for winter skate and estimated bycatch in groundfish and shrimp fisheries has been very low. There is considerable spatial-temporal overlap between the scallop fishery and the distribution of winter skate, however, the impact of discarding in the southern Gulf of St. Lawrence scallop (Placopecten magellanicus) fishery on winter skate mortality could not be quantified at the time of the RPA due to a lack of information on incidental catches (Swain et al. 2006b). In response, an at-sea sampling project aboard commercial scallop fishing vessels was conducted during the 2006 to 2008 scallop fishing seasons (Benoît et al. 2010). The goal was to estimate the amount of winter skate captured in the fishery, as well as to assess the potential survival rate of discarded skate.

Subsequently, DFO Fisheries and Aquaculture Management (FAM) requested advice on the contribution of discard mortality to overall adult winter skate mortality in the southern Gulf of St. Lawrence. Specifically, FAM requested that Science, “estimate the following quantities for the southern Gulf scallop fishery, within the limits of available data: the number and biomass of juvenile and adult winter skate captured annually, the potential survival rate of discarded winter skate, and the annual exploitation rate of bycaught winter skate”. FAM also requested that Science, “evaluate the estimated exploitation rate in light of other sources of mortality and evaluate the degree to which bycatch mortality in the scallop fishery may be impeding recovery”.

The Special Science Response review was held on April 6, 2010. In summary, the percentage of the winter skate population killed annually in the southern Gulf of St. Lawrence scallop fishery was about 0.14% for juveniles and 0.07% for adults (Benoît et al. 2010). In contrast, the estimated percent of the population that dies annually other than from bycatch mortality in the southern Gulf scallop, groundfish and shrimp fisheries is 75% for juveniles and 34% for adults.

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