Research Document - 2011/035

Grey seal reduction scenarios to restore the southern Gulf of St. Lawrence cod population

By D.P. Swain, H.P. Benoît, and M.O. Hammill

Abstract

Natural mortality (M) of older (ages 5+ yr) cod in the southern Gulf of St. Lawrence has been unusually high throughout the 1990s and 2000s. Examination of the evidence for a suite of hypotheses for the causes of this elevated M leads to the conclusion that predation by grey seals is the most likely cause of a major component of this high M.  However, due to data gaps, it is not possible to specify the probability that a given level of grey seal removals will result in a given level of reduction in cod M. Here we describe the effect of grey seal reduction on cod recovery given two alternate approaches for filling data gaps. Because many other approaches are possible, these results should be interpreted as ‘what-if’ scenarios regarding the impacts of seal removals on cod productivity given a range of estimates for the contribution of predation by seals to cod M. Based on one approach, predation by grey seals is estimated to account for only about 10% of the current M of 5+ cod. Given this estimate, seal removal cannot reduce M to a level that would permit cod recovery because M due to other unknown causes is too high to allow recovery. A second approach yields results more consistent with the weight of evidence that seal predation is an important component of cod M. This approach attributes about 50% of the current M of 5+ cod to predation by grey seals. In this case, seal removal can reduce M to a level that would permit cod recovery, though the necessary seal removals would be substantial. A lower seal reduction would be sufficient if grey seals show diet specialization and it is possible to target cod specialists. Based on a qualitative analysis of risk, grey seal reduction would appear to reduce ecological risk to the southern Gulf cod population without jeopardizing the security of the grey seal population.

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