Research Document - 2011/033

A risk analysis of the potential effects of selective and non-selective reductions in grey seal abundance on the population status of two species at risk of extirpation, white hake and winter skate in the southern Gulf of St. Lawrence

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

Abstract

Winter skate (Leucoraja ocellata) and white hake (Urophycis tenuis) both have populations that are endemic to the southern Gulf of St. Lawrence (sGSL). The population of winter skate may actually constitute a distinct, yet to be described, species. Elevated adult natural mortality (M) in both species is resulting in population declines, to the point that extirpation is possible within a few decades. Based on weight of evidence, predation by grey seals appears to be contributing significantly to the elevated M. However, existing grey seal diet estimates cannot be used reliably to directly determine if this is the case. Evidence for a predation effect is therefore largely indirect. Stochastic projections using models for the sGSL winter skate and white hake populations were used to determine the probable population trajectories under a number of scenarios including the status quo, increased predation from projected increases in grey seal abundance and grey seal population reductions. Three scenarios for grey seal removals were evaluated: 1) untargeted removals when seals are aggregated in the breeding colonies, 2) targeted removals of seals foraging in the winter skate or white hake ecosystem and 3) targeted removals of seals feeding in aggregation areas for the fish species. Based on simulations of hypothetical scenarios in which we assumed, among other things, that grey seals contribute considerably to adult fish M, we predicted that to have a 50% chance of stabilizing the abundance of winter skate at current low levels requires removing 174,300, 44,300 or 1,700 seals under removal scenarios 1, 2 and 3 respectively. Likewise, to increase age 3+ white hake abundance by 2020 to levels observed in the past decade would require removing 319,000, 79,000 or 13,000 under removal scenarios 1, 2 and 3 respectively. A risk analysis framework is then presented to evaluate the possible consequences of undertaking, or not, a grey seal population reduction, relative to whether or not grey seals actually are the main contributors to elevated fish M. Overall, ecological risks concerning white hake, winter skate and grey seals appear to be reduced by undertaking a reasonable reduction of grey seals in the NW Atlantic. Such reductions would pose minimal conservation risks to the grey seal population. For the fish, at best, reduced adult M will halt population decline and possibly promote recovery. At worst, the sGSL white hake and winter skate populations would be extirpated more rapidly than they would otherwise be.

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