Evaluating ecosystem responses to grey seal abundance changes in the southern Gulf of St. Lawrence

Northwest Atlantic ecosystems have been dramatically altered in recent decades; groundfish were overfished to low abundance and have largely failed to recover, while marine mammals, particularly pinnipeds, have increased rapidly in abundance and have emerged as top predators. These large-scale ecosystem changes have created considerable uncertainties in the assessment of wildlife populations and the development of sustainable management strategies, which typically assume that populations are driven by stationary processes. Pinniped culls have been proposed in some ecosystems, such as the southern Gulf of St. Lawrence, to improve groundfish survival. However, the response of groundfish to reduced pinniped abundance is difficult to predict because, in some ecosystems, the effect of pinnipeds on groundfish survival is unknown, whereas, in other ecosystems, groundfish productivity may be indirectly impaired by reduced pinniped abundance due to improved survival for pelagic species that compete with and/or consume young groundfish. Over three research chapters, I investigate nonstationarity in NW Atlantic ecosystems, particularly considering the effect of grey seal abundance reductions on prospects for groundfish recovery. First, I use age-structured models with time-varying parameters to demonstrate that the recovery of two groundfish populations on Georges Bank is impaired by high rates of adult natural mortality. Next, I develop an integrated assessment model for Northwest Atlantic grey seals (Halichoerus grypus) and demonstrate that the growth of this population has slowed in recent decades due to density-dependent reductions in juvenile survival. Finally, I develop a multispecies model of pinniped/groundfish/pelagic dynamics in the southern Gulf of St. Lawrence and demonstrate that the recovery of Atlantic cod (Gadus morhua) in this ecosystem is unlikely in the absence of strong and rapid reductions to the grey seal population. I did not find evidence of strong indirect effects on cod survival via pelagics, as seal predation only accounted for a minor portion of pelagic mortality. Taken together, these chapters highlight the difficulties in evaluating responses to pinniped culls and suggest that actions aimed at recovering groundfish may need to be taken before the effects are thoroughly understood.