Predation By Grey Seals Prevents Recovery of Atlantic Cod in the Southern Gulf of St. Lawrence: An Emergent Allee Effect

The population of Atlantic cod in the southern Gulf of St. Lawrence collapsed in the early 1990s due to overfishing. Fishing mortality has been at negligible levels since then, but the population has continued to decline. This lack of recovery results from dramatic increases in the natural mortality of adult cod. Among the potential causes for this high mortality, only the hypothesis that it reflects increased predation by grey seals received strong support. Grey seal abundance has increased roughly 40-fold since 1960. Grey seals feed intensively on cod aggregations in the southern Gulf and appear to prey preferentially on the large cod with high natural mortality. Grey seals and cod co-existed in the Northwest Atlantic in earlier centuries when both occurred at high abundance. The current high natural mortality of southern Gulf cod appears to represent an emergent Allee effect, resulting from the recovery of grey seal abundance at a time when cod abundance is at exceedingly low levels. Even in the absence of fishing, recovery of the southern Gulf cod population is not possible under current ecosystem conditions.