Predator-Prey Relationships and the Continued Decline of Atlantic Cod in the Southern Gulf of St. Lawrence

Atlantic cod (Gadus morhua) populations in eastern Canada have been exploited for hundreds of years. Most of these populations collapsed in the early 1990’s and have failed to recover. In contrast, the abundance of grey seals in Atlantic Canada has increased exponentially over the past 55 years. We examined the role that predation by grey seals may play in the lack of recovery of a cod stock in the southern Gulf of St. Lawrence that has negligible fishing mortality, but high natural mortality of adults. We used a cod population model incorporating a functional response of grey seals preying on cod to examine the contribution of grey seal predation to the elevated natural mortality of cod. We found that predation by grey seals could account for increases in natural mortality that are not attributable to other sources if adult cod comprise 30% of the average grey seal diet. We also used the model to project cod abundance forward under several grey seal reduction scenarios. Our results suggest that grey seal reductions from current levels are required to avoid the continued decline of cod from the southern Gulf of St. Lawrence.