The role of harp seals, fisheries and food availability in driving the dynamics of northern cod

The Atlantic cod Gadus morhua population off Newfoundland collapsed in the early 1990s due to over-exploitation, and despite marked reduction in fishing effort the stock remains depressed. Harp seal Pagophilus groenlandicus predation has been repeatedly proposed as an explanation for this lack of recovery, but other hypotheses include reduced prey availability and/or food quality (i.e. lack of capelin Mallotus villosus), as well as fisheries catches and environmental effects. Using a bioenergetics-allometric model, we gauged the relative contributions of these drivers on the lack of recovery and dynamics of the northern cod stock. Biomass dynamics were best explained by a combination of fisheries removals and capelin availability, whereas seal consumption was found not to be an important driver of the northern cod stock. Prey availability was linked to reduced somatic condition during the 1990s and 2000s. We discuss evidence that suggests that cod may be experiencing depensatory dynamics, but not related to a 'predator pit effect'. Our study suggests that trophic control is bottom-up, and that a depressed capelin stock could be a serious impediment for cod rebuilding.