The roles of capelin, climate, harp seals and fisheries in the failure of 2J3KL (northern) cod to recover

The Atlantic cod (Gadus morhua) fishery sustained the way of life of Newfoundlanders during centuries. Its collapse in the early 1990s represented a socio-economic and cultural crisis. Despite the fishing moratorium imposed in 1992, the northern cod stock (NAFO Divs 2J3KL) is still a shadow of its former self. Several hypotheses have been offered to explain this lack of recovery, many involving ecological interactions between harp seals (Pagophilus groenlandicus) and cod. However, these contentions are based on indirect evidence, while formal assessments are lacking. Availing of long term data series, I addressed the two simplest ways in which harp seals might impede cod's recovery: predation and competition for food resources, particularly for the major forage fish in the system: capelin (Mallotus villosus). I found no supporting evidence for either hypothesis. The biomass dynamics of northern cod during the period 1985-2007 were driven by an interplay between fisheries removals and food (capelin) availability, and potentially affected by water temperature. Harp seal predation was not an important driver of the stock. I provide evidence that a regime shift occurred on the Newfoundland and Labrador Shelf ecosytem during the early 1990s, linking community reorganization to physical perturbations. The capelin stock is regulated by bottom-up processes: ocean climate adn the regime shift, acting via food availability. I found also that Atlantic cod did not change its diet following the regime shift. In addition, the diet of Atlantic cod overlapped to a greater extent with the diet of Greenland halibut than with harp seals' diet. These findings constitute mounting evidence against the consumptive competition hypothesis. The results presented in this thesis are important for the development of ecosystem approaches to fisheries management and raise questions as to the potential impacts of climate change on the structure and productivity of the Newfoundland and Labrador Shelf marine ecosystem. Also, this suggests that reductions in the harp seal population are unlikely to enhance the rebuilding of the northern cod stock; management efforts should be focused on protecting cod itself and the lower trophic levels