Feeding by grey seals on endangered stocks of Atlantic cod and white hake

Abstract

High natural mortality is preventing the recovery of collapsed stocks of Atlantic cod and white hake in the southern Gulf of St Lawrence, Canada. Predation by grey seals has been proposed as an important cause of this high mortality. We determined the contribution of cod and hake to the diet of grey seals collected along the west coast of Cape Breton Island and in the Cabot Strait, an area where overwintering cod aggregate. Along the coast of Cape Breton Island, the contribution of hake and cod to the diet was 30 and 17%, respectively, by weight using stomach contents and 13 and 9%, respectively, based on intestine contents. In the Cabot Strait, when overwintering aggregations of cod were present, cod accounted for 68% (range 57-80%) of the male diet from stomachs, and 46% (range: 31-64%) of the diet determined from intestines. Among females, cod represented 14% (range: 0-34%)and 9% (range: 3-54%) of the diet from stomachs and intestines, respectively. InCabot Strait, white hake accounted for up to 17% of the diet by weight from stomachs, and up to 6% of the diet determined from intestines. The mean length of cod consumed by seals was 28 cm (SD = 8.6) along the coast of Cape Breton Island, and 39 cm (SD = 5.7) in Cabot Strait. The mean length of hake consumed by seals was 29 cm (SD = 7.0) along the coast of Cape Breton Island, and 35 cm (SD = 5.6) in Cabot Strait. Cod and hake are more important to the diet of males than that of females. The contribution of cod to the diet of grey seals foraging in the cod over wintering area is much greater than has been reported elsewhere.