Grey seal predation on the eastern Scotian Shelf: modelling the impact on Atlantic cod

Abstract

We modelled trends in the predation mortality of the Atlantic cod (Gadus morhua) stock on the eastern Scotian Shelf, Canada, by grey seals (Halichoerus grypus) and the potential effects of this mortality on cod recruitment over the period 1970n1994. Two predation models were explored: one in which the fraction of cod in the diet was constant and another in which the fraction of cod was proportional to cod abundance. The number of grey seals in the study area, from the Gulf of St. Lawrence and Sable Island populations, increased from about 5000 in 1970 to 64 000 in 1994, while total food consumption increased from 10 000 t·year n1 in 1970 to 121 000 t·year n1 . Under the constant ration model, grey seals consumed about 15 400 t of cod in 1994 compared with about 6300 t under the proportional ration model. Grey seal predation mortality was only 10n20% of estimated fishing until the fishery was closed in 1993, indicating that grey seals were not a major factor in the recent collapse of this stock.