GREY SEAL PREDATION ON THE EASTERN SCOTIAN SHELF - MODELING THE IMPACT ON ATLANTIC COD

We modelled trends in the predation mortality of the Atlantic cod (Gad us 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 1970-1994. Two predation models were expl ored: one in which the fraction of cod in the diet was constant and an other 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. Lawre nce 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(-1) in 1970 to 121 000 t . year(-1). 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 10-20% 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.