Research Document - 2002/054

Estimated Consumption of Atlantic Cod (Gadus morhua) and some Other Prey by Grey seals (Halichoerus grypus) and Harp seals (Phoca groenlandica), in the Southern Gulf of St. Lawrence (NAFO Division 4T).

By M. O. Hammill, and G.B. Stenson

Abstract

Consumption of Atlantic cod, Atlantic herring and some other species by seals in the southern Gulf of St. Lawrence (NAFO Division 4T) was estimated for the period 1975-2001. Estimates were obtained by combining information on abundance, energy requirements, diet composition and the distribution of animals. Consumption of Atlantic cod, and Atlantic Herring by grey seals in NAFO Division 4T is in the order of 5,700 t (SD=3,400), and 4,800 t (SD=2,500) respectively. Harp seal consumption based on diet samples from the Magdalen Islands and the St. Lawrence estuary is around 60 tonnes (SD=180), but sampling is limited to the winter months when cod are absent from the Gulf. Cod consumption by harp seals could be much higher but estimates are based on a key assumption that diet composition is similar to harp seal diet composition along the west coast of Newfoundland. Current estimates differ from earlier studies by attempting to incorporate variability in population estimates, seal distribution, and diet composition in our model. Harp seals appear to feed primarily on cod less than 31 cm in length, but variability in prey length in this study and elsewhere have been observed. Better diet sampling and information on the spatial distribution of seals will improve estimates of consumption.

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