Fish consumption by invading harp seals off the Norwegian coast in 1987 and 1988

Abstract

A large number of harp seals invaded the Norwegian coast in 1987 and 1988, when approximately 80 000 animals were drowned in the gillnet fisheries. The diet had the same composition and geographical variation as described for the previous invasion in 1902 and 1903, i.e. a dominance of cod and capelin in Finnmark, saithe in the Norwegian Sea, and herring in the Lofoten fjords. A physiological model is developed to study the sensitivity to variation in those parameters which are poorly known. An average seal is estimated to have consumed 4 ± 1 kg day ⁻¹ on the meagre diet (4.5% fat) during the invasion. In 1987 the harp seals consumed 215 000 ± 50 000 tonnes off Norway. Our estimate of the impact on the 1985 year-class of cod and the 1985 and 1986 year-classes of saithe is 110 000 000 ± 25 000 000 individuals eaten. This order of predation agrees well with the observed sudden decline of these three year-classes.