Comparing the diet of cod (Gadus morhua) and grey seals (Halichoerus grypus): an investigation of secondary ingestion

Abstract

Otoliths from the stomachs of 138 by-caught grey seals (Halichoerus grypus) from the west coast of Ireland were compared with otoliths from the stomachs of 364 commercially caught cod(Gadus morhua) to determine if there were any overlaps in type, number and size of prey that might be attributable to secondary ingestion. A total of 19 species/groups were common to both cod and seal stomachs accounting for 99·6% and 95·8% of the otoliths from cod and seal stomachs respectively. There were significant differences between the otolith/fish lengths of all six species/groups compared but there were overlaps in the size distributions. Analysis of the diet composition of the cod stomachs suggested that larger cod consumed mainly fish and smaller cod consumed mainly crustaceans. Cod and seals were utilizing the same fish prey but the seals were generally consuming larger fish. Overlaps between the size distributions of prey species/groups suggest that secondary ingestion was possible and should be considered in future seal diet studies.