Feeding Ecology of Harbour Seals (Phoca vitulina) in Southern Norway

Abstract

In order to understand ecosystem dynamics and functions it is vital to have knowledge about relationships between predator and prey. Harbour seals (Phoca vitulina) are piscivorous marine mammals found at or near the top of marine food webs. Being mainly piscivores, harbour seals have the potential to come into conflict with fishermen and fisheries. Recently, as the Skagerrak and Kattegat population of harbour seals has increased, claims that seals are depleting the cod population have surfaced. Using scat sampling and otolith identification, harbour seal diet was estimated in four separate locations along the coast of southern Norway (Kragerø, Risør, Tvedestrand and Fevik). This is the first time harbour seal diet has been estimated in this area. The results showed that seal diet was overall varied and included 20 different species and species groups. The most important prey groups were gadids and flatfish. There were regional differences mainly between Fevik and the other locations: Fevik showing a lower species richness. It was possible to divide results from Tvedestrand into two seasons (winter/spring and summer/autumn). Both species richness and diet composition changed between seasons at this location. Fish length and weight estimates showed that seals generally prefer small fish below the smallest allowed catch size. This suggests that harbour seals do not compete on a large scale with local fisheries.