Prey preference and niche overlap of ringed seals Phoca hispida and harp seals P. groenlandica in the Barents Sea

Abstract

Ringed seals Phoca hispida and harp seals P. groenlandica were collected for diet analysis along the ice edge in the Barents Sea to investigate possible niche overlap between these 2 seal species. The diet analysis is based on contents from stomachs and intestines. A resource availability survey was conducted concurrently based on an echo survey combined with demersal and pelagic trawling. This survey showed that the potential prey biomass was dominated by pelagic crustaceans (99% of total biomass) -principally Themisto libellula and Thyssanoessa spp. Despite the prevalence of these crustaceans both seal species showed a strong preference for fish of various species, which constituted only 1 % of the biomass in the area. The most common fish in their diet was polar cod Boreogadus saida, which had a Manly's prey preference index of 0.87 for ringed seals and 0.42 for harp seals. Pianka's niche overlap index for the 2 seal species in this area was 0.985, indicating an almost complete niche overlap. However, harp seals prey on significantly larger polar cod than ringed seals, and the larger cod were distributed in deeper water than smaller cod. Thus it appears that the 2 seal species exploit different fractions of the same resource.