Habitat and resource segregations of two sympatric seals in the North Sea

In the North Sea, sympatric grey and harbour seals may compete for food resources impacted by intense fishing activities and a recent increase of seal populations. In order to reduce inter-specific competition, sympatric species must segregate at least one aspect of their ecological niches: temporal, spatial or resource segregation. Using isotopes and Se and Hg concentrations, the foraging resources of grey seals and harbour seals and the potential competition between these species in the North Sea was studied. δ¹³C, δ¹⁵N and δ³⁴S values were combined with the concentrations of Hg and Se in blood samples of harbour and grey seals from the North Sea. Blood samples were collected on 45 grey seals and 37 harbour seals sampled along German and Scottish coasts. This multi-tracer approach showed spatial and resource partitioning within grey and harbour seals. Data indicated the offshore foraging distribution of grey seals as reflected by the lower δ¹⁵N values and T-Hg concentrations and higher Se concentrations, and the inshore foraging distribution of harbour seals because of higher δ¹⁵N values and T-Hg concentrations and lower Se concentrations. The SIAR mixing model revealed a more selective diet of grey seals compared to harbour seals, and the importance of sandeels in grey seal diet reflected by their high δ³⁴S values. Lastly, diet ellipse overlaps between grey seals and harbour seals sampled along the German coasts suggested a potential sharing of food resources, possibly due to the increase number of grey seals number in this area during the foraging season - all year except breeding and moulting periods. The multi-tracer approach provided a more robust discrimination among diet resources and spatial foraging distributions of grey seals and harbour seals in the North Sea.