Diet of the social groups of long-finned pilot whales (Globicephala melas) in the Strait of Gibraltar

The Strait of Gibraltar is inhabited throughout the year by a group of pilot whales (Globicephala melas), but their spatial distribution varies between Summer and Autumn. In this paper, we have used carbon (C-13/C-12) and nitrogen (N-15/N-14) stable isotope signatures to investigate the differences in diet amongst seasons, sex and stable social units. Skin samples were collected from 56 individually photo-identified pilot whales during Autumn 2005 and Summer 2006. These individuals were genetically sexed and their isotopic signature determined. The level of inter-individual association both within and between stable social units were compared to Euclidean distances between individual isotopes signatures. No differences in either delta N-15 or delta C-13 were found according to the sex of individuals, but significant seasonal differences were found in delta N-15, although not in the delta C-13 values. This suggests that pilot whales are resident year round in the Strait, a finding supported by independent photo-identification. The variation in delta N-15 could reflect a shift in pilot whale diet through the year, with pilot whales feeding at a higher trophic level in Autumn compared to Summer. This could also represent a change in the diet of pilot whale prey species. The delta C-13 values were significantly different amongst the four stable social units sampled and individual delta C-13 values were significantly related to the level of inter-individual association, while no relationship was found for delta N-15. These results suggest that within the same general area (i.e. the Strait of Gibraltar), there is some level of specialisation in habitat or prey choice between pilot whales social units.