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

Abstract

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 (13C/12C) and nitrogen (15N/14N) 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 δ15N or δ13C were found according to the sex of individuals, but significant seasonal differences were found in δ15N, although not in the δ13C values. This suggests that pilot whales are resident year round in the Strait, a finding supported by independent photo-identification. The variation in δ15N 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 δ13C values were significantly different amongst the four stable social units sampled and individual δ13C values were significantly related to the level of inter-individual association, while no relationship was found for δ15N. 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.