Fatty acid signatures and stable isotopes as dietary indicators in North Sea seabirds

Fatty acid signatures (FAS) of plasma and stable isotopes of carbon (δ13C) and nitrogen (δ15N) of red blood cells were determined in northern gannetsMorus bassanus, great skuasStercorarius skua, shagsPhalacrocorax aristotelis, and common guillemotsUria aalgefrom colonies in the North Sea (collected 2002 to 2003) in order to compare foraging ecologies, and especially to assess the extent to which birds feed on demersal or pelagic prey. The biochemical markers in great skua and gannet indicated that these species feed at a relatively high trophic level, and high variance, especially in great skua, demonstrated either a wide range of food types, individual dietary specialisation or both. The biochemical markers suggested that demersal fish are important constituents of great skua and gannet diets, and thus changes in fisheries discard rates probably influence these populations. In contrast, clear pelagic characteristics and low variance in the markers showed that the diet of common guillemots and shags is pelagic and varies little in composition. Comparison with the reference FAS data for North Sea fish confirmed the dependence of common guillemots on few shoaling pelagic species of fish, probably mainly young sandeelsAmmodytes marinus.