Pelagic or benthic prey? Combining trophic analyses to infer the diet of a breeding South American seabird, the Red-legged Cormorant (Phalacrocorax gaimardi)

Abstract

The available information about the feeding habits and preferences of the Red-legged Cormorant (Phalacrocorax gaimardi) generally suggests that this near-Threatened South American cormorant is a sedentary benthic forager that also incorporates pelagic prey in its diet. In order to describe the dietary composition and assess the importance of certain prey types (pelagic vs benthic), we studied the diet of this cormorant on the Argentine Atlantic coast during four breeding seasons, using a combined technique of conventional diet assessment (pellets) and stable isotope analysis. Our results show that the Red-legged Cormorant forages mainly on pelagic and demersal-pelagic prey. Results of both techniques showed the main prey to be the Patagonian Sprat, a small high-energy-content pelagic forage fish, and the Patagonian Squid, a low-energy-content demersal-pelagic invertebrate. We also found an overall low prey diversity and important interannual variation for the main prey types, as well as variation between the different breeding stages. This study therefore contributes new and unambiguous information about the Red-legged Cormorant's use of trophic resources and suggests that the combination of a conventional technique with stable isotope analysis provides a solid framework for this seabird diet assessment.