Stomach content analysis: Comparison amongst different sources of sampling in the study of the diet of sea lions in Argentina

The South American sea lion, Otaria flavescens, has proven to be an excellent biological sampler of the marine biodiversity since it is possible, through the examination of its diet, to determinate the distribution and abundance of marine resources in the northern Patagonian ecosystem. There are different methods to analyze the diet of pinnipeds, such as the use of scats, stomach contents, fatty acids and stable isotopes. The aim of the present study was to compare the type of information obtained from stomach contents analysis according to different sources of sampling (1- stomach contents from animals found dead on the coast, 2- natural regurgitations and 3- stomach lavage of previously anesthetized individuals). Between 2005 and 2022 a total of 48 stomach contents of O. flavescens were obtained at the sea lions rookeries of Punta Bermeja (41°09′S, 63°05′ W), Promontorio Belén (41° 09′S; 63° 48′W) and Caleta de los Loros (41°02′ S 64°10′ W) (San Matías gulf, Argentina). These three sources of sampling provided different dietary information. Stomach contents from dead animals presented more diverse prey than those coming from the two other sources. Overall, the fish species Raneya brasiliensis was the main prey item found, followed by cephalopods and crustaceans, coinciding with previous dietary reports based on scat analysis. The results obtained suggest that scat analysis is still the most appropriate method for monitoring the diet of sea lions on a temporal and spatial basis, being stomach content analysis a good complementary source of information.