Foraging Resource Partitioning in the California Sea Lion (Zalophus californianus) from the Southwestern Gulf of California

California sea lion (CSL, Zalophus californianus) abundance has declined in different localities across this species’ Mexican distribution. However, Los Islotes rookery in the southwestern Gulf of California (GoC) deviates from this pattern. It is vital to gather ecological knowledge of this CSL settlement and its surroundings to better understand its population in the GoC. This study aimed to determine the foraging habits of different CSL sex and age classes. Sixty-five CSL samples were collected in Los Islotes and its surroundings for stable isotope analysis (δ13C and δ15N). The data were analyzed using a hierarchical Bayesian model, and isotopic areas were estimated using the SIBER package in R. Our findings evidenced resource partitioning. Adult females had lower δ15N values than most classes, reflecting the regional 15N-enrichment of the GoC. Conversely, subadult males showed low δ15N values, carrying foraging information from the 15N-depleted Pacific Ocean into the GoC. Adult males presented the highest δ15N values (after pups), suggesting a higher trophic position than adult females and values corresponding to the GoC. Moreover, juveniles had the most negative δ13C values and the largest isotopic areas, indicating offshore foraging habits and a mixed consumption of maternal milk and their first prey. Pups showed the highest mean δ15N value due to maternal milk consumption, reflecting the mother’s δ15N value and their enrichment. Our findings suggest that segregation is explained by unique life history traits and a possible strategy to avoid potential competition