Trophic level and overlap of sea lion (Zalophus californianus) in the Gulf of California, Mexico

Abstract

Stable isotope and scat analyses were used in concert to determine trophic level and dietary overlap among California sea lions from different rookeries in the Gula of California. Isotopic analysis of the fur of sea lion pups revealed differences in δ15N and δ13C values among rookeries during the breeding season. Mean δ15N and δ13C values varied from 20.2‰ to 22.4‰ and from −15.4‰ to −14.0‰, respectively. The pattern of differences among rookeries was similar between years in most cases. Isotopic variations among rookeries were associated with differences in prey consumption. There was a significant correlation between δ15N value and trophic level, as determined by scat analysis. Joint application of isotopic and scat analyses allowed us to identify how the feeding habits of sea lions vary with location. Our results suggest the presence of spatial structure in available prey as well as the localized use of prey by sea lions across the Gulf of California