Analysis of California sea lion (Zolophus californianus) feeding habits using stable isotopes: A potential record for El Niño occurrences

Climate change poses the critical question of how species will alter distribution patterns in response to environmental warming; a clear answer will be required to improve long-term planning for the protection of threatened and endangered species. The California sea lion (Zalophus californianus) has a growing population while close congeneric relatives, the Galapagos sea lion (Zalophus wollebaeki) and the Japanese sea lion (Zalophus japonicus), are vulnerable and extinct, respectively. Increased conflict with fisheries requires improved management and better projections of population response to climate change. Here, stable isotope analysis (δ¹³C δ¹⁵N) of annual growth layers in teeth is used to examine variations in foraging patterns and their connection to short-term warming events, such as El Niño. Results indicate that ontogenetic shift associated with weaning is the most marked change in foraging pattern common to all animals, although such data cannot be used to definitively indicate environmental events such as El Niño. Furthermore, responses for adults could be attributable to facultative feeding behavior, individual variability, sub-adult male migration, or combinations of these factors. Nevertheless, in some individual sea lions, strong El Niño events seemed correlated with a decline in primary productivity (δ³C) as well as average trophic position (δ¹⁵N), whereas weaker El Niño events were correlated with a decline in primary productivity with an increase in average trophic position in certain animals. Illustrating how California sea lion populations may respond to warming during short-term El Niño events has the potential to provide insight for predicting responses of this species to different warming scenarios and for informing policy decisions about risks to species presented by long-term population management in the context of global climate change predictions.

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