Impact of anomalous ocean warming on the abundance and foraging habits of the California Sea Lion (<i>Zalophus californianus</i>) from the San Benito Archipelago in the Mexican Pacific

Abstract The California sea lion (Zalophus californianus; CSL) reflects environmental conditions of the Northeastern Pacific, where resource availability was importantly reduced for different taxa during climatic variations such as the North Pacific heatwave (The Blob 2014 to 2015) or El Niño 2015 to 2016 due to the effect of these anomalies on primary productivity. Our objective was to determine the abundance and foraging habits of CSLs from the San Benito Archipelago (Mexican Pacific) in a period (2013 to 2019) that involved warm anomalies. Annual counts were conducted by sex/age classes; scats were sampled to identify the main prey consumed; and pup fur (maternal foraging indicators) was collected for analysis of δ13C and δ15N. The prey importance index (PII) and trophic level were calculated and the Siber assessment in R was used to estimate isotopic niches. Colony abundance decreased by approximately 50% from 2013 to 2019. Values of δ13C showed significant interannual differences (P &amp;lt; 0.001) with more negative values (−17.2‰ to −16.7‰) from 2015 onwards, indicating offshore foraging habits. Significant interannual differences (P &amp;lt; 0.001) were found for δ15N, with a declining trend (20.4‰ to 19.07‰), and a difference of more than 1‰ between 2013 and 2019, evidencing a probable shift of foraging grounds toward higher latitudes. Based on the PII, Strongylura exilis (30%), Merluccius productus (24%), Ommastrephes bartramii (10%), and Sardinops sagax (10%) were the most important prey, showing significant interannual differences (P &amp;lt; 0.001). CSLs had a mean trophic level of 4.89, with a slight increasing trend toward the end of the period. Dietary plasticity was found in CSLs, possibly caused by environmental changes that modified prey availability and led to variations in foraging grounds, and consequently in the colony abundance over time.