Abundance, isotopic amplitude, and pups body mass of California sea lions (Zalophus californianus) from the Southwest Gulf of California during anomalous warming events

The California sea lion ( Zalophus californianus ) population in the Gulf of California has shown a significant abundance decline. However, its 13 breeding colonies have distinct individual dynamics modulated by regional environmental conditions. Thus, the relationship between environmental variability and population growth is complex and must be analyzed regionally. Los Islotes is the only colony that has steadily increased since the 1980s. The goal of this study was to determine the interannual changes in abundance and foraging habits (trophic breadth/habitat use) of adult females, and the interannual abundance and body mass of pups of the California sea lion colony from Los Islotes (Southwest Gulf of California) and its relationship with anomalous warming events. For this purpose, monthly population counts were carried out during the breeding seasons (2013–2018) and their interannual variability was analyzed. In addition, 152 pups were captured and weighted, and a small portion of hair was sampled to analyse the stable isotopes of nitrogen (δ 15 N) and carbon (δ 13 C) to obtain annual isotopic areas with the program R (SIBER routine). A 50–60% decrease in pups abundance was documented in warm years, such as 2014 and 2015, compared to 2013. Furthermore, pup body mass was negatively affected (by around 3 kg) during anomalous warming events in 2014 and 2018. The largest isotopic areas (3.18‰ 2 and 2.88‰ 2 ) were observed in 2016 and 2018, as a probable result of greater dispersion of adult females during their foraging trips. These results suggest adult females show adaptive responses to environmental variability. It is relevant to continue monitoring and understanding the dynamics of this colony, especially under current environmental change scenarios.