Influence of El Niño-Southern Oscillation on the population structure of a sea lion breeding colony in the Gulf of California

• ENSO phenomenon causes changes in the population structure of California sea lion.
• Population structure is a good indicator of the demographic conditions.
• Residuals between model and census data are descriptors of environmental effects.
• Male groups are more susceptible than female groups to the ENSO effects.
• The BEST index is a good descriptor for the ENSO influence in La Paz Bay.

The El Niño-Southern Oscillation (ENSO) phenomenon has a significant influence on Pacific marine ecosystems from primary trophic levels to top predators that cause fluctuations in their populations. Based on this fact we analyzed the sea lion Zalophus californianus population structure variability using censuses performed from 1979 to 2004 in Los Islotes breeding colony located at La Paz Bay as well as concomitant the ENSO phenomenon variability. To discriminate variations in the population structure not ascribable to the population attributes, a virtual population was created and compared to the census population. The residuals obtained from this comparison were correlated with the MEI (Multivariate ENSO Index) and BEST (Bivariate ENSO Time Series) indices, descriptors of the ENSO variability. The results showed that the population structure is an adequate descriptor of the conditions of the population instead of the abundance, and the total population is affected by the ENSO. The adult, subadult and pup male groups were the most sensitive groups to this phenomenon due to their intrinsic development and behavior. Likewise the BEST index is a better descriptor than the MEI index of the ENSO influence in the region where the breeding colony Los Islotes resides. Therefore we demonstrate in this work that changes caused by the ENSO not directly affect the sea lion due to its homeothermic capacity but affects the habitat where this organism performs its biological functions, producing behavioral changes in the population.