Intracolony niche partitioning among California sea lions in the Gulf of California, Mexico

Background/Question/Methods

Many generalist populations contain subsets of individuals that specialize in different resources and habitats. To better understand the potential drivers of intrapopulation niche partitioning in generalist predators, we compared the number of distinct foraging strategy groups among adult female California sea lions (Zalophus californianus) at three breeding colonies in different parts of the Gulf of California - Isla Granito (n=52), Los Islotes (n=83), and San Jorge (n=81) - using stable isotope analysis (δ13C and δ15N) of fur collected from their suckling pups. We hypothesized that colonies with higher intraspecific competition will exhibit greater niche partitioning.

Results/Conclusions

Using a hierarchical cluster analysis, we identified six distinct foraging groups at Isla Granito (assignment accuracy=95.5% based on discriminant function analysis), which is a declining colony that is supported by low prey diversity. Los Islotes and San Jorge, which are both increasing or stable in abundance and have access to more diverse prey bases, contain two distinct foraging groups each (accuracy of assignment=96.3% and 98.1%, respectively). The preliminary results presented here are in line with our hypothesis. This initial analysis suggests that female sea lions in the GoC partition foraging resources over relatively small spatial scales; however, the degree of partitioning at each colony varies across the species' range. Characterizing this heterogeneity may facilitate conservation planning.