Abstract: The San Benito Islands in Mexico host a population of about 7,000 California sea lions (Zalophus californianus) and have been recolonized by Guadalupe fur seals (Arctocephalus townsendi) since 1997. Due to similarities in natural history between the two species, we undertook a study to determine their feeding habits, measure diversity of their diets, examine trophic feeding level and overlap as indicators of competition, and estimate ability to adjust to changes in prey availability. During winter and summer 2001 and 2002, 289 sea lion scats and 218 fur seal scats were collected. To identify prey species, samples were sieved to recover otoliths and cephalopod beaks. A total of 1,495 structures were recovered from the sea lion scats: 83.8% otoliths and 16.2% cephalopod beaks. The most prevalent prey was in fish species (Argentina sialis, Merluccius angustimanus, and Sebastes spp.) and the squid (Loligo opalescens). Of the 1,866 structures recovered from the Guadalupe fur seal scats, 95.6% were cephalopod beaks and 4.4% were otoliths, with L. opalescens as the most prevalent prey. The diversity of the trophic spectrum (H') of the sea lion was greater than the fur seal in every one of the samples, placing it as a "generalist predator" (Levins Index B = 4.65) in comparison to the fur seal (B = 1.53). The only significant trophic overlap (Morisita-Horn Index) occurred during the summer of 2001 (CH = 0.73). Both species consumed prey at similar trophic levels (sea lion = 4.42; fur seal = 4.22), which placed them as secondary-tertiary carnivores. The evidence suggests that the California sea lion forages in both benthic and pelagic habitats, resulting in a broader feeding spectrum and better adaptations to cope with changes in prey availability than the Guadalupe fur seal.


Document Type: Research article

DOI: 10.1578/AM.33.3.2007.315

Page Numbers: 315 - 326

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