Stable isotopes indicate differing foraging strategies in two sympatric otariids of the Galapagos Islands

The feeding behavior of marine predators varies with the level of competition to which they are exposed. Populations living in the same or overlapping geographic regions (sympatric) are generally subject to inter-specific competition, which can lead to the development of differing trophic strategies that maximize both nutritional and reproductive efficiency. The otariids of the islands in the western Galapagos Archipelago represent appropriate subjects for studying the response mechanisms of sympatric species exposed to strong competition, both trophic and spatial. The present study has focused on evaluating the possible differences in the trophic niches of the Galapagos sea lion (Zalophus wollebaeki) and the Galapagos fur seal (Arctocephalus galapagoensis) over time, based upon the analysis of stable isotope ratios of carbon and nitrogen (δ ¹³C and δ ¹⁵N), and observation of both species at sea. A comparative analysis of the isotopic signal of skin samples from pups of both species was performed for rookeries on Fernandina Island between 2003 and 2009. Analyses of the isotopic signal present in prey covering the entire trophic range of these predators were also performed, in order to relate this information to otariid sighting records collected during offshore cruises in 1993-1994 and 2000. The δ ¹³C and δ ¹⁵N values defined differences in the feeding zones and in the trophic level of each species; which were related with the interannual variability of the ecosystem, i.e. El Niño and La Niña events. Whereas Z. wollebaeki showed a coastal (inshore) foraging strategy, relying on the consumption of small epipelagic fishes, A. galapagoensis turned out to be a more oceanic (offshore) predator, with a preference for small squid. These results are in good agreement with the distribution of these animals at sea, as indicated by the sighting data. The findings of this study provide new insights to our understanding of how sympatric species exposed to strong inter-specific competition can develop foraging strategies leading to a decreasing level of food competition and facilitate their survival in a stochastic environment that is highly demanding in terms of the availability of resources.