Abstract
Analysis of fish otoliths in scats and vomits of Galápagos fur seals (Arctocephalus galapagoensis) and Galápagos sea lions (Zalophus californianus wollebaeki) was used to determine the numerical composition of the diets for the post-El Niño year 1983, the cold seasons in 1984 and 1985, and the warm season in 1986. Between 84 and 99% of all otoliths in fur seal scats were from myctophids and bathylagids. The fur seals' diet included 26 species. Only 3 species contributed more than 1% of otoliths. No seasonal differences in diet were found. Sea lion samples contained a mean of 14 otoliths. Sardines (Sardinops sagax) contributed 75-85% of otoliths. Sea lions preyed on 24 species, but only 3 surpassed 1% abundance. Seasonality was not reflected in the sea lions' diet. After the 1982-1983 El Niño, the diets of both species deviated from those in all other years of the study. Food-niche overlap between the two sympatric species was almost non-existent. This is hard to understand, based on our knowledge of the diving capabilities of the two species, but reflects the fact that Galápagos fur seals are nighttime foragers and sea lions are daytime foragers.
Highlights
Much is known about the foraging behavior and diet of otariid pinnipeds, the fur seals and sea lions, in areas where just one species occurs (Antonelis et al 1984; Croxall et al 1985; Green et al 1991; Gales and Pemberton 1994; Harcourt et al 1994)
In 1983, the strongest El Niño of the century had just ended in July and marine conditions had not yet returned to normal (Fahrbach et al 1991)
All fur seal species investigated to date are mainly nocturnal feeders (Gentry et al 1986; Gentry 1998), taking vertically migrating shoaling fish and cephalopods
Summary
Much is known about the foraging behavior and diet of otariid pinnipeds, the fur seals and sea lions, in areas where just one species occurs (Antonelis et al 1984; Croxall et al 1985; Green et al 1991; Gales and Pemberton 1994; Harcourt et al 1994). The extent or importance of competition between species that are similar in their diving and swimming abilities (Ponganis et al 1990) has rarely been explored (Antonelis et al 1990; Green et al 1990). No long-term information is available on the division of foodresource space between sympatric fur seals and sea lions.
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