Stable isotope and scat analyses indicate diet and habitat partitioning in northern fur seals Callorhinus ursinus across the eastern Pacific

Abstract

We used stable isotope (SI) and scat analyses to describe and compare the foraging ecology of northern fur seals Callorhinus ursinus from different rookeries throughout their North American range, including rookeries on the following islands: Bogoslof Island (BI), Alaska; Reef and Vostochni on St. Paul Island (SPI), Alaska; and San Miguel Island (SMI), California. SI samples were collected from 36 adult females and 37 juveniles in Alaska, and 9 adult females and 7 pups on SMI during fall 2006. Isotopic analyses of blood and fur indicated differences in stable carbon (δ13C) and nitrogen (δ15N) isotope values within an individual, between age classes, and among locations. At all sites, adult females generally had higher δ13C compared to their younger conspecifics for all tissues, suggesting that they forage in different locations. Mean δ15N values of adult females were lower compared to those of pups at SMI, higher than those of juveniles on SPI, and similar to those of juveniles on BI, suggesting differences in trophic level between age classes at all locations except on BI. We found differences in δ13C values at all islands, suggesting that animals at each location forage in different oceanic domains. The δ15N values of all age classes indicated that animals at SMI and Vostochni feed at similar trophic levels within their respective communities, but feed at higher trophic levels than animals at Reef and BI. Scat analysis supported SI results in that animals from each location were found to feed on species associated with unique oceanic features. By using scat and SI analyses, we were able to acquire a better understanding of the foraging ecology of different-aged conspecifics from multiple locations. © Inter-Research 2010.