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 eco- logy of northern fur seals Callorhinus ursinus from different rookeries throughout their North Amer- ican range, including rookeries on the following islands: Bogoslof Island (BI), Alaska; Reef and Vos- tochni 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 (δ 13 C) and nitrogen (δ 15 N) isotope values within an individual, between age classes, and among locations. At all sites, adult females generally had higher δ 13 C compared to their younger conspecifics for all tissues, suggesting that they forage in different locations. Mean δ 15 N values of adult females were lower com- pared 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 δ 13 C values at all islands, suggesting that animals at each location forage in dif- ferent oceanic domains. The δ 15 N values of all age classes indicated that animals at SMI and Vos- tochni 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 loca- tion 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.