Foraging habitats based on the diet of female northern fur seals (Callorhinus ursinus) on the Pribilof Islands, Alaska T. K. Zeppelin & R. R. Ream National Oceanic and Atmospheric Administration, Alaska Fisheries Science Center, National Marine Mammal Laboratory, Seattle, WA, USA Keywords northern fur seal; Callorhinus ursinus; diet; foraging; Pribilof Islands; resource partitioning. Correspondence Tonya K. Zeppelin, National Oceanic and Atmospheric Administration, Alaska Fisheries Science Center, National Marine Mammal Laboratory, 7600 Sand Point Way NE, Seattle, WA 98115, USA. Tel: 206-526-4036; Fax: 206-526-6615 Email: Tonya.Zeppelin@noaa.gov Received 25 January 2006; accepted 16 March 2006 doi:10.1111/j.1469-7998.2006.00122.x Abstract Scats (fecal samples) collected between 1987 and 2000 on northern fur seal Callorhinus ursinus rookeries of St Paul (n =2968) and St George Islands (n = 1203), Alaska, were used to examine the relationship between breeding sites and food habits of adult female seals. On the basis of the frequency of occurrence (FO) and per cent minimum number of individual prey (%MNI) in scats, juvenile walleye pollock Theragra chalcogramma and gonatid squid Gonatopsis borealis/ Berryteuthis magister and Gonatus madokai/Gonatus middendorffi were the domi- nant prey species consumed overall. Other primary prey (FO45%) included Pacific sand lance Ammodytes hexapteus, Pacific herring Clupea pallasi, northern smoothtongue Leuroglossus schmidti, Atka mackerel Pleurogrammus monoptery- gius, Pacific salmon (Oncorhynchus spp.) and other squid of the Gonatus genus. We identified five rookery complexes from a cluster analysis of the FO of primary prey in scats. Rookery complexes were separated geographically and each was further defined by characteristic patterns in the representation of prey types typically associated with specific hydrographic domains. Diet differences were observed among rookeries on the north and south side of St George Island and on the east, south and south-west side of St Paul Island. The rookery clusters observed in this study provide evidence of resource partitioning among adult female northern fur seals and have important implications for fur seal conservation and management. Introduction Post-parturient northern fur seals Callorhinus ursinus have dependent offspring and alternate between foraging at sea and nursing their pups on land (Gentry & Kooyman, 1986; Gentry, 1998). This central place foraging strategy limits their foraging range and may result in intra-specific compe- tition during at-sea foraging trips. Habitat and prey parti- tioning reduce intra-specific competition for marine predators, especially when resources are limited and the environment is patchy or unpredictable (Bowen et al., 2002; Bradshaw et al., 2003; Field et al., 2005). Additionally, animals that return to successful foraging habitat may minimize energy spent searching for suitable foraging opportunities. Telemetry studies of northern fur seals on the Pribilof Islands (St Paul and St George), Alaska, show that during the summer breeding season, adult females exhibit separa- tion of foraging area by island and breeding rookery (Robson et al., 2004). Sterling & Ream (2004) observed a similar spatial relationship among juvenile male foraging areas, and suggested that foraging habitat is also segregated by sex because of the longer distance and duration of juvenile male foraging trips compared with adult females. Telemetry studies demonstrate that fur seal foraging habi- tats are characterized by unique marine environments de- fined by hydrographic domains associated with the continental shelf (Goebel et al., 1991; Robson et al., 2004; Sterling & Ream, 2004). Likewise, northern fur seal dive patterns vary, depending on their foraging habitat. Fur seals feeding off-shelf typically make shallow nighttime dives and animals feeding on-shelf make deeper dives throughout the day (Gentry & Kooyman, 1986; Goebel et al., 1991; Goebel, 2002; Sterling & Ream, 2004). The diet of adult female northern fur seals also varies according to the hydrographic structure surrounding breed- ing islands (Sinclair, 1988; Sinclair, Loughlin & Pearcy, 1994; Antonelis et al., 1997; Perez, 1997) and the environ- ment they encounter during their winter migration into the North Pacific Ocean (Kajimura, 1984; Perez & Bigg, 1986). Using scats (fecal material) collected between 1987 and 1990, Antonelis et al. (1997) found differences in prey species consumption by adult female fur seals among three breeding islands of variable hydrographic structure: St Paul Island, St George Island and Medny Island (Commander Islands, Russia). Fatty acid signature analysis of northern fur seal milk, although unable to identify specific prey items, has also suggested differences in adult female diet between St Paul and St George Islands as well as among rookeries on St Paul Island (Goebel, 2002). Journal of Zoology 270 (2006) 565–576 c  2006 The Authors. Journal compilation c  2006 The Zoological Society of London 565 Journal of Zoology. Print ISSN 0952-8369