454 



Fishery Bulletin 104(3) 



samples have revealed that northern fur seals primarily 

 consume juvenile walleye pollock (Sinclair et al.. 1994, 

 1996), indicating that there is limited competition be- 

 tween northern fur seals and the commercial trawl fish- 

 ery. When walleye pollock age class was estimated from 

 scats collected from 1990 to 2000, we found minimal 

 overlap between the age classes of pollock consumed by 

 northern fur seals and age classes of pollock caught by 

 the commercial fishery during this time (Fig. 3). How- 

 ever, when spew samples were used to estimate age/size 

 of fur seal prey, a high degree of overlap between age 

 classes of pollock consumed by northern fur seals and 

 pollock caught by the commercial fishery was observed 

 (Fig. 3). 



Because of the protective measures that resulted in 

 closures of fisheries in Steller sea lion (Eiunetopias 

 jubatus) critical habitat, there is concern that fishery 

 pressures in waters surrounding the Pribilof Islands 

 may increase (NMFS^). Approximately 75% of the glob- 

 al northern fur seal breeding population inhabits the 

 Pribilof Islands during the summer months (Loughlin 

 et al., 1994) and recent estimates indicate a population 

 decline. The total number of adult males on St. George 

 and St. Paul Islands decreased by 13.4% and 2.8%, re- 

 spectively, from 2002 to 2003 (Towell et al., 2006; York 

 et al., 2005). Pup production also declined by >5% per 

 year between 1998 and 2002 on both islands (York et 

 al., 2005). At present, the cause of decline is uncertain 

 but there is concern that increased fishing activity in 

 waters surrounding the Pribilof Islands may adversely 

 affect the northern fur seal population. We found that 

 the occurrence and the age or size of some important 

 prey species (such as walleye pollock) has been under- 

 estimated in previous northern fur seal diet studies in 

 which only scat samples were used. Because some of 

 these species are also commercially important, northern 

 fur seal conservation and fishery management decisions 

 should incorporate diet-related information acquired 

 from multiple noninvasive sampling methods. This ap- 

 proach would allow better interpretation of northern 

 fur seal dietary requirements, thereby providing a more 

 accurate estimation of the extent to which protective 

 measures in and around the Pribilof Islands should be 

 instituted. 



Scat and spew samples were collected simultaneously 

 from the same rookeries on the Pribilof Islands during 

 the breeding season; therefore we do not believe our 

 results were affected by sampling effort. The discrepan- 

 cies in sample sizes observed in this study may be the 

 result of northern fur seal foraging behavior. Satellite 

 tracking and behavioral studies of female northern fur 

 seals have indicated that individual at-sea foraging 

 trips may range between 6 and 10 days on average 

 (Loughlin et al., 1987; Gentry, 1998; Robson et al., 

 2004). The accumulation of large cephalopod beaks and 



3 NMFS (NationalMarine Fisheries Service). 2001. Steller 

 sea lion protection measures: final supplemental environ- 

 mental impact statement, 3 volumes. NMFS, NOAA, Alaska 

 Region, P.O. Box 21668, Juneau, AK 99802-1668. 



fish bones near the pyloric sphincter during this time 

 may have irritated the stomach, causing the fur seals 

 to regurgitate prey remains while at sea prior to their 

 return to the rookeries. Although we do not know the 

 regurgitation rates of fur seals, a tendency to regurgi- 

 tate food during active digestion would explain the dif- 

 ferences in sample size between spew and scat samples 

 found on rookeries of the Pribilof Islands. Differences 

 in proportions of sample types also have been observed 

 on northern fur seal haul-outs (Kiyota et al., 1999), 

 as well as on California sea lion haul-outs (Lowry and 

 Carretta, 1999) and on Australian fur seal rookeries 

 and haul-outs (Gales et al., 1993). Despite the skewed 

 sample sizes between scats and spews in the present 

 study, the observed differences in F0% and estimated 

 age or size of prey between sample types were similar 

 for both islands and were consistent with previous pin- 

 niped diet studies comparing scat and spew samples 

 (e.g., Gales et al., 1993, Kiyota et al., 1999). 



The various sampling methods used in pinniped diet 

 analyses each have associated biases and sources of 

 error that must be considered. The reliance upon any 

 one sample type in diet assessments will limit one's 

 ability to completely describe prey composition and prey 

 size. Although our results corroborate the findings of 

 previous diet studies with regard to the primary prey 

 consumed by northern fur seals, this study demon- 

 strates that using multiple sampling methods allows 

 for a more accurate assessment of occurrence and age 

 or size of prey. 



Acknowledgments 



We thank W. Walker for assistance with otolith and 

 cephalopod identification, as well as Gb-Bm regression 

 data; J. Thomason and E. Sinclair for help with cepha- 

 lopod identification; J. Berger for providing commercial 

 trawl fishery catch data; S. Crockford and Pacific Iden- 

 tifications for identification offish bones; and J. Laake 

 for statistical advice. Reviews and helpful comments on 

 this manuscript were provided by W. Walker. K. Call, T. 

 Loughlin, E. Sinclair, S. Melin, W. Jenkins, and three 

 anonymous reviewers. 



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 Bigg, M. A., and I. Fawcett. 



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