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Fishery Bulletin 105(2) 



ing in the Gulf of Alaska and Aleutian Islands during 

 summer had diversity indices of 2-3, compared to 5.3 in 

 Southeast Alaska (Fig. 5, Merrick et al., 1997; Sinclair 

 and Zeppelin, 2002). Summer diets were dominated in 

 the Aleutian Islands by a single species (Atka mackerel) 

 and there were small amounts of other prey in the diet. 

 In the Gulf of Alaska, the dominant prey was pollock, 

 followed by salmon (Sinclair and Zeppelin, 2002). Di- 

 etary diversity remained low in the Gulf and Aleutian 

 Islands from summer to winter (Sinclair and Zeppelin, 

 2002), but dropped in Southeast Alaska from 5.3 to 

 4.1 (Fig. 5). Winter diets in Southeast Alaska were 

 dominated by pollock. However, the average scat from 

 Southeast Alaska contained at least two prey species. 

 In other words, pollock was rarely consumed alone and, 

 when consumed, was usually accompanied by at least 

 one other species type, such as herring, salmon, sand 

 lance, flatfish, or skates (Figs. 2 and 6). 



The inclusion of our data from Southeast Alaska 

 with those from Merrick et al. (1997) provided the 

 same conclusion, namely that the numbers of sea lions 

 declined more slowly and even increased as diversity 

 of diet increased (Fig. 9). Steller sea lions that con- 

 sumed the least diverse diet experienced the greatest 

 population declines. However, it is not clear whether 

 diet diversity is a proxy for energy content of the sea 

 lion diet as suggested by Winship and Trites (2003), or 

 whether it captures some other biologically meaning- 

 ful measure of nutrition. Nor is it clear whether the 

 diet diversity index reflects depths of nearest ocean 

 passes to rookeries (with diet diversity increasing with 

 shallower depths; Sinclair et al., 2005), or whether it 

 could be a relative measure of prey distribution and 

 density. 



The relative importance of pollock in the diet of Stell- 

 er sea lions in Southeast Alaska was not expected. How- 

 ever, pollock is not as dominant in the sea lion diet in 

 Southeast Alaska as it is in other regions, and appears 

 to usually be accompanied by other types of energy-rich 

 prey (Fig. 4). It may be easier for sea lions with a more 

 diverse or energy-richer diet to obtain sufficient prey 

 to meet their energy requirements (Trites, 2003; Rosen 

 and Trites, 2004; Trites et al., 2006). They may also be 

 less sensitive to changes in overall prey abundance and 

 may spend less time foraging under risk of predation. 



The increase in Steller sea lion numbers in South- 

 east Alaska since the 1970s contrasts sharply with 

 the declines observed in the Gulf of Alaska. The dif- 

 ference between the diets of Steller sea lions in the 

 two regions is one possible explanation underlying 

 the population trends. Stomach samples collected in 

 the Bering Sea and Gulf of Alaska before the popula- 

 tion decline (1950s-mid-1970s) indicate that their diet 

 might have once resembled that of sea lions in South- 

 east Alaska during the 1990s (Alverson, 1992; Merrick 

 et al., 1997; Sinclair and Zeppelin, 2002). However, the 

 small number of stomachs sampled and the nonstan- 

 dard methods used to collect them make it difficult 

 to compare pre- and post-decline periods over broad 

 areas. A change in diet during the population decline 



may be related to large-scale changes in oceanographic 

 conditions (regime shifts) that may have affected the 

 relative abundances of different suites of species (Wil- 

 derbuer et al., 2002; King, 2005; Trites et al., 2007). 

 In terms of oceanic regimes, the marine ecosystems 

 of the eastern North Pacific appear to group into two 

 broad domains (California to Southeast Alaska, and the 

 Gulf of Alaska to western Aleutian Islands) that are 

 out of phase with each other as they alternate between 

 anomalous warm and cool states (regimes). Finer-scale 

 analyses should be undertaken to determine how the 

 declines and increases of different prey and preda- 

 tor species line up in time and space with changes in 

 oceanographic events. 



Conclusions 



A comparison of our dietary data with dietary data 

 collected from other regions of Alaska indicated that 

 Steller sea lions consumed a relatively similar suite of 

 schooling species, most notably pollock, salmon, herring, 

 sand lance, rockfish, and squid. However, in terms of 

 frequency of occurrence, there were marked differences 

 between Southeast Alaska during the 1990s and regions 

 where sea lions have declined. Diets in Southeast Alaska 

 were more diverse and may have had a higher energy 

 content overall. Pollock is part of a normal sea lion diet 

 but is less dominant in Southeast Alaska than in the 

 Gulf of Alaska and Bering Sea where population declines 

 occurred. The difference in diets between the regions is 

 potentially a useful clue for determining why population 

 trends of Steller sea lions have diverged in Alaska. This 

 difference in diets also underlines the overall importance 

 of continuing to assess and monitor sea lion diets. 



Acknowledgments 



We are grateful to the many people who helped col- 

 lect and clean the scat, and would particularly like to 

 acknowledge and thank W. Cunningham, D. McAllister, 

 B, Porter, D. Porter, and S. Stanford. We thank P. Rosen- 

 baum for managing our scat collection, and S. Crockford 

 (Pacific IDentifications) for identifying the prey and 

 patiently answering our questions. We are also grateful 

 to M. Rehberg and S. Heaslip for assisting with the map, 

 and M. Sigler for his insights into pollock distribution 

 in Southeast Alaska. Useful comments and suggestions 

 on earlier drafts of our manuscript were made by D. 

 Tollit, D. Rosen, and three anonymous reviewers. Labo- 

 ratory space was provided by the Vancouver Aquarium 

 Marine Science Centre, and funding was provided by the 

 National Oceanic and Atmospheric Administration to 

 the North Pacific Marine Science Foundation and to the 

 Alaska Department of Fish and Game. Additional sup- 

 port was also provided by the North Pacific Marine Sci- 

 ence Foundation through the North Pacific Universities 

 Marine Mammal Research Consortium. Research was 

 conducted under U.S. Marine Mammal Act and Endan- 



