Zeppelin et al.: Sizes of walleye pollock and Atka mackerel consumed by Eumetopias jubatus 



519 



and 1986 in the central Gulf of Alaska contained pri- 

 marily juveniles (mean FL=25.4 cm; SD = 12.4) and had 

 a weak mode of adult-size fish (39-43 cm; Merrick and 

 Calkins, 1996; Calkins and Goodwin 1 ). Mostly adult-size 

 fish were found in stomachs recovered from Steller sea 

 lions caught in trawl nets in the central Gulf of Alaska 

 11983-84; Loughlin and Nelson, 1986) and in stomach 

 samples collected from 1994 to 1995 in Japanese waters 

 (Goto and Shimazaki, 1998). However, in both these 

 studies the samples of prey size may have been biased 

 by the selectivity of the fishing gear for larger fish. 



Using identical methods to those of our study, Tollit 

 et al. (2004a, this issue) estimated the size of wall- 

 eye pollock consumed by the eastern stock of Steller 

 sea lions between 1994 and 1999. The average size of 

 walleye pollock consumed, estimated from all grade- 

 corrected structures (mean=42.4 cm; SD = 11.6), was 

 similar to the average size found in our study of the 

 western stock of Steller sea lions. Furthermore, Tollit 

 et al. (2004a, this issue) also found a greater occur- 

 rence of adult pollock in scats collected on rookery sites 

 than from scats collected on haul-out sites. However, 

 Steller sea lions from the western stock consumed a 

 greater proportion of juvenile and adolescent fish and 

 less adult fish than those from the eastern stock dur- 

 ing summer (June-July) and similar-size fish were 

 consumed on haul-out sites in winter (March) in both 

 regions. Neither study indicated the high occurrence 

 of juvenile walleye pollock reported in the 1970s and 

 1980s. The greater occurrence of juvenile walleye pol- 

 lock in historical studies may be a result of prey avail- 

 ability or differences in sampling methods. 



By examining the relative size-frequency distributions 

 of prey selected by Steller sea lions and those taken 

 in the commercial trawl fishery, we found considerable 

 overlap (689r walleye pollock and 53% Atka mackerel). 

 Likewise, high levels of potential overlap in size were 

 found between walleye pollock selected by Steller sea 

 lions from the eastern stock and caught by the small 

 commercial fishery bordering Southeast Alaska (Tollit 

 et al., 2004a, this issue). The extent of overlap through- 

 out the range of Steller sea lions between the size of 

 prey consumed by sea lions and the size of fish targeted 

 and taken by the pollock and Atka mackerel trawl fish- 

 eries could result in competition between fisheries and 

 foraging sea lions if resources are limited. 



Acknowledgments 



Fish specimens for the regression equations were pro- 

 vided by the National Marine Fisheries Service, the 

 University of Victoria, and the University of British 

 Columbia bone reference collections. Fish remains were 

 identified by Pacific Identifications, Victoria, BC. We 

 thank J. Laake, A. York, and R. Joy for statistical advice, 

 K. Chumbley, E. Sinclair, and S. Crockford for help in 

 initiating the study, A. Browne and M. Wilson for wall- 

 eye pollock otolith data, and S. Heaslip for graphics and 

 laboratory assistance. Reviews by E. Sinclair, S. Melin, 



W. Walker, B. Robson, and three anonymous reviewers 

 greatly improved this manuscript. 



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