530 



Fishery Bulletin 102(3) 



sizes of Atka mackerel {Pleurogrammus monopterygius) 

 consumed in western Alaska. 



Comparing samples collected at rookeries from the 

 eastern and western populations reveals that sea lions 

 in the western stock ate significantly greater numbers 

 of smaller pollock and fewer adults in summer than sea 

 lions in Southeast Alaska (Zeppelin et al., 2004, this is- 

 sue; and our study). However, both eastern and western 

 stock sea lions using haul-outs in March (winter) ate 

 similar size pollock. Adult pollock occurred more fre- 

 quently in scats collected from rookeries along the open 

 ocean coastline of Southeast Alaska during June and 

 July (74% adults) than they did in scats from haul-outs 

 located in inside waters between October and May (51% 

 adults). Scats collected at rookeries can be considered to 

 be from adult female sea lions and to a lesser extent from 

 adult males, whereas those collected at haul-outs during 

 other times of the year contain a more diverse mix of age 

 groups, including greater numbers of younger sea lions. 

 Thus it is uncertain whether observed size differences 

 in pollock between these two groups are seasonal or due 

 more to size preferences of different aged animals. Lim- 

 ited support for the former comes from the similar size 

 pollock observed in the scats between the two groups in 

 June and July of 1999. Overall, however, it is unknown 

 whether the consumption patterns observed are a result 

 of an actual size selection of prey or if they result from co- 

 incidental distributions of sea lions and prey-size classes. 

 Some pinnipeds may select prey of particular sizes (Sin- 

 clair et al., 1994) and may encounter difficulties if they 

 cannot switch to other sizes or species if the abundance 

 of preferred prey is reduced. Fine-scale studies are now 

 being undertaken to address such uncertainties. 



There are few assessments of pollock stock size for 

 the 1990s in Southeast Alaska (Martin, 1997). However 

 the biomass is believed to have been low compared to 

 other regions of Alaska. Juvenile pollock are known to 

 congregate in the shallow inside waters of Southeast 

 Alaska during winter (Sigler ) but are also known to 

 occur in significant numbers in the summer in waters 

 shallower than 200 meters on the outer coastline (Mar- 

 tin, 1997). Recruitment of 1-year-old fish was found to 

 be high during acoustic studies in 1994 and 1999 in the 

 Gulf of Alaska (Guttormsen et al., 2003). 



Steller sea lions using rookeries in Southeast Alaska 

 consumed mainly adult pollock between 1994 and 1999 

 and showed no evidence of tracking any abundant age 

 class of pollock. However, the trend in increasing length 

 estimates for inside haul-outs after 1995 (Fig. 3 1 does 

 suggest that sea lions might be tracking a particu- 

 lar age class of prey. Certainly a greater range of age 

 classes were consumed at these haul-outs (Fig. 3). 



Scientific trawls in 1996 indicated that the larger pol- 

 lock on the outside coastline occurred generally in wa- 

 ters 201-300 m deep during daylight hours (Martin, 



B Sigler, M. F. 2003. Unpubl. data. Auke Bay Lab, National 

 Marine Fisheries Service. 1 L305 Glacier Highway, Juneau. 

 AK 99801. 



1997) and that smaller pollock were present in shallower 

 depths. Larger pollock tend to disperse and move to 

 shallow waters to feed at night (Smith, 1981). Thus, the 

 observed crepuscular and nighttime foraging by lactating 

 Steller sea lions (Higgins et al., 1988; Trites and Porter, 

 2002) would be a logical foraging strategy to capture 

 adult pollock. Other important factors, in addition to 

 depth, that likely influence size selection include prey 

 density and spatial distribution in relation to rookeries 

 and haul-outs. Given both the greater mass and energy 

 content of adults compared with juveniles (Perez, 1994; 

 Anthony et al., 2000), the selection of adults would be an 

 energy efficient strategy — all other things being equal. 



Overlap in sizes of pollock consumed by Steller sea lions 

 and sizes of pollock caught by fisheries 



There was no commercial fishery for pollock in South- 

 east Alaska during the 1990s. However, a small fishery 

 occurred in nearby Dixon Entrance, B.C., that might 

 indicate sizes that could have been caught in Southeast 

 Alaska if a fishery had occurred. Overlap in sizes of 

 pollock caught by the B.C. fishery with those taken by 

 Steller sea lions further north (our study) more than 

 doubled after applying digestion correction factors (from 

 24% to 52%). Similarly, high levels of overlap were also 

 found between the sizes of pollock consumed by the 

 western stock (1998-2000) and those caught in the 

 same region by fisheries (after our DCFs were applied to 

 structures recovered from scats — Zeppelin et al., 2004. 

 this issue). A high degree of overlap in size highlights 

 a potential conflict between fisheries and sea lions, but 

 this overlap cannot be considered indicative of competi- 

 tion unless the resource that fisheries and sea lions seek 

 is limited across the space and time in question (Krebs 

 and Davies, 1991). 



Conclusions 



Our study provides the first substantial description of 

 the size of pollock eaten by Steller sea lions in South- 

 east Alaska. It also shows the benefits of using bones 

 other than otoliths to estimate the sizes of prey eaten 

 by Steller sea lions, and the importance of correcting 

 for degree of digestion. Accurately reconstructing the 

 sizes of bones and otoliths recovered from scats has a 

 significant bearing, in turn, on accurately determining 

 the mass of prey consumed, and on the extent of overlap 

 of sizes of prey consumed and sizes of the same resource 

 caught in commercial fisheries. 



We found that Steller sea lions in Southeast Alaska 

 consumed a large proportion of adult pollock and few 

 juveniles between 1994 and 1999. Although greater 

 proportions of juvenile and adolescent pollock were con- 

 sumed over the same period, during the summer in the 

 Gulf of Alaska and Bering Sea, larger size fish still were 

 the most abundant prey item in the diet of sea lions. 

 A comparison of these estimates with the lengths of pol- 

 lock consumed during the 1970s and 1980s shows that 



