576 



Fishery Bulletin 101(3) 



80% 



70% - 



60% 



50% - 



40% 



30% 



20% 



10% 



0% 



a t c d c ' g h 



21 00-2 59 



 a; Haul out 

 n b 4 meters 

 c 4-6 meters 

 Hd: 6-10 meters 

 lae 10-20 meters 

 Hf: 20-34 meters 

 Hg: 34-50 meters 

 Eh: 50-74 meters 

 H i: 74-100 meters 



??aTTi>-.^ 



a c 6 (■ I g t\ \ atcdefgh 



3:00-8:59 900-1459 



Period 



Figure 6 



Percentage of time spent at depth for seven young-of-the-year Steller sea lions approximately (A) 7-10 months of age in 

 Alaska, and percentage of time spent at depth for three young-of-the-year Steller sea lions approximately (B) 11-12 months 

 of age (during May and June) in Alaska. This figure suggests that as young sea lions approach one year of age they tend 

 to spend less time hauled out and that a greater proportion of their dives are deeper. 



mass-specific metabolic rates than their smaller counter- 

 parts and thus expend less energy and use less oxygen 

 stores (Schreer and Kovacs, 1997). Our sample size of sea 

 lions of comparable age is small; however, we compared the 

 mean mass of three Washington sea lions to the mean mass 

 of three Alaska sea lions of approximately the same age 

 (Table 1) and found that the Alaska animals had less mass 

 than those in Washington (108 kg vs. 145 kg). Whether or 

 not this difference in mass can account for the differences 

 we saw in diving characteristics for animals of similar age 

 (Fig. 3, A and B) is unknown. 



The differences in diving characteristics between animals 

 tracked in coastal waters of Puget Sound, Washington, and 

 those tracked in Alaska waters are most likely linked to 

 localized differences in prey habitat. The primary prey of 

 Steller sea lions across their range are fish and ccphalo- 

 pods, both of which have a broad but predictable range 

 in temporal, spatial, and seasonal nearshore availability. 

 Typically, each species makes predictable migrations sea- 

 sonally from pelagic to nearshore waters where they form 

 large spawning concentrations. The prey are then further 

 concentrated by local transition boundaries such as frontal 

 zones and bathymetric features such as submarine chan- 

 nels (Sinclair et al., 1994). Steller sea lions appear to have 



the foraging flexibility to take advantage of both the pre- 

 dictable behavioral traits of these prey species, as well as 

 the localized oceanographic conditions that enhance prey 

 concentrations (Sinclair and Zeppelin, 2002). 



The primary prey of Steller sea lions in Alaska waters 

 is walleye pollock (Theragra chalcogramma ), which is con- 

 sumed year-round (Sinclair and Zeppelin, 2002). Walleye 

 pollock is replaced as a dominant year-round prey item by 

 Pacific whiting (Mer/(/cc/(/.s productus ) in Pacific Northwest 

 waters (Gcarin et al., 1999). Both species are semidemersal 

 and can be found from near surface waters to depths >1200 

 m, depending on localized conditions (Hart, 1973; Esch- 

 meyer et al., 1983). The greatest abundances of both species 

 are available to Steller sea lions in nearshore waters over 

 the continental shelf and perhaps as the prey become more 

 available during nighttime diurnal vertical movements. 



The physical features of Puget Sound, along with its com- 

 plex bathymetry and the extensive channels and canyons, 

 provides extensive microhabitat for both predator and 

 prey species to express the full extent of their depth range. 

 In this respect, Puget Sound is comparable to the Gulf of 

 Alaska where Pacific cod (Gadus macrocephalus) is the 

 predominant winter prey item for Steller sea lions. Pacific 

 cod is thought to be consumed during spawning when it ap- 



