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Fishery Bulletin 105(1) 



must also be adequate prey on a consistent basis within 

 the foraging range of lactating females. Perhaps the lim- 

 ited availability of such sites has restricted the establish- 

 ment of new rookeries at other locations. 



Changes in the ocean environment, particularly to- 

 wards warmer water temperatures (Field et al., 2006), 

 have also been proposed as a factor that has favored the 

 California sea lion and other pinnipeds over the Steller 

 sea lion in the southern part of their range (Bartholomew 

 and Boolootian, 1960). Environmental conditions can 

 affect sea lion populations directly or indirectly. Tem- 

 perature could directly affect the survival of animals and 

 such effects would be expected to be most evident at the 

 latitudinal extremes of the range. The ocean environment 

 can also act indirectly by affecting marine food webs, and 

 thus the quantity and quality of prey available to sea 

 lions. Unfortunately, with historical survey data being so 

 scant, and with sea lions having been artificially reduced 

 below natural levels, one can only speculate about the 

 long-term effects of environmental conditions on the east- 

 ern Steller sea lion population, but conditions currently 

 appear to be favorable through much of their range. 



A somewhat similar change in Steller sea lion distribu- 

 tion and the establishment of new rookeries have been 

 noted along the Asian coast. There the southern range 

 limit has moved northward by 500-900 km over the past 

 50 years and several new rookeries have been established 

 (Burkanov and Loughlin, in press). 



Based on the population-wide survey in 2002, pup 

 production for the eastern population is currently esti- 

 mated to be about 11,000, and total abundance on the 

 order of 46,000-58,000. It should be emphasized that 

 this should be regarded as a "general" estimate because 

 several factors can affect the accuracy of pup counts and 

 correction factors. Following Trites and Larkin (1996), 

 we added 10% to pup counts to estimate pup production 

 (i.e., actual number of births), which seems reasonable, 

 but the adjustment is subjective and arbitrary, and in 

 reality the adjustment probably varies from site-to-site 

 and year-to-year. The sex and age structure of popula- 

 tions, and hence the ratio of pups to nonpups, may differ 

 between populations and change with population status 

 in ways we do not understand. We attempted to delineate 

 the possible range of changes in the correction factors 

 by using sensitivity analyses, which showed the multi- 

 plier could either decrease if population productivity is 

 controlled by fecundity or age at maturation, or increase 

 if population productivity is controlled by mortality. As- 

 sessments for the western North Pacific population have 

 indicate that the population declines were primarily due 

 to poor juvenile survival (York, 1994), and if this is in 

 fact the main determinant of population growth, the pup 

 multiplier and estimated abundance of the eastern popu- 

 lation may lie toward the high end of our range. 



During the 2002 population-wide survey, a surpris- 

 ingly large number of nonpups were observed (75-100% 

 of the number expected based on our life table analy- 

 sis). Because one would expect appreciable numbers of 

 juveniles and adults to be dispersed at sea and missed 

 during surveys, the actual size of the eastern population 



may be near the upper end of our estimated range. On 

 the other hand, 2002 may merely have been an excep- 

 tional year for pup production, although the more recent 

 pup counts available for California (2003 and 2004) and 

 southeastern Alaska (2005) indicate that pup numbers 

 have continued to increase. The apparent surplus of non- 

 pups observed during the 2002 survey could also be 

 indicative of the presence of nonbreeding animals asso- 

 ciated with the western population in our survey area. 

 Studies (where sea lions have been branded) have shown 

 there is some overlap in the nonbreeding range of the two 

 populations (Raum-Suryan et al., 2002), although there 

 is no reason to expect a higher degree of movement from 

 west to east. Moreover, the observed ratios of total counts 

 to pup counts was uniformly high over the entire range 

 of the eastern population (4.1 in southeastern Alaska, 

 4.7 in British Columbia, 4.7 in Oregon, and 5.4 in Cali- 

 fornia), and if anything decreased slightly towards the 

 north where one would expect the greatest overlap with 

 the western population. The high nonpup to pup ratios 

 indicate that high survival rather than high fecundity 

 may be the primary mechanism responsible for popula- 

 tion growth. 



Steller sea lions in the eastern population currently 

 breed at 13 major rookeries (>50 pups born), and the 

 highest concentration of breeding animals is in south- 

 eastern Alaska, northern British Columbia, and near the 

 Oregon-California border. Currently there is a large gap 

 (993 km) between the Scott Islands rookery off north- 

 western Vancouver Island and the Orford and Rogue 

 Reef rookeries in southern Oregon. There are no records 

 of rookeries along this coastline, and natives hunting 

 sea lions along the Washington coast had no knowledge 

 of rookeries in that state (Scheffer, 1950). However, it 

 would not be surprising to see new rookeries founded 

 or re-established at haulout sites along this gap, as has 

 occurred in southeastern Alaska, if the eastern popula- 

 tion continues to increase in the northern part of its 

 range. Nonbreeding animals use approximately 59 major 

 haulout sites (>50 animals during) during the breeding 

 season, plus numerous smaller sites and many seasonal 

 haulout sites. The major haulouts are widely distributed 

 from Cape Fairweather (58.8°N, 137.9°W) to Ano Nuevo 

 Island (37.1°N, 122. 3°W), providing Steller sea lions with 

 access to coastline spanning about 22° of latitude or 

 2400 km. 



During the 1970s the eastern population represented 

 only about 10% of the total number of Steller sea lions 

 along the North American coast. With the large decline 

 in the western population in conjunction with the in- 

 crease in the east, this percentage has changed dramati- 

 cally; about 55% of pup production in North America now 

 occurs in the eastern population. We anticipate that con- 

 tinued monitoring and comparisons of the growing east- 

 ern population with the western population will provide 

 insight into factors that ultimately regulate Steller sea 

 lion populations, and we hope this synthesis for the east- 

 ern population will contribute toward better coordination 

 of surveys and standardization of counting methods over 

 the distribution range of the species. 



