Pitcher et at. Abundance and distribution of Eumetopias /ubotus 



103 



500 



In recent years, attention has focused on 

 the western Alaskan population because of 

 a precipitous decline since the 1970s (Lough- 

 lin et al., 1992; Trites and Larkin; 1996) 

 resulting in an "endangered" classification 

 under the U.S. Endangered Species Act. The 

 eastern population is currently classified as 

 "threatened." Abundance from southern Or- 

 egon through southeastern Alaska has gen- 

 erally shown an increasing trend (Calkins et 

 al., 1999; Brown et al.-; DFO, 2003), whereas 

 numbers in southcentral California have de- 

 clined substantially (Le Boeuf et al.-^; Hast- 

 ings and Sydeman, 2002). This is the first 

 detailed population-wide status evaluation 

 of abundance, trend, and distribution with a 

 historical perspective for the eastern popula- 

 tion. We also present the results of the first 

 population-wide census of pup production con- 

 ducted in 2002 and apply life-table analysis 

 to estimate total population size. 



In our study, we reviewed records of Steller 

 sea lion abundance, with particular emphasis 

 on data collected at rookeries. Some counts 

 date back to the early 1900s, but early sur- 

 veys were not systematic and methods lacked 

 standardization, and some of the counts may 

 have been affected by culling and hunting 

 activities. Although these earlier survey 

 methods preclude formal statistical analyses, 

 the historical data provide a general sense 

 of gross changes in abundance and distribu- 

 tion. Systematic surveys began in most regions along the 

 west coast in the 1970s, but counting techniques varied 

 among the researchers and agencies conducting the sur- 

 veys, and surveys were not coordinated between jurisdic- 

 tions. Nevertheless, these time series indicate changes 

 in relative abundance within each geopolitical region. 

 In recent years, there has been an effort to compare 

 and calibrate counting techniques, especially for pups 

 (Snyder et al., 2001; P. F. Olesiuk, unpubl. data), and to 

 synthesize survey results (Loughlin et al., 1992). 



Materials and methods 



Count data used to estimate population trends between 

 the late 1970s and 2004 were of two types: 1) counts of 

 pups obtained between late June and early July (at the 

 end of the pupping season) when most pups are <1 month 

 of age, and 2) counts of juveniles and adults al year of 



White Sisters 



Hazy Is. 



Scott Is. 



t: Noilh Danger Rocks 



Orford Reef 



Rogue Reef 



Sugarloaf Is., Cape Mendocino 



Ano Nuevo Is 



500 1000 



Kilometers 



65=N 



35° 



145°W 



125° 



115° 



135° 



Figure 1 



Geographic range of the eastern Steller sea lion [Eumetopms jubalus) 

 population showing locations of major (>50 pups born) breeding 

 rookeries. 



2 Brown, R. F., S. D. Riemer. and B. E. Wright. 2002. Pop- 

 ulation status and food habits of Steller sea lions in Oregon. 

 Report from Oregon Dept. of Fish and Wildlife to Oregon 

 State Univ. Contract F0225A-01, 17 p. Oregon Department 

 of Fish and Wildlife, Marine Mammal Research Program, 

 7118 NE Vandenberg Ave.. Corvallis, OR 97330. 



3 Le Boeuf, B. J., K. Ono, and J. Reiter. 1991. History of the 

 Steller sea lion population at Ano Nuevo Island, 1961-1991. 

 NOAA Admin. Report NMFS-SWFSC LJ-91-45C, 9 p. 



age (i.e., nonpups) obtained from mid June to early July 

 (mid to late in the breeding season). Steller sea lions 

 normally give birth between late May and early July and 

 breed between late May and mid July, although timing of 

 these events varies somewhat geographically (Pitcher et 

 al., 2001). Counts of pups are the preferred index to popu- 

 lation size for many species of pinnipeds (Berkson and 

 DeMaster, 1985). For the Steller sea lion, the vast major- 

 ity of births occur at traditional rookeries, and because 

 pups are confined to land for the first month of life, sur- 

 veys of rookeries at the end of the pupping season provide 

 a nearly complete estimate of annual pup production. 

 Pups are more difficult to count than nonpups be- 

 cause of their small size and dark color. This disad- 

 vantage is especially pronounced for counts made at 

 oblique angles from aircraft circling rookeries or from 

 vessels adjacent to the sites. From the mid 1970s to 

 the late 1990s, pups were usually counted by placing 

 people on rookeries, herding nonpups into the water, 

 and tallying the number of pups while walking through 

 the rookery (Calkins and Pitcher, 1982). However, the 

 methods of obtaining such counts are disruptive to sea 

 lions (Lewis, 1987), and counts may not be possible 

 where rookeries are protected in parks or ecological 

 and nature reserves. More recently, vertical 126-mm 

 format aerial photography has been shown to be as 

 accurate and far less disruptive (Snyder et al., 2001) 

 for counting pups. Depending on the physical size, 



