Pitcher et al : Abundance and distribution of Eumetopias /ubalus 



105 



Department of Fish and Wildlife has conducted 

 numerous aerial surveys of haulout sites dur- 

 ing the breeding season using oblique 35-mm 

 format photography since 1978. In Oregon, the 

 Oregon Department of Fish and Wildlife has 

 conducted state-wide aerial surveys of nonpups 

 on rookeries and haulouts using oblique 35- 

 mm format photography on a nearly annual 

 basis since the mid-1970s and has periodically 

 obtained ground, or more recently vertical 126- 

 mm format or high-resolution digital 35-mm 

 format, pup counts. In California, the National 

 Marine Fisheries Service, Southwest Fisheries 

 Science Center, conducted statewide surveys 

 during early July beginning in 1996 using 

 vertical 126-mm format photography to count 

 pups and nonpups at all rookeries and haulout 

 sites. Time series of counts that were obtained 

 with assorted methods were also available for 

 some rookeries in California dating back to the 

 1970s. Although these surveys provide reliable 

 information on changes in relative abundance 

 within each region or at a particular rookery, 

 they are difficult to synthesize into a popula- 

 tion-wide assessment because of uncoordinated 

 survey schedules and methods. Given the consistency 

 within, but inconsistency between, these geo-politi- 

 cal jurisdictions, we assessed trends in abundance by 

 region (southeastern Alaska, British Columbia, Wash- 

 ington, Oregon, and California). Counts for each re- 

 gion were converted to natural logarithms and then 

 regressed on year to determine average annual popula- 

 tion growth rates. 



We estimated the total population size in 2002 from 

 the predicted ratio of pups to nonpups in the population 

 (Calkins and Pitcher, 1982; Trites and Larkin, 1996). 

 From life tables for a stable sea lion population in the 

 Gulf of Alaska, Calkins and Pitcher (1982) estimated 

 total population size to be about 4.5 times the number 

 of pups born. In order to apply this approach to the 

 eastern population, which was not stable but increas- 

 ing (see "Results" section), we conducted sensitivity 

 analyses to determine how this multiplier varies with 

 population growth rate (A) by incrementally chang- 

 ing each of the life history parameters that affect it, 

 namely juvenile mortality rates, adult mortality rates, 

 age at maturation, and fecundity rates (Lotka, 1907; 

 Cole, 1954) . 



We also reviewed historical records of Steller sea 

 lion abundance in an attempt to relate current popu- 

 lation size with abundance prior to the initiation of 

 standardized surveys. Although these records provide 

 insights into relative population levels, caution must 

 be used because the older counts were obtained by a 

 variety of methods and the seasonal timing of counts 

 was inconsistent. In most cases the counts were made 

 by professional biologists or naturalists hired by govern- 

 ment agencies to conduct sea lion investigations, and 

 special trips were made to rookeries to obtain first-hand 

 counts; therefore it is unlikely numbers were grossly in- 



accurate. Because of the ad hoc nature of these counts, 

 it was difficult to synthesize them into even a regional 

 estimate of abundance, or to conduct statistical analy- 

 ses; therefore these counts were generally examined on 

 a rookery-by-rookery basis (Appendix). 



Results 



Southeastern Alaska 



Counts of Steller sea lion pups in southeastern Alaska 

 increased from 2219 in 1979 to 5510 in 2005 (Fig. 3A), 

 representing an average annual rate of increase of 3.2% 

 (r2=0.91; n = 10; P<0.001). Prior to the early 1980s, the 

 only rookery in southeastern Alaska was the Forrester 

 Island complex. Only 50-100 animals were recorded 

 when the site was first noted in the 1920s, and 350 

 animals were recorded when the site was revisited in 

 1945, and there was no mention of pupping in either case 

 (Rowley, 1929; Imler and Sarber, 1947). Thus, although 

 count data are extremely limited, it appears that Steller 

 sea lion abundance was probably quite low in south- 

 eastern Alaska during the first half of the 20"' century. 

 Counts are not available, but the Forrester Island rook- 

 ery must have grown dramatically through the 1950s 

 and 1960s (Fig. 4A). By the time the first aerial survey 

 was conducted in 1961, Forrester Island had grown to 

 about one-third its current size in terms of both the 

 numbers of pups and nonpups (Bigg, 1985). However, 

 increases at Forrester Island appear to have slowed 

 since the late 1970s, showing only a slight increase in 

 pup production (0.6% per year; r2=0.40; n=13; P=0.021) 

 and no discernible increase in the number of nonpups 

 (r2 = 0.22;;i=12;P=0.125). 



