Research Document - 2008/063
Abundance of Steller sea lions (Eumetopias jubatus) in British Columbia
By P.F. Olesiuk
Abstract
Recent trends in the abundance of Steller sea lions (Eumetopias jubatus) in British Columbia were assessed based on a series of 10 province-wide aerial surveys conducted during the breeding season (27-June to 06-July) between 1971 and 2006. Numbers of non-pups (juveniles and adults) increased at an average rate of 3.5% per annum and pup production at a rate of 3.9% per annum during the study period, resulting in a three-fold increase in abundance since the species was protected in 1970. In both cases, numbers on B.C. rookeries appeared to be relatively stable during the 1970s and early 80s, with most of the increases occurring since the mid-1980s. Steller sea lions recently resumed breeding on the Sea Otter Group, a rookery that had been eradicated by predator control programs in the 1920s and 1930s, and the number of year-round haulout sites has almost doubled from about 12 to 23. During the most recent province-wide survey in 2006, we counted 4,118 pups and 15,700 non-pups (7,171 on rookeries and 8,529 on haulouts). Applying correction factors of 1.05 to account for pups obscured in oblique 35mm photographs (Olesiuk et al. 2007) and 1.1 for pups not included in censuses (Trites and Larkin 1998; Pitcher et al. 2007), total pup production in B.C. was estimated to be about 4,800 pups. Based on life tables for a stable population (Calkins and Pitcher 1982; Trites and Larkin 1996) and taking into account the uncertainty in how demographics differ for an increasing population, it was estimated that total abundance could range from 4.0 to 5.8 times the number of pups born. Its thus estimated that at least 20,000 and perhaps as many as 28,000 Steller sea lion currently inhabit coastal waters of B.C. Despite the recent increases, the proportion of the population occupying breeding rookeries appears to have remained relatively constant at about 61% (range 51-67%) over the last 35 years, suggesting that numbers on rookeries provides an index of total abundance. A review of historic counts at rookeries (Bigg 1985) indicated that control programs and commercial harvests conducted in B.C. during 1912-1967 eradicated one breeding area and reduced numbers on the remaining rookeries to about 25-30% of peak levels observed in the early 1900s. Abundance of Steller sea lions in SE Alaska has also increased in recent years, where 5 new rookeries have become established, including what is now the largest Steller sea lion breeding site at Forrester Island just a few kilometres north of the B.C.-Alaska border (Calkins et al. 1999; Pitcher et al. 2007). These recent increases likely represent the recovery of populations from control programs and harvests, but abundance in this region now appears to have surpassed peak historic levels by a factor of two.
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