Research Document 2018/006
Recent trends in Abundance of Steller Sea Lions (Eumetopias jubatus) in British Columbia
By Olesiuk, P.F.
Abstract
Recent trends in the abundance of Steller sea lions (Eumetopias jubatus) in British Columbia were assessed based on a series of 13 province-wide aerial surveys conducted during the summer breeding season (27-June to 09-July) between 1971 and 2013. Numbers of non-pups (juveniles and adults) increased at an average rate of 3.8% per annum and pup production at a rate of 4.8% per annum over the study period, resulting in more than a 4-fold increase in abundance since the species was protected in 1970, with most of the increase occurring since the mid-1980s. A review of historic counts for rookeries indicated that control programs and commercial harvests in B.C. from 1912 to 1967 eradicated one breeding site and reduced numbers on the remaining rookeries to about 25-30% of the peak levels observed in the early 1900s. During the most recent province-wide survey in 2013, 6,317 pups and 22,135 non-pups (10,969 on rookeries and 11,166 on haulouts) were counted. Applying correction factors to account for pups missed during surveys or obscured in oblique 35mm photographs, total pup production in B.C. was estimated to be about 7,300 pups. Based on life tables with adjustments for higher survival and reproductive rates in an increasing population, it was estimated that pup production on B.C. rookeries in 2013 would indicate a total population of about 32,900 (range 31,200 to 33,900) animals. Based on the distribution of non-pups observed during the 2013 survey, an estimated 35,600 (range 33,800 to 36,700) sea lions currently inhabit the coastal waters of B.C. during the summer breeding season. A second abundance estimate was made using survey correction factors based on satellite telemetry data, which indicated that about 67% of non-pups would be hauled out during summer surveys. Adjusting for the 33% of animals that were at sea and missed during summer surveys, it was estimated that 39,200 (95% confidence interval 33,600 to 44,800) animals inhabit coastal waters of B.C. during the breeding season. An average of 17,679 animals were counted during winter surveys conducted in B.C. during 2009 and 2010. Satellite telemetry indicated that only about 37% of animals (53% of yearlings and 33% of older animals) would be hauled out during daylight hours in winter. Adjusting for animals at sea during winter surveys, an estimated 48,500 (95% CI 38,100 to 58,900) Steller sea lions winter in coastal waters of B.C. The higher winter count suggests there is net immigration of sea lions into B.C. during the non-breeding season. Steller sea lions in California and Oregon are displaced northward by migrating California sea lions after the breeding season, and some animals dispersing from the large rookery on Forrester Island just north of the BC-Alaska border probably winter in B.C.
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