Abundance and distribution of Steller Sea Lions (Eumetopias jubatus) in British Columbia : Updates from 2016-17 aerial surveys

Abstract

An aerial survey was conducted during the 2017 breeding season to assess trends in abundance and distribution of Steller Sea Lions (Eumetopias jubatus) in British Columbia (BC). This survey was part of a range-wide assessment of the Eastern population of Steller Sea Lions conducted to fulfill requirements under the Canadian Species at Risk Act and the United States (US) Endangered Species Act. Additional surveys were conducted in fall 2016 and winter 2017 to examine seasonal changes in abundance and distribution. A total of 31,753 (6,640 pup and 25,113 non-pup) Steller Sea Lions were counted during the breeding season survey. Models fit to the counts indicate a possible slowing in the annual rate of pup production since 2013, but not in the rate of growth in the non-pup component of the population. An estimate of the total population size was obtained by applying a correction factor of 1.48 to non-pup counts to account for animals that were at sea and missed during surveys. The adjusted 2017 breeding season population estimate was 43,200 (95% CI of 38,700 to 48,200) suggesting no significant change from the previous assessment. However, there continues to be an increase in the number of rookeries and year-round haulout sites being used by Steller Sea Lions in BC waters. There was evidence of five new major year-round haulout sites and two year-round haulouts were re-classified as rookery sites since 2013. Surveys conducted outside of the breeding season indicated a continuing trend of increased numbers of animals overwintering in BC waters from 2010–17. A total of ~31,400 animals were counted in October 2016, as compared to ~28,700 in fall 2012, while a total of ~22,400 animals were counted during surveys conducted in February 2017, as compared to the average of ~17,700 animals counted from multiple surveys flown during the winters of 2009-10. After applying a correction factor of 2.43 to account for animals that were at sea and missed during winter surveys, there were an estimated 52,700 (95% CI 41,000 to 67,800) Steller Sea Lions overwintering in the coastal waters of BC in 2017. Larger numbers of Steller Sea Lions estimated from winter surveys compared to breeding season surveys appear to be due to a net influx of animals from rookeries outside of BC. The Potential Biological Removal (PBR) for Steller Sea Lions during the summer breeding season is 2,474 for Canadian waters.