Research Document - 2009/105

An assessment of population trends and abundance of harbour seals (Phoca vitulina) in British Columbia

By Peter F. Olesiuk

Abstract

Population trends and abundance of harbour seals in British Columbia are assessed based on aerial surveys conducted during 1966-2008 (197 flights). The assessment is an update of earlier assessments published in the Canadian Journal of Fisheries and Aquatic Sciences (Olesiuk et al. 1990a) and as a Canadian Science Advisor Secretariat Research Document (Olesiuk 1999). Progress since the previous assessment include: 1) an analysis of recent population trends based on census data collected during 1999-2008 (38 flights); 2) expansion of baseline survey coverage to include the central and northern mainland coast, the Discovery Passage area, and more extensive coverage on the west coast of Vancouver Island and in the Queen Charlotte Islands (an increase in coverage from 36% to 82% of the total British Columbia coastline) (26 flights); 3) updated population estimates with confidence limits that account for the inherent variability of aerial counts, uncertainty in correction factors to account animals in the water that were not counted, and – for areas yet to be surveyed – the variability in the density of seals observed among surveyed areas; and 4) a reconstruction of historic population trends to assess whether recent population increases represent the recovery of population that had been depleted by bounty kills and commercial harvests prior to the species being protected in 1970. Its estimated that the number of harbour seals in the Strait of Georgia, the area with the longest survey time-series, increased ten-fold from 3,760 (95% confidence interval of 3,200 to 4,320) when the first standardized censuses were conducted in 1973 to about 39,100 (95% confidence interval of 33,200 to 45,000) by 1994-2008. Populations grew at an annual rate of about 11.5% (95% confidence interval of 10.9 to 12.6%) during the 1970s and 1980s, but the growth rate began to slow in the mid-1990s and the population now appears to have stabilized. Based on counts conducted in Index Areas distributed throughout the province, the trend observed in the Strait of Georgia appears to be indicative of harbour seal populations throughout British Columbia. Total abundance of harbour seals on the B.C. coast in 2008 was estimated to be on the order of about 105,000 (95% confidence interval of 90,900 to 118,900) seals. Historic reconstructions indicate the population was depleted by a period of commercial harvesting during 1879-1914, and subsequently maintained below natural levels by predator control programs until the early 1960s. Already depleted, the population could not sustain a second period of intense commercial harvesting during 1962-1968 and was further depleted, but now appears to have fully recovered.

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