Research Document - 1999/033

An assessment of the status of harbour seals (Phoca vitulina) in British Columbia.

By P. Olesiuk

Abstract

An assessment of population trends and abundance of harbour seals in British Columbia is presented based on aerial surveys conducted during 1966-98 (133 flights). It is an update of an earlier assessment published in the Canadian Journal of Fisheries and Aquatic Sciences (Olesiuk et al. 1990a). Progress since the original assessment includes: 1) an analysis of more recent population trends based on census data collected during 1989-98 (an additional 58 flights); 2) expansion of survey coverage to include the central and northwest coast of Vancouver Island, the south and southwest Queen Charlotte Islands, northwestern Queen Charlotte Strait, and the Broughton Archipelago (an increase from 24 to 36% of the total British Columbia coastline); 3) an improved correction factor to account for animals not hauled out and hence missed during surveys based on an analysis of haulout patterns using time-depth recorders; and 4) a consideration of the statistical precision of the population growth rate and abundance estimates. It is estimated that abundance of harbour seals in the Strait of Georgia increased about ten-fold from 3,570 (95% confidence interval of 2,480 to 4,650) animals when the first standardized censuses were conducted in 1973 to about 37,300 (95% confidence interval of 28,200 to 46,300) by 1996-98. Populations grew at a rate of about 11.5% (95% confidence interval of 11.1 to 12.2%) per annum during the 1970s and 1980s, but the growth rate began to slow in the early 1990s and the population now appears to have stabilized. These trends appear to be indicative of harbour seal populations throughout British Columbia, and probably represent the recovery of populations that had been depleted by predator control programs and especially commercial harvests prior to the species being protected in 1970. Total abundance of seals in British Columbia in 1996-98 was estimated to be on the order of about 108,000 based on an extrapolation of the observed density of seals in surveyed areas to the entire province and on the relative distribution of historical bounty kills. This estimate is about 35% greater than our previous estimate of 75,000-88,000 in 1988 (Olesiuk et al. 1990a) due in part to the continued population growth, but also because time-depth recorders indicated that a greater proportion of animals were not hauled out and hence missed during surveys than was previously believed (39% as opposed to 12-20%). Recommendations are made for future aerial survey require-ments to monitor population trends and to derive more reliable estimates of province-wide abundance.

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