Research Document 2022/060

Abundance and distribution of Harbour Seals (Phoca vitulina) in the Strait of Georgia, British Columbia; synthesis of the 2014 aerial survey and long-term trends

By Majewski, S. P. and Ellis, G.M.

Abstract

The Strait of Georgia (SOG) supports the highest density of Harbour Seals (Phoca vitulina) on the British Columbia (BC) coast and has been the primary index site for population surveys since the protection of the species in 1973. There were an estimated ~39,000 Harbour Seals in the SOG as of 2008, representing 37% of the BC population. Harbour seals have been identified as the primary prey species of threatened Transient (aka Bigg’s) killer whales along the coast of BC. It is suspected that the recovery of Harbour Seal populations has contributed to the increased observations of Transient Killer Whales in the SOG in recent years, with Harbour Seals having been found to account for more than half of their diet in this area. The Recovery Strategy for Transient Killer Whales identifies the need to determine the quantity, quality and distribution of Transient Killer Whale prey necessary to sustain or increase the current population level. In support of this recovery objective, an updated assessment of Harbour Seal abundance and distribution in the SOG is provided based on counts from aerial surveys conducted near the end of the pupping season in 2014. Based on survey logistics, the SOG was partitioned into 7 subareas and seals were counted from images of individual haulout sites. Counts were adjusted for haulout sites that were not surveyed and unborn pups, and subsequently corrected for animals that were in the water and therefore missed during surveys. Abundance estimates were generated by 1) summing corrected counts across all haulout sites, and 2) by fitting the corrected counts to a generalized logistic model of the time series (1973-2014). The calculated abundance estimate of 39,300 (95% Confidence Intervals (CI) 33,400-45,200, rounded to the nearest hundred) was similar to the modeled estimate of 39,000 (95% CI 35,000-42,100) for the SOG. These are not significantly different from the population size of 39,100 (95% CI 33,200 to 45,000) reported for 2008. However, there was evidence of continuing redistribution among subareas and individual haulout sites, with further increases in the proportion of animals in the southern Gulf subarea in 2014.

Research Document 2022/060 (PDF, 11,924 KB)

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