Abundance estimate of the South and East Hudson Bay Walrus (Odobenus rosmarus rosmarus) stock from an aerial survey flown in September 2022

Abstract

From September 1 to 13, 2022, an aerial photographic survey was flown to estimate the abundance of walruses from the South and East Hudson Bay (SEHB) stock. The entire SEHB range was covered. A total of 130 walruses were counted from aerial photographs within the survey area, 129 of which were from a single haul-out site on Kidney Island, and one individual on Eddy Island, in the Ottawa Islands archipelago. Correcting raw counts using the mean proportion of hauled out animals from the literature (P = 0.3, CV = 0.072) resulted in an abundance estimate of 432 (95% CI = 157–1188) animals. A second abundance estimate was calculated from 30 cm resolution satellite photographs of the main haul-out sites within the survey area, and that were obtained between August 11 and October 15, 2022. On satellite images, walrus aggregations were detected on Kidney Island, as well as on a small reef northwest of Cape Henrietta Maria. An average walrus aggregation density of 0.446 (CV = 0.027) individuals · m-2 was derived from georeferenced aerial photographs from Kidney Island collected during the 2022 survey. This density was multiplied by the area covered by walrus aggregations on satellite images to estimate abundance. This yielded uncorrected abundances of 61 and 129 walruses on Kidney Island and at a haul-out site near Cape Henrietta Maria, respectively. Adjusting these indices for the proportion of animals hauled out at the time satellite photographs were taken resulted in an abundance estimate of 633 (95% CI = 226–1770) individuals. Considering that the aerial survey and satellite imagery estimates were independent, a combined estimate of 494 (95% CI = 231–1054) walruses was derived for the SEHB stock. This abundance estimate is larger, but not significantly different, from the estimate produced by the last survey conducted in 2014 (i.e., 200 animals; 95% CI = 70–570). The potential biological removal (PBR) estimate for the SEHB stock was estimated at 4 animals per year.