Abundance estimates for Beluga (Delphinapterus leucas) in James Bay and the Belcher Islands-Eastern Hudson Bay area in Summer 2021

Abstract

Systematic line-transect surveys were flown in James Bay and the Belcher Islands-eastern Hudson Bay area from July 22 to August 23, 2021. A total of 357 groups of belugas were detected by primary observers, but only 330 groups remained after the left truncation of groups closer than 120 m from the track line to account for the reduction in detection probability near and underneath the plane. A single hazard-rate detection function was selected to model the probability of detection in both surveyed areas from the ungrouped distribution of perpendicular distances, which estimated an average effective strip half-width of 771 m (CV = 4%). A total of 249 groups with an average size of 1.78 (CV = 7%) animals were detected in James Bay over 4,272 km of survey lines, resulting in a surface abundance estimate of 5,043 (95% CI: 3,494–7,279) belugas. The Belcher Islands-eastern Hudson Bay area was split into a high coverage stratum, surveyed twice, and two low-coverage strata located to the north and in Richmond Gulf. On the first and second survey of the high coverage area, 72 and 35 groups with average sizes of 1.86 (CV = 12%) and 3.31 (CV = 26%) detected over 8,897 km and 8,828 km of transects, respectively. These produced respective surface abundance indices of 766 (95% CI: 413–1,423) and 669 (95% CI: 244–1,832). In the northern low-coverage stratum, only one individual was detected over 1,030 km of survey lines resulting in a surface abundance index of 12 (95% CI: 2–63). No beluga were observed in Richmond Gulf. New correction factors for availability and perception biases were calculated to account for the proportion of belugas visible at the surface and to account for animals at the surface but missed by observers. For the 2021 survey in James Bay, an availability correction factor of 0.590 (CV = 7.7%) and a perception bias correction factor of 0.601 (CV = 10.5%) resulted in a corrected abundance estimate of 14,213 (95% CI: 9,208–21,938). For the 2021 survey in Belcher Islands-eastern Hudson Bay, an availability correction factor of 0.549 (CV = 7.7%), a perception bias correction factor of 0.601 (CV = 10.5%), and the addition of 289 belugas observed in the Little Whale River Estuary, resulted in a corrected abundance index of 2501 (95% CI: 1,439–4,344). New correction factors were also estimated from and applied to the previous surveys (1985–2015) conducted in the James Bay and Belcher Islands-eastern Hudson Bay area.