Availability bias adjustment for calculating aerial survey abundance estimates for Belugas (Delphinapterus leucas) in the Eastern Beaufort Sea

Abstract

Abundance estimates of whales from aerial surveys need to be adjusted for animals that are underwater during the survey and cannot be counted by survey observers. In 2019, an aerial survey was conducted to estimate the abundance of the Eastern Beaufort Sea beluga (Delphinapterus leucas) population. To complement the survey, belugas were equipped with satellite transmitters to record their time at depth and determine adjustment factors for the survey estimate. Two types of adjustments factors, instantaneous and non-instantaneous, were computed depending on the type of survey (photographic vs. with visual observers) and the area surveyed (inshore vs. offshore). The instantaneous adjustment factor was calculated for use in the photographic strata of the survey of the inshore areas based on the proportion of time belugas spent within 1 m of the surface. The non-instantaneous adjustment factor was computed for the observer-based strata of the survey in the offshore areas. The non-instantaneous adjustment factor was based on the proportion of time belugas spent within 5 m of the surface and was computed using the Laake method. The resulting adjustment factors were 1.56 (S.D. = 0.592) for the instantaneous and 1.94 (S.D. = 0.521) for the non-instantaneous.