Summer abundance estimates for St. Lawrence Estuary Beluga (Delphinapterus leucas) from 52 visual line transect surveys and 11 photographic surveys conducted from 1990 to 2022

Abstract

The St. Lawrence Estuary (SLE) beluga population has been surveyed repeatedly via photographic and visual aerial surveys to evaluate its abundance and trends following similar survey designs since 1988. Published abundance indices from both types of surveys up to 2014 were corrected for availability bias only, using a fixed correction factor. The present study presents results from recently-conducted photographic strip-transect (four surveys, in 2019), and visual line-transect surveys (16 surveys, in 2015, 2016, 2018, 2019, 2020, 2021, and 2022) of the SLE beluga population, as well as a re-analysis of past surveys conducted since 1990 using newly-estimated survey-specific correction factors for both availability (photographic and visual) and perception biases (visual surveys only). The latest photographic survey in 2019 resulted in a total average abundance estimate of 2,119 whales (SE = 267). The latest visual survey in 2022 resulted in a total abundance estimate of 1,257 whales (SE = 400). The 2022 visual survey has the lowest abundance estimate of all visual surveys conducted since 2001. Mean abundance estimates from visual surveys were consistently higher than for photographic surveys once fully corrected for both availability and perception biases. In past survey analyses, no perception bias correction factors were applied due to lack of data, but their use in the present study yielded fully corrected abundance estimates which are 1.5 to 2 times greater than the abundance indices corrected only for availability. While past analyses were adequate to produce abundance indices for this population, the current study represents a major step forward not only for an improved precision and accuracy of abundance estimates for the SLE beluga population but also for understanding the caveats associated with different methodological and analytical approaches to aerial surveys.