Research Document - 2007/080

A review and re-analysis of Cosens et al. (2006) aerial survey assessment of bowhead whale abundance for the eastern Canadian Arctic

By Dueck, L., P. Richard, and S.E. Cosens

Abstract

A review of Cosens et al. (2006) single-platform analysis of double-platform aerial surveys of bowhead whale populations in the Eastern Arctic was conducted. A similar single-platform analysis was conducted with revised data, accounting for errors and certain potential biases in the original analysis. Errors and potential sources of bias in the original analysis included use of off-transect sightings, duplicate sightings, sightings in narrow fiords, sightings with uncertain angle readings, distance calculation errors, and poorly fitting detection functions. A reanalysis of the single-platform approach, using a global detection function, resulted in about half the numbers estimated by Cosens et al. Supplementary analyses were conducted, including double-platform mark-recapture distance sampling with covariates, to account for perception bias and variables that influence sighting detectability with distance. Recapture rate was low, demonstrating the likelihood that observers missed significant numbers of animals that were available to be seen. Ice was the only covariate to improve the fit of the detection function. The double-platform analysis resulted in an estimate nearly twice that estimated by Cosens et al. single platform method. The results of the double-platform approach suggest that there is a sizeable bowhead whale population, but large confidence intervals reflect the uncertainties due to small sample size in sightings, mark-recapture and dive behaviour, and the compounding of these uncertainties when used to correct for detection probability, missed animals, and availability bias. Potential biases in both directions are discussed.

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