Correction factors account for the availability of bowhead whales exposed to seismic operations in the Beaufort Sea

Abstract

The accuracy of estimates of cetacean density from line-transect survey data depends in large part on how visible the target species is to the observer. Behavioural data (i.e. surface and dive times) from government- and industry-funded aerial observation programmes (1980–2000) were used to calculate availability correction factors needed to estimate the number of bowhead whales (Balaena mysticetus) from aerial survey sighting data. Correction factors were calculated for bowhead whales exposed and not exposed to seismic operations. Travelling non-calf whales were found to be less likely to be available for detection than other whales, and their availability further declined in the presence of seismic operations. Noncalves were also less available to observers during autumn when exposed to seismic operations than when not exposed, regardless of activity (travelling or otherwise). Such differences in availability appear to reflect behavioural responses to the sound of seismic operations that alters the surfacing and diving patterns of bowhead whales. Localised abundance estimated from aerial surveys may range from 3% to as much as 63% higher in areas ensonified by seismic operations if correction factors are applied to account for differences in availability associated with the presence of seismic operations, compared to abundance estimates derived from assessments that only account for changes in availability of undisturbed whales. These results provide the first empirical estimates of availability for bowhead whales exposed to seismic operations and highlight the implications of not correcting for disturbance-related availability in density assessments in the vicinity of seismic operations.

Report of the Scientific Committee

“…Trackline detection probability was estimated using markrecapture distance-sampling methods for trial configurations of observers, with independent data from imagery collected concurrently from a belly port camera during some line-transect aerial surveys. Availability bias was derived from estimates of observation time based on specialised field-of-view experimental flights and estimates of bowhead whale surface and dive duration from Robertson et al (2015). A bootstrap sensitivity analysis suggested that the largest contributors to CV(N) were sampling variance in the line-transect encounter rates and the estimate of trackline detection probability.…”