Research Document - 2013/016
Abundance estimate of eastern Hudson Bay beluga and James Bay, summer 2011
By J.-F. Gosselin, T. Doniol-Valcroze, and M.O. Hammill
Abstract
The management of beluga whales hunted around Nunavik relies on the estimation of abundance of summering stocks, including the endangered eastern Hudson Bay stock. Systematic aerial line-transect surveys to estimate abundance of beluga whales were conducted in James Bay and eastern Hudson Bay from 19 July to 18 August 2011. The flights followed east-west lines with a spacing of 18.5 km in all strata except in the central portion of eastern Hudson Bay, a high coverage area where spacing was reduced by half, i.e. 9.3 km. Unlike 2008, this stratum could not be surveyed twice because of unfavourable weather conditions. A total of 232 beluga clusters were detected between perpendicular distances of 190 m and 2173 m from the track line. When fitted to the ungrouped perpendicular distance data, the hazard-rate model (AIC = 3306.43) was selected over the half-normal model (AIC = 3308.99) and yielded an effective strip half width of 765 m (CV 6.8%). A total of 173 beluga groups with an average size of 3.38 (CV 15.7%) were detected on 4,182 km of lines in James Bay providing a surface abundance index of 7,154 (CV 27.3%). No animal was seen over the 995 km surveyed in the low coverage area of eastern Hudson Bay. Sixty-three groups of belugas were detected on the survey of the high coverage area of eastern Hudson Bay (6,684 km), with an average size of 3.21 (CV 37.2%), resulting in a surface abundance index of 1,433 (CV 47.1%). Abundance indices were corrected for the proportion of time animals are available at the surface to be detected (availability bias) but not for the proportion at the surface that are missed by observers (perception bias). The abundance index for James Bay, after correcting for submerged belugas, was 14,967 (CV 30.2%, 95% CI: 8,316 – 26,939). The abundance index in eastern Hudson Bay corrected for submerged animals and adding the 354 individuals counted in the Little Whale River estuary during coastal survey was 3,351 beluga whales (CV 48.9%, 95% CI: 1,552 – 7,855). This was the sixth visual systematic survey of James Bay and eastern Hudson Bay. High abundance indices in James Bay seem to be linked to the presence of high densities in the northwestern portion of the Bay. More information on movements and genetic identity from this large number of individuals is required to evaluate the stock relationship of belugas in this area. Belugas have a clumped distribution and abundance indices, especially for small populations such as eastern Hudson Bay, may be strongly influenced by the detection of a small number of large groups. Increasing sampling effort is one way of reducing the clumping problem, but the second survey planned for the high coverage area of eastern Hudson Bay could not be completed in 2011. The six available survey indices are comparable for the assessment of population trends. However, abundance indices of belugas from visual surveys could be improved by increasing sampling effort, by improving the estimation of group size and by developing more specific correction factors for availability and perception biases.
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