Research Document - 2009/111
Distribution of bowhead whales in the SE Beaufort Sea during late summer, 2007-2009
By L.A. Harwood, J. Auld, A. Joynt and S.E. Moore
Abstract
Strip-transect systematic aerial surveys were conducted over the Canadian Beaufort Sea in late August of 2007, 2008 and 2009 to examine the distribution of bowhead whales (Balaena mysticetus). A total of 24-26 N-S transect lines were flown in each survey, all under favourable survey conditions, along lines of longitude spaced at 15’. A total of 334 bowhead whales (244 sightings) including 10 calves, were observed on-transect by primary observers, mostly as individuals (76.6%) and groups of two (14.3%). The study area was divided into 20 km x 20 km grid cells, with the grid cell dimensions equal to the transect spacing. Transect segments within each grid cell were the basic sampling unit for subsequent calculations. The mean regional density of surfaced, visible bowhead whales was 1.81 bowheads/100 km² in 2007, 2.61/100km² in 2008 and 0.66/100 km² in 2009. Extrapolation of visible, surfaced whale counts on transect segments to unsurveyed areas in each grid cell were summed to yield an estimated 1,320 (95% CI 1036 to 1603) bowheads visible at the surface during the 2007 survey. When corrected for submerged whales, an estimated 4,884 to 5,280 bowheads or approximately 50% of the current estimate of stock size was estimated to have been in the study area at the time of the 2007 survey.
The distribution of bowheads was clumped in all years, with variance/mean ratios of 4.13, 5.38, and 2.63 in 2007, 2008 and 2009, respectively (p<0.0001). The proportion of the surveyed grid cells in which bowhead densities were >5 bowheads/100 km² (our working definition of an aggregation within a grid cell) were 11.1% (2007), 13% (2008), and 4.9% (2009). Bowheads aggregated in nine geographic locations within the study area in the 2007-2009 survey series. Not all of the nine areas were used in all years, and no more than six areas were used in any given year. The shallow, shelf waters offshore of the Tuktoyaktuk Peninsula were the most attractive to bowheads in all years of the 2007-2009 survey series, with 47.3% of all whales sighted (66.5% of sightings). The other eight areas where bowheads aggregated had from 1.5% to 6.3% of the total on-transect bowhead whales, and in total (28.1%). The importance of these other areas combined did not equal that of the Tuktoyaktuk Peninsula aggregation area, at least in terms of number of bowheads sighted.
In summary, bowhead whales aggregate in the SE Beaufort Sea each summer for feeding, and appear to do so starting in early August and through to late September or early October. They utilize several different areas for feeding, moving amongst these locations to some extent over the course of the summer. Up to 50% of the population may use the Canadian Beaufort Sea at any one time, and of those in Canadian waters, the majority tends to feed on continental shelf waters offshore of the Tuktoyaktuk Peninsula in waters 20-50 m deep. The propensity of bowheads to aggregate, and a real-time knowledge of the aggregation areas they use in a given season, provides a framework for the establishment of mitigative measures relating to seismic surveys in the Beaufort Sea.
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