Composition, biomass and energetic content of biota in the vicinity of feeding bowhead whales (Balaena mysticetus) in the Cape Bathurst upwelling region (south eastern Beaufort Sea)

Abstract

Bowhead whales (Balaena mysticetus) of the Bering-Chukchi-Beaufort population form annual late-summer feeding aggregations offshore of the eastern Tuktoyaktuk Peninsula and Cape Bathurst. This region is strongly influenced by episodic upwelling events. A systematic aerial survey (10% coverage) was flown over the eastern shelf on 2 August 2008, revealing large numbers of bowheads (est. 3500 individuals) in a clumped distribution over the continental shelf. Between 7 and 11 August 2008, zooplankton samples and hydrography were obtained from the vicinity of two bowhead feeding aggregations, one at the northeast edge of the continental shelf, and the second off the northeast shore of the Cape Bathurst. Dense aggregations of zooplankton were found in the lower part of the water column (below 40m), on the shelf, in water with an upwelling signature. The zooplankton were largely resting phases of calanoid copepods (Calanus hyperboreus and C. glacialis) and collectively had twice the abundance, twice the biomass, three times the caloric density and thus six times the energy content of contemporaneous zooplankton samples from the western Canadian Beaufort Shelf. Although upwelling at Cape Bathurst is wind-driven and thus episodic in nature, its consistent delivery of zooplankton to the bowhead foraging area creates conditions attractive to bowhead whales, with an estimated 33% of the bowhead population present on the eastern portion of the shelf at the time of the August 2008 aerial survey.