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Abstract
Background: Little is known about bowhead whale (Balaena mysticetus) foraging behavior and what concentrations of prey are required to balance the energetic trade-offs of feeding. We used satellite telemetry, archival depth recorders, and water column echo sounding data to study bowhead whale diving behavior relative to prey depth and concentration in Disko Bay, West Greenland. Results: Between March and May 2008 to 2011, nine bowhead whales were tagged in Disko Bay, West Greenland with instruments that collected data on location and diving over a period of 1 to 33 days. The frequency of U-dives (presumed to be foraging dives) was low during winter months but more than doubled in spring concurrent with a decrease in diving depth. The mean speed of the horizontal bottom phase of the U-dives was 0.9 ms -1 and on average, whales spent 37% of their time at the bottom phase of the dive. In March, bowhead whales presumably fed on copepods (Calanus spp.) close to the seabed (between 100 and 400 m). In April and May, after the copepods ascended to shallower depths, bowhead whales also dove to shallower depths (approximately 30 m) more often. However, echo sounding surveys in the vicinity of feeding whales in early May indicated that patches of copepods could still be found close to the seabed. Conclusions: There was a marked change in diving behavior from winter through spring and this was likely in response to the changes in sea ice conditions, primary production and potential copepod abundance in the upper part of the water column. Depth and duration of dives changed significantly during this period; however, other dive parameters (for example the proportion of time spent feeding on the bottom of U-dives) remained fairly constant indicating a constant feeding effort. Bowhead whales target copepods at or close to the seabed in winter months in Disko Bay and continue feeding on copepods when they migrate to the surface. However, bowhead whales leave West Greenland before peak abundance of copepods occurs at the surface.
Highlights
Little is known about bowhead whale (Balaena mysticetus) foraging behavior and what concentrations of prey are required to balance the energetic trade-offs of feeding
Bowhead whales range over large areas in the Eastern Canadian Arctic, Baffin Bay, and West Greenland
With a spring abundance of 1,410 whales, Disko Bay is currently the only area in West Greenland where bowhead whales congregate in large numbers [5]
Summary
Little is known about bowhead whale (Balaena mysticetus) foraging behavior and what concentrations of prey are required to balance the energetic trade-offs of feeding. Why bowhead whales (Balaena mysticetus), primarily females without calves, spend time in Disko Bay, West Greenland in spring [1,2,3] is unknown. High vocalization rates suggest the area is a mating ground [4]; there are few males suggesting that there may be other explanations for whales spending time there. An alternative explanation is spring feeding because of the productivity of the area; it is unclear why other age and sex classes do not feed there [5]. With a spring abundance of 1,410 whales, Disko Bay is currently the only area in West Greenland where bowhead whales congregate in large numbers [5]. This, in concert with early sea ice retreat and complex oceanographic and bathymetric conditions, make Disko Bay productive and attractive to many species [7]
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