With a Bowhead Whale Feeding Hotspot Near Barrow, Alaska

Stephen R. Okkonen, Carin J. Ashjian, Robert G. Campbell, Janet T. Clarke, Sue E. Moore, Kevin D. Taylor

Satellite imagery acquired by earth-orbiting satellites provides spatial and temporal reference frames for illustrating the character of and changes to oceanographic features and phenomena. In the Arctic, where persistent cloud cover limits usable visible-band imagery of the ocean, radar imagery is often the best visualization tool for the ocean surface. This all-weather capability was exploited as part of an interdisciplinary study of a bowhead whale feeding hotspot that develops in late summer and early autumn of most years near Point Barrow, Alaska (Figure 1). The intensity of this feeding hotspot varies with wind-driven changes to the local ocean circulation. When winds are weak (less than 5 m s-1, ~10 kts), a strong oceanographic front forms along the southern edge of Barrow Canyon where the Alaska Coastal Current flows east-northeastward adjacent to the southern flank of the canyon. This front is of particular local interest because it is indicative of aggregation and retention of zooplankton prey in the shallow coastal waters of the western Beaufort Sea and, as a result, is a locus for bowhead whales pausing to feed during their westward fall migration.

Free-floating sea ice serves as a proxy for the distribution of zooplankton prey and is a convenient tracer to illustrate the location of this oceanographic front and the attendant aggregation and retention of zooplankton in the shallow coastal waters of the western Beaufort Sea under weak wind conditions. For example, a RADARSAT-1 image, acquired on 23 October 2007 (Figure 2a), when average winds were weak (4.2 m s−1) clearly shows a frontal boundary along the southern edge of Barrow Canyon separating open water over the canyon from grease/new ice that is largely confined to the shallow western Beaufort Shelf. A second front separates more consolidated sea ice from less consolidated sea ice. A second RADARSAT-1 image, acquired on 30 October 2007 (Figure 2b, i.e., one week after the Figure 2a image), during strong winds (7.4 m s−1) from the east shows the grease/new ice that was previously confined to the western Beaufort Shelf has been pushed northwestward off-shelf into the deeper waters across Barrow Canyon. The fronts along the southern edge of
Barrow Canyon and parallel to the Beaufort Coast that were present during weak wind conditions are no longer present.

Based on the spatial distributions of sea ice shown in these two synthetic aperture radar (SAR) images, it is inferred that the local ocean circulation promotes the aggregation and retention of zooplankton prey in the shallow waters of the western Beaufort Sea when winds are weak, but that local circulation is altered by strong easterly winds so as to not retain or aggregate zooplankton on the western Beaufort Shelf. If the ocean circulation associated with the weak wind regime, in fact, concentrates and retains zooplankton in sufficient quantities to support efficient feeding by groups of bowhead whales, then whale groups should more likely be observed over the Beaufort Shelf when winds are weak. Conversely, if, as indicated by Figure 2b, strong winds from the east do not promote accumulation and retention of zooplankton on the western Beaufort Shelf, then observations of whale groups should be less common during these windier conditions.

These inferences were validated by parsing aerial observations of bowhead whales, obtained during September-October 1982-2009, by the Bowhead Whale Aerial Survey Project (BWASP) according to the wind regime associated with each observation. Figure 3a shows that 62 large groups (4 or more individuals) of whales were observed when winds were weak (5 m s-1). Accordingly, it is reasonable to infer that weak winds lead to the establishment of a shelf-break front adjacent to Barrow Canyon and that the presence of this front is both an indicator and promoter of better feeding opportunities for bowhead whales on the western Beaufort Shelf.