MEPS 272:281-300 (2004)  -  doi:10.3354/meps272281

Cetacean populations on the SE Bering Sea shelf during the late 1990s: implications for decadal changes in ecosystem structure and carbon flow

Cynthia T. Tynan*

Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA

ABSTRACT: Baleen whales typically migrate to high-latitudes to feed during summer in regions of predictably high concentrations of their zooplankton or fish prey. Therefore, their presence in shelf systems is a good indicator of productive food webs. Line-transect surveys of cetaceans conducted during the late 1990s in the SE Bering Sea (160.32 to 171.08°W) indicate that the middle shelf (50 m < z < 100 m) now supports the highest whale biomass in this region. In recent decades, whales were rare and hence their ecological role on the shelf was limited. During July 1997, the density and abundance (N) of humpback whales Megaptera novaeangliae and fin whales Balaenoptera physalus were highest on the middle shelf between 163.7 and 168.1°W: 0.0236 humpback whales km-2, N = 1308 (coefficient of variation, CV = 0.65), and 0.00924 fin whales km-2, N = 513 (CV = 0.61), respectively. Similarly, during June 1999, fin whales were the most numerically dominant species of large whale in the SE Bering Sea, with highest densities and abundance in the Œcentral¹ middle shelf (163.7 to 168.1°W): 0.0169 fin whales km-2, N = 938 (CV = 0.54). During the summers of 1997 and 1999, 65 and 79% of the prey consumption by fin whales in the SE Bering Sea occurred on the central middle shelf. Prey biomass consumed by fin and humpback whales on the SE Bering Sea shelf and slope in 1997 (264474 to 723504 metric tons, mt) is equivalent to 34-94% of the 1997 echo-integration trawl estimate of walleye pollock Theragra chalcogramma biomass (age 1+) for the shelf east of 170°W (0.77 ×106 mt). Large whales were much more abundant on the middle shelf during the late 1990s than during preceding decades and, therefore, their foraging may have impacted trophic dynamics, carbon pathways and nutrient cycling. The partitioning of available carbon and energy in the pelagic and benthic systems of the shelf, as determined previously during the 1970s, when whales were rare, should now be reexamined relative to the increased ecological impact of these top-level predators. Fluxes of carbon that involve pathways through whales should be incorporated in shelf and oceanic carbon budgets.

KEY WORDS: Bering Sea · Carbon · Cetaceans · Coccolithophore · Ecosystem changes · Whales

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