Swarming benthic crustaceans in the Bering and Chukchi seas and their relation to geographic patterns in gray whale feeding

Abstract

Swarming benthic crustaceans were widespread in the Chukchi and Bering seas. Swarms differed in their geographic extent, local biomass, and life stages of swarming individuals and thus in their availability to feeding gray whales (Eschrichtius robustus). Immature amphipods apparently swarmed for dispersal, whereas cumaceans probably swarmed for mating. All life stages of the hyperbenthic mysids occurred above the sea floor. Although the geographic spread of mysid swarms and shrimp communities was much greater than for the amphipod and cumacean swarms, the latter swarmed in denser patches to produce higher local biomass. Crustacean swarms are important in describing the geographic patterns of gray whale feeding from the Chukchi Sea to Baja California, including the primary, secondary, and tertiary feeding grounds. The primary feeding ground is in the southern Chukchi Sea and especially the northern Bering Sea, where gray whales suck infaunal amphipods from fine sand, producing an extensive record of feeding excavations. The primary feeding ground is divided into a relatively deep zone (> 20 m), where tube-dwelling ampeliscid amphipods are the major prey, and a shallow zone (< 20 m), where burrowing pontoporeid amphipods dominate. The secondary feeding ground is in the southern Bering Sea along the eastern Alaska Peninsula and adjacent Alaskan mainland where shrimp and mysids are the major prey. The tertiary feeding ground is at the periphery of the primary and secondary feeding grounds in Alaskan waters and south of the Bering Sea where there is a general decrease in the availability of prey and their use by gray whales from Canada to Baja California. The tertiary prey communities include swarms of amphipods, cumaceans, and mysids as well as infaunal polychaete worms, but mysids are used the most by whales. The primary gray whale feeding ground was much smaller during low sea levels when the extensive Beringian Platform was exposed to air. This shallow shelf is a unique habitat that presently harbors the largest ampeliscid amphipod community in the world. At low sea level, swarming crustaceans like those sampled in the present study may have been equally or more important to gray whales than infaunal prey. These historical changes in prey availability may account for the catholic diet of the gray whale.