Gray whale and walrus feeding excavation on the Bering Shelf, Alaska

Abstract

Sidescan sonar has been used to delineate benthic feeding structures of the California gray whale ( Eschrichtius robustus ) and Pacific walrus ( Odobenus rosmarus divergens ) on the northeastern Bering Shelf. The gray whales (average mouth length, 2.0 m), when suction feeding on infaunal amphipods, create shallow pits in the sea floor, typically 2.5 m x 1.5 m x 10 cm deep, which are distinct and mappable on sidescan sonographs. Similarly, walrus, when foraging for shallow clams, create long, linear feeding furrows that average 47 x 0.4 x 0.1 m (length-width-depth). The distribution of the whale pits over 22,000 km 2 of the Bering Shelf closely matches 1) sightings of feeding whales identified by mud plumes; 2) the distribution of ampeliscid amphipods, the gray whale's main prey; and 3) the distribution of a transgressive inner-shelf fine sand that serves as a substrate for the amphipods. The walrus' furrows are recognized over 6,600 km 2 of variable muddier or coarser-grained substrate with clam-rich benthic communities that surround the fine sand substrate of whale feeding areas. The whale feeding pits are commonly enlarged and oriented by seasonal storm-related scour. Nonenlarged pits (less than 5.3 m 2 in area) form a discrete statistical population that we define as fresh . We estimate that a minimum of 5.6 percent (1,200 km 2 ) of the feeding area of the northeastern Bering Shelf (22,000 km 2 ) was covered by fresh pits made by whales during the 1980 feeding season. Assuming that the average pit depth is 10 cm, a minimum of 120 x 10 6 m 3 (172 x 10 6 metric tons) of sediment, equivalent to about three times the yearly sediment load of the Yukon River, is excavated and injected into the water column by as many as 16,000 gray whales feeding in northeastern Bering Sea each season. As a result of 1) sediment resuspension by whales, 2) average current speeds of 10.7 cm/s northward during the feeding season, and 3) enhanced post-feeding current scour because of bottom roughening, the following occur: the majority of the clay fraction (4.3 x 10 6 metric tons) of resuspended sediment is advected to the Chukchi Sea each year; sand gradually is transported northward and fills old feeding pits; modern mud does not accumulate in this region; and the whale-disturbed sand lacks physical sedimentary structures and matrix mud. Walrus feeding features are smaller, formed in higher-energy environments, and modified more rapidly than whale feeding pits. The amount of sediment reworking by walrus feeding may nearly equal that of whale feeding, but this cannot be quantified accurately.