SEABIRD FEEDING ON BENTHIC AMPHIPODS FACILITATED BY GRAY WHALE ACTIVITY IN THE NORTHERN BERING SEA

Ampeliscid amphipods are the dominant benthic fauna in the northern Bering Sea and the major food of the California gray whale. Field studies indicate that benthic amphipods brought to the surface during gray whale feeding provide a food source for surface-feeding birds (northern fulmars, red phalaropes, black-legged kittiwakes) as well as diving birds (thick-billed murres). A sampling grid was used to document zooplankton and benthic population structure. Neuston tows, behavioral observations and collections of birds for stomach content analyses were made within and outside of 'whale slicks', the muddy plumes at the sea surface marking a site where a feeding whale has surfaced. Results indicate that infaunal amphipods are abundant at the surface in whale slicks, and that abundance and size-class of these floating amphipods directly reflect relative abundance and size-class of the major amphipod species found in the underlying benthos. Surface-feeding birds ate mostly small amphipods at the slicks, which were the dominant amphipod size-class observed in the tows. Birds as divergent in size as northern fulmars (700 g) and red phalaropes (60 g) consumed small amphipods or fragments less than 2 mm in length. Thick-billed murres, which dive to feed, caught the larger amphipods that sank. Location and abundance of northern fulmars and red phalaropes feeding in the study area were positively related to occurrence of feeding whales. The breeding thick-billed murres and black-legged kittiwakes were in a more restricted distribution on the east side of the grid, nearest the seabird colonies on King Island, Alaska, USA. The local distribution of all 4 species is shaped by whale feeding activity, and their diet is a direct reflection of benthic population structure of the northern Bering Sea.