Seabird densities and species and hydrographic features across Amchitka Pass, Aleutian Islands

To examine mesoscale physical and biological factors affecting seabird communities, boat-based seabird censuses were conducted during a single cruise from the North Pacific to the Bering Sea across Amchitka Pass, central Aleutian Islands, during summer. Acoustic surveys of potential prey biomass and currents, and conductivity-temperature-depth profiler measurements for vertical profiles of the temperature, salinity, and fluorescence, were conducted simultaneously. The seabird density and number of species were highest in Amchitka Pass (283 birds km−2, 17 species), where the acoustically determined biomass was the highest, followed by the Bering Sea basin (100 birds km−2, 13 species) and North Pacific basin (4 birds km−2, seven species). Crested auklets Aethia cristatella were dominant (95% in number) in Amchitka Pass, while short-tailed shearwaters Ardenna tenuirostris were dominant (92% in number) in the Bering Sea basin. Surface fluorescence was higher in the Bering Sea basin (median: 1.3 volts) than in Amchitka Pass (0.9) and the North Pacific basin (0.6). Within the pass, the number of crested auklets was highest over the northern part of the sill, where the water column was well mixed, and the northward-flowing tidal currents increased the concentration of acoustically determined biomass in the subsurface layer. Thus, we hypothesize that small alcids were attracted to an aggregation of prey that resulted from tidal currents impinging on the sill.