FORAGING ECOLOGY OF SHORT-TAILED SHEARWATERS NEAR THE PRIBILOF ISLANDS, BERING SEA

We studied short-tailed shearwaters Puffinus tenuirostris foraging nea r the Pribilof Islands, Alaska, USA, during the summers of 1987, 1988, and 1989. Their foods were almost exclusively the euphausiid Thysanoe ssa raschii, which they obtained both from near-surface swarms and fro m epibenthic layers. Near-surface mating swarms of euphausiids occurre d in areas of elevated phytoplankton standing stocks near inshore tida l fronts. Many of these euphausiids had attached spermatophores. Shear waters also obtained euphausiids over shallow reefs and inshore of the fronts where euphausiids were trapped in water shallower than 40 m by irregularities in bottom topography ('bathymetric traps'). We hypothe size that the largely inshore distribution of shearwaters in the south eastern Bering Sea described by previous workers is the result of attr action of shearwaters to frontal areas where euphausiids may forage on phytoplankton stocks throughout the summer. These areas, when shallow er than 40 m, would also permit shearwaters to access epibenthic aggre gations of euphausiids during daylight, when euphausiids not engaged i n mating swarms usually migrate to depth.