Murre foraging, epibenthic sound scattering and tidal advection over a shoal near St. George Island, Bering Sea

Abstract

High concentrations of 2 murre species (Uria aalge and U. lomvia) were observed foraging over a submarine ridge on the east side of St. George Island. Bering Sea, usually in the morning. Simultaneous measurements of tidally generated current velocity and acoustically determined biomass along transects over the ridge demonstrated that the murres were concentrating over a dense epibenthic sound-scattering layer near the ridge crest on southward ebb tide currents. Samples taken by net tows through the layer contained high numbers of euphausiids (Thysanoessa inerrnis and T. raschii). Stomachs of murres collected in the area contained predominantly euphausi~ds. When tidal currents reversed from southward to northward, the sound-scattering layer was carried northward off the ridge and the murres left the area. We hypothesize that the euphausiid aggregations resulted from the interaction of the die1 downward migration of euphausiids in the morning, bottom topography and tidal currents.