MURRE FORAGING, EPIBENTHIC SOUND SCATTERING AND TIDAL ADVECTION OVER A SHOAL NEAR ST-GEORGE ISLAND, BERING SEA

MARINE ECOLOGY PROGRESS SERIES Mar. Ecol. Prog. Ser. Published July 2 Murre foraging, epibenthic sound scattering and tidal advection over a shoal near St. George Island, Bering Sea Kenneth 0. Coylel, George L. Hunt, ~ r Mary ~ , Beth ~ e c k e r Thomas J. weingartner1 'Institute o l Marine Science, School of Fisheries and Ocean Sciences. University of Alaska Fairbanks, Fairbanks. Alaska 99775-1080, USA 'Department of Ecology and Evolutionary Biology, University of California, Irvine. Irvine, California 92717, USA ABSTRACT: High concentrations of 2 murre species (Uria aalge and U. lomvia) were observed forag- ing 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 epi- benthic 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. INTRODUCTION Energy flow through marine ecosystems depends on a number of variables, one of which is prey concentra- tion. While high primary production may result in favorable conditions for grazers, the efficiency of energy transfer from grazer stocks to higher trophic levels is sensitive to physical processes which influ- ence the concentration of prey. The complex inter- action of animal behavior patterns and physical processes can produce a patchy distribution of zoo- plankton and micronekton (Omori & Hamner 1982, Sameoto 1983), thereby affecting feeding conditions of high level consumers. Physical processes can transport weakly swimming or drifting organisms horizontally (Banse 1986, Schel- tema 1986, Stancyk & Feller 1986). Tidal currents and wind-induced turbulence can vertically disperse hori- zontal layers (Haury et al. 1990, Incze et al. 1990), or interact with bathymetry to upwell organisms to the surface (Vermeer et al. 1987, Brown & Gaskin 1988). In addition, physical processes and behavior may interact O Inter-Research 1992 to affect the horizontal or vertical distribution of some species (Norcross & Shaw 1984, Epifanio 1987, Genin et al. 1988). Schneider et al. (1987) hypothesized that the interaction of flow gradients with bathymetry would result in concentrations of prey that were pre- dictable in time and space. Whatever the origin of zooplankton aggregations, they are an important source of energy to predators. For example, baleen whales seek out and prey on zoo- plankton aggregations (Kenney et al. 1986, Wishner et al. 1988) and the diets of fish are known to be influ- enced by prey distribution (Owen 1981, Genin et al. 1988). A variety of marine birds prey upon zooplankton aggregations associated with frontal zones (Bradstreet & Brown 1985, Schneider et al. 1987, 1990, Hunt 1990, Hunt & Harrison 1990), and tidal upwellings (Vermeer et al. 1987, Brown & Gaskin 1988). Spatially and temporally predictable prey aggregations may be of particular importance to foraging birds (Schneider et al. 1987). In this paper we examine the effects of tidal currents, bathymetry and the diurnal vertical migra- tion of euphausiids on the feeding behavior of thick-