Foraging at a front: hydrography, zooplankton, and avian planktivory in the northern Bering Sea

We studied hydrographic structure, zooplankton distributions, and foraging by planktivorous seabirds in the Anadyr Strait, northern Bering Sea, during 4 summer cruises (1984-1986 1993). The western portion of the strait was o ccupied by cold, dense Anadyr water that was mixed from top to bottom. This mixed water was separated from the stratified Bering Shelf water on the ea stern side of the strait by a sharp surface front (the 'Anadyr Front'). Net sampling indicated that calanoid copepods were the numerically dominant co mponent of the zooplankton, and that densities of several species were elev ated in the frontal zone, apparently due to mechanical accumulation resulti ng from surface convergence. Hydroacoustic surveys showed that overall zoop lankton biomass was concentrated along the thermocline and at the front. Al though the location of the Anadyr Front was highly variable over time scale s as short as 1 d, large numbers of least auklets Aethia pusilla often flew 25 to 50 km from their breeding colonies to feed at the front. Diet sample s indicated that the copepod Neocalanus plumchrus was the principal prey ta ken by least auklets both at the front and away from it, indicating that he avy use of the distant frontal habitat was due to the higher densities of t heir preferred prey (i.e. rather than absence of suitable prey species clos er to shore). Whenever aggregations of least auklets were found away from t he front, there was evidence that they were exploiting near-surface high-de nsity patches of zooplankton, though the exact mechanisms responsible for t he formation of such patches are unclear. In contrast to least auklets, cre sted auklets Aethia cristatella were usually found away from the front. In several eases, compact aggregations of crested auklets were located over ac oustically observed epibenthic layers of zooplankton. Hydrographic data sug gested that intense subsurface jets and/or upwelling along the eastern side of the strait might have increased the availability of the crested auklets ' preferred euphausiid prey. Thus, spatial segregation of the 2 principal p lanktivores in Anadyr Strait Likely arises because different physical mecha nisms cause concentrations of preferred prey originating at different depth s.