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 occupied 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 eastern side of the strait by a sharp surface front (the 'Anadyr Front'). Net sampling indicated that calanoid copepods were the numerically dominant component of the zooplankton, and that densities of several species were elevated in the frontal zone, apparently due to mechanical accumulation resulting from surface convergence. Hydroacoustic surveys showed that overall zooplankton biomass was concentrated along the thermocline and at the front. Although the location of the Anadyr Front was highly variable over time scales 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 samples indicated that the copepod Neocalanus plumchrus was the principal prey taken by least auklets both at the front and away from it, indicating that heavy use of the distant frontal habitat was due to the higher densities of their preferred prey (i.e. rather than absence of suitable prey species closer to shore). Whenever aggregations of least auklets were found away from the front, there was evidence that they were exploiting near-surface high-density patches of zooplankton, though the exact mechanisms responsible for the formation of such patches are unclear. In contrast to least auklets, crested auklets Aethia cristatella were usually found away from the front. In several eases, compact aggregations of crested auklets were located over acoustically observed epibenthic layers of zooplankton. Hydrographic data suggested 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 planktivores in Anadyr Strait Likely arises because different physical mechanisms cause concentrations of preferred prey originating at different depths.