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Oceanographic features are known to influence the distribution of marine predators by affecting the abundance and distribution of their prey. We tested the hypothesis that oceanographic features also affect predator distribution by enhancing the profitability of smallsized prey. During July and August 1999, short-tailed shearwaters feeding in Akutan Pass, Alaska (Aleutian Islands) fed upon small (11.6 ± 0.2 mm) euphausiids present in high density near the sea surface. Conductivity-temperature-depth (CTD) casts, hydroacoustic surveys, and net tows revealed that high densities of small euphausiids were associated with a tidal front on the north side of Akutan Pass. At most sites elsewhere in the Bering Sea, away from tidal fronts, shearwaters selected larger (14.2-20.1 mm) euphausiids, even when small euphausiids were present. This study provides evidence that, by promoting high densities of easily accessible prey, oceanographic features can broaden the range of prey sizes taken by marine predators.

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The purpose of this study was to examine whether variation in the extent to which marine birds track prey over small spatial scales can be attributed, in part, to fluctuations in regional prey abundance. The distributions of 4 marine bird species were compared to the distribution of acoustically determined prey biomass on days with contrasting prey abundances measured over a 10 × 20 km (regional) spatial scale. Spatial associations were measured at smaller (local) spatial scales, ranging from 0.2 to 7.6 km. Spatial concordance (i.e. overlap) between the distribution of acoustic biomass and the rhinoceros auklet Cerorhinca monocerata and between acoustic biomass and the Pacific loon Gavia pacifica at the smallest spatial scale (0.2 km) was greater on days when regional prey abundance was relatively low than on days when regional prey abundance was relatively high. This pattern was not evident in Brandt's cormorant Phalacrocorax penicillatus or the common murre Uria aalge. The densities of all 4 marine bird species examined were, however, only significantly correlated with densities of acoustic biomass on days when regional prey abundance was relatively low. I propose that short-term fluctuations in regional prey abundance account for some of the widespread variation previously observed in the strength of spatial associations between marine birds and their prey.