Forage fish play a vital role in marine ecosystems by funneling biomass and energy from lower trophic levels to higher marine vertebrates, including commercial fish, seabirds and marine mammals. Often it is useful to investigate factors influencing forage fish populations from the bottom up, and is equally fruitful to monitor the status of predators that influence them from the top down. Seabirds are conspicuous, highly mobile consumers of forage fish that go to great distances and depths to locate ephemeral prey. They can be effective samplers of regional food webs, providing a valuable complement to traditional fisheries sampling. We used the diet of Tufted Puffins (Fratercula cirrhata) to characterize forage fish community composition in waters surrounding their colonies across the Aleutian Archipelago during mid-August in 2012 and 2013. At colonies, we measured puffin chicks to obtain an index of their body condition, which integrated the seasonal success of adults in finding and delivering food to their young. Additionally, we conducted at-sea surveys around each puffin colony to measure seabird abundance and distribution, and measured sea surface temperature and salinity simultaneously. Walleye pollock (Gadus chalcogrammus) dominated the prey community in the eastern Aleutians in 2012, representing 74% of individual prey items and 67% of total biomass delivered to puffin chicks. In the western Aleutians in 2013, walleye pollock were the most prevalent prey (29%), although they were less abundant than in the east. Atka mackerel (Pleurogrammus monopterygius) led prey composition in terms of biomass (62%) in the western islands. Tufted Puffin chick condition was significantly greater in the western then the eastern islands, and condition of chicks in the eastern islands was influenced significantly by the size of their primary prey. The density and composition of marine birds at sea varied among >570 km of strip-transect surveys: Northern Fulmars (Fulmarus glacialis) were observed in highest overall abundance, followed by Tufted Puffins, shearwaters, and auklets. Tufted Puffins were observed in significantly higher proportions and densities in the eastern than the western islands. Oceanographic sampling suggested that puffin colonies were clustered into four distinct water masses. The influence of these different water masses on and around islands may explain spatial heterogeneity in biological characteristics of puffin and other seabird communities and the forage fish they depend on along the Aleutian chain.
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