Résumé
Black guillemots (Cepphus grylle) are one of the few ice-obligate seabirds, dependent on prey associated with the poorly sampled cryopelagic ecosystem associated with arctic sea ice. During the past four decades guillemot diet and breeding success, a proxy for prey availability, has been studied on Cooper Island, 35 km east of Point Barrow, Alaska. Parent guillemots provision nestlings from late July to early September and before 2002, when sea ice was commonly within the 30 km foraging range, arctic cod (Boreogadus saida) was the primary prey and breeding success was typically high. In the last decade, however, increasing retreat of summer sea ice has resulted in seasonal prey shifts from adult to first-year arctic cod followed by a shift in nearshore demersals, primarily four-horned sculpin (Myoxocephalus quadricornis). Deployment of temperature-depth recorders on provisioning parents combined with analysis of physical oceanographic data indicate that adult cod availability decreases with the disappearance of sea ice with first-year cod persisting at depth in ice-free waters until sea surface temperature (SST) exceeds 4[degrees]C. Sculpin show dramatic daily shifts in availability late in the summer associated with rapid changes in SST. As arctic summer sea ice continues to decrease annually, the Cooper Island black guillemot colony can be an important monitor of changes in nearshore forage fish populations in the western Beaufort Sea, providing a level of sampling not available with traditional shipboard methods. Additionally, it can provide direct evidence of how the changes in fish populations are affecting upper trophic level predators.