While decreases in Arctic sea ice affect all marine communities in the Arctic Basin, the effects are greatest on the cryopelagic ecosystem and species with critical life history stages dependent on the presence of sea ice. During the recent and ongoing period of rapid sea ice loss these species have been subject to spatial and temporal disruptions requiring behavioural plasticity. Mandt’s Black Guillemots (Cepphus grylle mandtii) is one of the few ice-obligate Arctic seabirds. Polar cod (Boreogadus saida) is their preferred prey. We monitored their prey selection and diving behaviour during the annual period of chick provisioning from 2011 to 2017, to assess their ability to respond to the now common seasonal loss of sea ice and increased water temperature in their nearshore foraging area. The percentage of polar cod fed to nestlings decreased with increasing SST, with fourhorn sculpin (Myoxocephalus quadricornis), a nearshore demersal, becoming common (20% of deliveries) with SST > 2.0 °C and comprising more than half of the prey when SST > 3.4 °C. This prey-switch coincided with a marked increase in dives and time underwater per day and a decrease in dive duration as birds switched to nearshore, benthic habitats. Sea ice is declining and SST increasing throughout the Arctic Basin and other upper-trophic level predators dependent on polar cod could be expected to be exhibiting similar prey-switching and modifications in foraging effort.
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