Timing of sea-ice retreat affects the distribution of seabirds and their prey in the southeastern Bering Sea

The potential impacts of future climate warming on marine ecosystems can be assessed by examining the effects of present-day variation in climate. Here we report how the cross-shelf distributions of seabirds and their potential prey responded to interannual variation in the timing of sea-ice retreat in the southeastern Bering Sea. We expected that in years of early sea-ice retreat, prey resources would be scarce over the shelf and that seabird species would concentrate in frontal regions where availability of zooplankton and forage fish might be enhanced. To test this hypothesis, we used a 40 yr database of the distribution of marine birds and recently available data on the distribution of zooplankton and forage fish. We found that although there were substantial changes in the distribution of seabird species between years with early and late sea-ice retreat, there was no overall shift into frontal regions. Instead, in years with early sea-ice retreat, there was a strong tendency for seabird species that foraged off the shelf to move toward, or onto, the shelf, whereas inshore-foraging species shifted seaward. Further, the cross-shelf centers of abundance of the copepod Calanus mar shallae/glacialis shifted seaward, but there was little change in the cross-shelf distributions of Neocalanus spp. copepods, euphausiids (primarily Thysanoessa spp.), and age-0 pollock Gadus chalcogrammus. Shifts in seabird distributions, as demonstrated in this study, indicate the importance of sea-ice retreat for structuring trophic interactions and could present both opportunities and challenges for central-place-foraging breeding seabirds and long-distance migratory species.