Responses of piscivorous seabirds at the Pribilof Islands to ocean climate

Climate variability and change are expected to influence the seasonal cycle in North Pacific sub-arctic marine ecosystems. The hypothesis that timing of nesting and productivity of piscivorous seabirds [kittiwakes (Rissa tridactyla and Rissa brevirostris) and murres (Uria aalge and Uria lomvia)] at the Pribilof Islands are related to variation in ocean climate as indexed by changes in sea-ice concentrations (SICs) and sea-surface temperatures (SSTs) was tested. To test this hypothesis, timing and productivity of the seabirds were correlated with the winter sea-ice extent, defined as area-averaged SICs, and with winter, spring, and summer SST near the Pribilofs over a 32-yr period, 1975–2006. Timing and productivity for the two species of kittiwake were strongly correlated with each other and between the two breeding locations, St. George and St. Paul islands. Similar results were found for the two species of murres, although kittiwakes and murres were not strongly correlated with each other. Kittiwakes bred progressively earlier, advancing their hatching dates by 0.58 to 0.88 d/yr over the study period. In contrast, there were no overall trends for murres, with the exception of thick-billed murres at St. Paul that nested progressively later, delaying breeding by 0.47 d/yr. Because of these trends, residuals from quadratic regressions were used to “detrend” the data (including cases where no significant trend was observed) for interannual comparisons with climate variables. Detrended kittiwake timing was inversely correlated with maximum SIC and positively correlated with winter SST. We found no strong relationships between the timing of murre nesting and SIC or SST with either raw or detrended data. Both raw and detrended data revealed a positive relationship between kittiwake productivity and SIC, and an inverse relationship with winter and spring SST. Murre productivity was correlated negatively with summer SST, but unrelated to SST in other seasons and to SIC. Timing and productivity were correlated inversely for murres and kittiwakes at both sites, but were significant only for raw (i.e. non-detrended) murre data and for detrended kittiwake data. Detrending emphasizes high-frequency (interannual) variability by suppressing low-frequency (interdecadal) variability, indicating that low-frequency change in murre timing has an effect on productivity, whereas interannual variation is responsible for the timing–productivity relationship for kittiwakes. These contrasting responses of surface-foraging kittiwakes and deep-diving murres provide insight into how the eastern Bering Sea ecosystem is responding to climate variability and change, and suggest that the food web has changed more substantially in the upper water column than at depth.