Fine-scale oceanographic features characterizing successful Adélie penguin foraging in the SW Ross Sea

Abstract

According to central place foraging theory, breeding seabirds should energetically optimize prey acquisition and, therefore, foraging is expected to be located where prey are most available, within limits defined by the energetics of the species. We have shown this previously for Adélie penguins Pygoscelis adeliae, using foraging intensity as a proxy for prey patch quality, but we have yet to assess the habitat characteristics where foraging success is highest. Here, we report an effort using biologging instruments that recorded location and an index of foraging success, allowing us to characterize aspects of more or less successful foraging locations on the basis of sea-surface temperature, chlorophyll concentration, sea ice cover, water column stratification, and bathymetry. We retrieved data from 162 breeding Adélie penguins over 5 austral summers, 2005−2008 and 2012, and used a machine-learning algorithm to model the relationship between the number of undulations (>1 m) penguins made (i.e. our index of foraging success) and oceanographic conditions at the fine scale (5 km). We found that most oceanographic features were not predictive of foraging success, although light availability and thermocline strength as measured at the scale of individual penguin foraging dives were both relatively strong predictors. Contrary to previous results obtained at larger scales, we showed that at the fine scale, sea ice concentration is not an important predictor of foraging success, although the associated effect of sea ice cover, i.e. a stratified water column as indicated by thermocline strength, was important. We also confirmed that penguins traveled farther to achieve the same foraging success later in the season despite consistent oceanographic features, indicating that prey become depleted as the breeding season progresses. Our findings suggest that finer spatial and temporal scale data, including from underwater, are necessary to accurately describe the environmental variables that correlate with penguin foraging success, reinforcing the promise of small, animal-borne sensors for evaluating ecosystem processes.