Emergence of functional prey depletion halo through penguin-krill behavioural dynamics

Predators can influence prey through both consumption and non-consumptive effects arising from their behaviour. In colonial central-place foraging systems, repeated predator activity near breeding sites can generate spatial gradients in prey availability, termed prey-depletion halos (Ashmole's halo). However, it remains unclear whether these patterns are driven primarily by numerical (abundance) or functional (accessibility) depletion of prey. We addressed this question using fine-scale behavioural data from breeding Adélie penguins (Pygoscelis adeliae) feeding on krill in East Antarctica. Combining GPS tracking, three-dimensional dive paths and feeding events, we quantified prey accessibility and patch quality across foraging scales. Penguins foraged most intensively near the colony, where dive depths and under-ice feeding distances were greater, indicating reduced accessibility without corresponding changes in patch quality. Within bouts, penguins progressively exploited deeper or more distant portions of the same patch while maintaining similar feeding rates, consistent with krill redistribution as an escape response. These within-bout processes can accumulate near the colony, generating colony-scale gradients in accessibility. Our results suggest that predator-induced changes in prey accessibility can contribute to the emergence of a functional prey depletion halo around the colony, supporting a role of non-consumptive behavioural processes.