Large seabird colonies are surrounded by a boundary known as Ashmole’s halo, where food sources are depleted and birds must travel farther to feed. The pattern has traditionally been explained by prey depletion — predators simply eating down the local food supply — but a new study of penguins and krill, published in Proceedings of the Royal Society B, points to another factor: prey may become harder to catch even when they haven’t necessarily become scarcer.

“Traditionally, this pattern has been mainly explained by prey depletion. However, prey may also become harder to catch if they change their behavior or distribution in response to predators.”

Hina T. Watanabe, National Institute of Polar Research, Japan

Because fine-scale predator-prey interactions are difficult to observe underwater, Watanabe and colleagues fitted breeding Adélie penguins at an East Antarctic colony with bio-logging devices to track their movements, dives and feeding beneath the sea ice. Landfast ice around the colony meant penguins could only reach open water through a handful of shared openings, concentrating foraging activity in predictable spots.

Across 30 foraging trips and more than 6,000 dives from 23 penguins, the researchers found that during repeated dives from the same ice opening, birds had to dive progressively deeper and travel farther to encounter prey — while feeding rates, once prey were found, stayed unchanged. The same pattern held across the colony: penguins foraging closer to the colony dove deeper and traveled farther, despite little change in feeding success. Because feeding rates held steady once prey were located, the results better fit reduced prey accessibility than simple depletion.

“Food can become harder to obtain even when it has not necessarily been depleted,” Watanabe said, suggesting that repeated diving activity near colonies may cause krill to shift position over time, producing a functional depletion halo even where prey remain present. The next step, she said, is to combine animal-borne sensors with technology that can directly track krill distribution beneath the ice.

Publication

The study, “Emergence of functional prey depletion halo through penguin–krill behavioural dynamics,” was published in Proceedings of the Royal Society B: Biological Sciences (DOI: 10.1098/rspb.2026.0490), funded by the Japanese Antarctic Research Expedition, the National Institute of Polar Research, and the Japan Society for the Promotion of Science.