Large seabird colonies have a surrounding boundary known as Ashmole's halo, where food sources are depleted, forcing the birds to travel farther to gather the food they need. The reason seems obvious — the more birds, the more they eat, which reduces the available prey. But that might not be the only factor at play, according to a new study of penguins and krill published on July 15 in the Proceedings of the Royal Society B.
"Traditionally, this pattern has been mainly explained by prey depletion: Predators consume prey near the colony, reducing prey abundance," said corresponding author Hina T. Watanabe, a postdoctoral scholar at the National Institute of Polar Research in Japan. "However, prey may also become harder to catch if they change their behavior or distribution in response to predators."
Watanabe explained that these fine-scale predator-prey interactions are difficult to observe in marine environments, resulting in limited field evidence for how predators influence prey behavior. To better understand these interactions, Watanabe and the research team outfitted breeding Adélie penguins at an East Antarctic colony with bio-logging devices to track their movements, dives and feeding behaviors beneath Antarctic sea ice. Because the bay around the breeding colony was covered by landfast sea ice, breeding penguins could access the ocean only through a limited number of shared sea-ice openings, creating localized areas of repeated foraging activity.
"We used high-resolution bio-logging data to reconstruct three-dimensional underwater dive paths and identify feeding events beneath Antarctic sea ice (Fig.1). This allowed us to investigate how prey accessibility changed as penguins repeatedly foraged from the same sea-ice opening during a foraging dive bout and whether these fine-scale changes could contribute to the formation of Ashmole's halo," Watanabe said, explaining that they specifically instrumented nesting birds so that the penguins would need to forage and return to the colony to feed their young.
The study yielded data from 30 foraging trips and more than 6,000 dives across 23 individual penguins. During repeated dives from the same sea-ice opening, penguins had to dive progressively deeper and travel farther beneath the ice to encounter prey (Fig.2a–c), while feeding rates remained unchanged once prey were encountered (Fig.2d). The same pattern also emerged across the breeding colony: penguins foraging closer to the colony likewise had to dive deeper and travel farther beneath the ice (Fig.3a,b), despite little change in feeding rates (Fig.3c). Because feeding rates remained unchanged once prey were encountered, the results were more consistent with reduced prey accessibility than with prey depletion alone.
"Food can become harder to obtain even when it has not necessarily been depleted," Watanabe said. "We found that penguins had to dive progressively deeper and farther to encounter prey, but once prey were encountered, feeding rates remained unchanged. This suggests that prey accessibility — not only prey abundance — can shape predator foraging patterns. Because repeated diving activity is concentrated near breeding colonies, local prey displacement may accumulate over time, contributing to functional prey depletion, where prey remain present but become progressively less accessible."
According to Watanabe, by combining reconstructed three-dimensional dive paths with video-validated feeding events, the study provides empirical support for the hypothesis that repeated predator disturbance may alter prey behavior. There are some limitations to the work, though.
"We inferred changes in prey accessibility from penguin behavior, but we did not directly observe krill movements. The next step is to combine animal-borne sensors with technologies that can directly measure prey distribution beneath sea ice," Watanabe said. "Ultimately, I hope to understand how interactions between predators and prey generate ecological patterns across scales, from individual foraging behavior to colony-scale resource landscapes."
Co-authors are Junichi Takagi, School of Platforms and Field Science Education and Research Center, Kyoto University; and Akinori Takahashi, National Institute of Polar Research and Polar Science Program, Graduate Institute for Advanced Studies, SOKENDAI.
The Japanese Antarctic Research Expedition, the National Institute of Polar Research and the Japan Society for the Promotion of Science funded this work.
