Full Breakdown
Penguins Push Krill Deeper, Creating Ashmole’s Halo
7/17/2026, 10:56:09 AM
Study of Adélie Penguins at Hukuro Cove
Researchers studied a small Adélie penguin colony of 217 breeding pairs at Hukuro Cove, East Antarctica, to examine why a ring of reduced prey density—Ashmole’s halo—forms around seabird colonies. The team, led by postdoctoral scholar Hina T. Watanabe of Japan’s National Institute of Polar Research (NIPR), considered both prey depletion and changes in prey accessibility.
High-Resolution Bio-Logging Methodology
Twenty-three penguins carried lightweight sensors (?1.6 % body mass) that logged depth, motion, GPS position and feeding strikes; some also filmed underwater. The devices recorded 30 foraging trips, over 6,000 dives, an average depth of 61 feet, and trips up to 2.6 miles from the colony. Stomach samples showed krill made up 99.9 % of prey by weight, with Antarctic krill accounting for about 85 %.
Findings on Prey Accessibility
Repeated dives through the same sea-ice openings forced penguins to dive deeper and swim farther before encountering krill. Catch rates stayed steady once prey were found, indicating krill were not depleted but displaced vertically or laterally. The authors suggest that localized prey displacement can accumulate, creating functional depletion that appears as the halo pattern.
Broader Ecological Implications
Krill are central to the Southern Ocean food web, supporting whales, seals and other seabirds. Misreading reduced accessibility as scarcity could distort ecosystem health assessments, and deeper dives increase energy costs for feeding chicks.
Verbatim Quotes
- “Traditionally, this pattern has been mainly explained by prey depletion: Predators consume prey near the colony, reducing prey abundance,” — Hina T. Watanabe, postdoctoral scholar, NIPR
- “We found that penguins had to dive progressively deeper and farther to encounter prey, but once prey were encountered, feeding rates remained unchanged.” — Hina T. Watanabe, postdoctoral scholar, NIPR
- “This suggests that prey accessibility — not only prey abundance – can shape predator foraging patterns.” — Hina T. Watanabe, postdoctoral scholar, NIPR
- “What the work cannot show “We inferred changes in prey accessibility from penguin behavior, but we did not directly observe krill movements.” — Hina T. Watanabe, postdoctoral scholar, NIPR
