The Krill Never Vanished: So Why Are Penguins Struggling to Eat?

Antarctic krill have not disappeared from the waters around Adélie penguin colonies in East Antarctica. Yet a new study shows the birds are working noticeably harder to catch them — a finding that is forcing scientists to rethink a long-held assumption about how predators and prey interact in extreme environments.

The research, led by Hina T. Watanabe, a postdoctoral scholar at Japan’s National Institute of Polar Research, and published on 15 July in the Proceedings of the Royal Society B, set out to test the cause behind Ashmole’s halo, a well-documented pattern in which food appears scarcer in the waters immediately around large seabird colonies. ‘Traditionally, this pattern has been mainly explained by prey depletion: predators consume prey near the colony, reducing prey abundance,’ Watanabe said. ‘However, prey may also become harder to catch if they change their behaviour or distribution in response to predators.’

To test that idea, her team fitted breeding Adélie penguins with advanced bio-logging devices that recorded dive depth, movement and feeding events in three dimensions, backed by video footage. Thick sea ice covering the bay meant the penguins could only enter and leave the ocean through a handful of shared holes, concentrating their hunting in the same locations trip after trip.

Over 30 foraging trips, the team tracked 23 penguins across more than 6,000 dives. Every time a penguin used the same opening in the ice repeatedly, it had to dive deeper and travel farther underwater to find prey on each successive attempt. But once it found the krill, it fed just as successfully as it always had, meaning the krill had not shrunk in number — they had simply moved.

‘We found that penguins had to dive progressively deeper and farther to encounter prey, but once prey were encountered, feeding rates remained unchanged,’ Watanabe said. ‘This suggests that prey accessibility, not only prey abundance, can shape predator foraging patterns.’ Penguins hunting close to the colony made significantly deeper and longer dives than those foraging farther away, even though nearby waters still held abundant krill.

Watanabe’s team describes the long-term version of this effect as ‘functional prey depletion’ — food that technically remains present in the ecosystem but becomes progressively harder for predators to reach as hunting pressure repeats in the same spots, a dynamic that could matter increasingly as climate change and shifting sea ice reshape the Southern Ocean.

Wikimedia Commons/by Jason Auch