For penguins that nest in Antarctica, finding food is no longer just a matter of how many fish or krill there are in the ocean. It’s also about how difficult it is to catch those animals.Scientists who dived more than 6,000 penguins beneath Antarctic sea ice found that prey may be much harder to reach, even if their overall numbers did not decline. Instead of eating their food until it almost disappears, the repeated presence of hunting penguins causes krill and fish to change their behavior, moving deeper into the water or spreading out to avoid the birds. This forces the penguins to work harder to find food.The study, published July 15 in the Proceedings of the Royal Society B, changes the way scientists understand predator-prey relationships in extreme environments. This suggests that how easy it is to access prey is as important to survival as the availability of food.
mystery of empty fields
For many years, scientists have been studying a phenomenon called Ashmole’s halo. This is the area around large seabird colonies where food becomes scarce.Traditionally, researchers believed this happened because thousands of birds living in one place ate most of the prey nearby. As a result, the birds had to move away from the colony to find enough food.“Traditionally, this pattern has been explained primarily by prey scarcity: predators eat prey near the colony, reducing prey abundance,” said Hina T. Watanabe, a postdoctoral scholar at the National Institute of Polar Research in Japan. “However, if the prey changes its behavior or distribution in response to predators it may also be harder to catch.”Because it is extremely difficult to observe these small and rapid interactions underwater, scientists have little direct evidence to show how predators change the behavior of their prey.To investigate, Watanabe and his team studied breeding Adélie penguins in East Antarctica. The bay around the colony was covered with thick sea ice, meaning that the penguins could only enter and leave the sea through a very small number of shared holes in the ice. Due to this, their hunting activity became concentrated in those places.
Measuring dive in three dimensions
The researchers equipped the penguins with advanced bio-logging equipment. These trackers recorded bird movements, dive depths and feeding events in detailed three-dimensional data supported by video recordings.In total, the team collected information from 30 search trips, tracking 23 penguins on more than 6,000 dives beneath the sea ice.The results showed a clear pattern. Each time penguins enter the water repeatedly from the same hole, they have to dive deeper and swim farther under the ice to find prey during each new dive.However, once they reached krill, they fed as successfully as before. If too much of the krill were depleted, the penguins would find less of them and their feeding rate would decrease. Instead, they continued to feed at the same pace, but had to spend more time searching as the krill moved away from the area where the penguins were hunting.
Schematic illustration showing how repeated dives from shared sea-ice openings lead penguins to encounter krill at progressively greater depths and farther beneath the Antarctic sea ice, while feeding rates remain unchanged.
The same pattern was found in the breeding colony. Penguins searching for food closer to the nesting site had to dive deeper and longer than penguins feeding farther away, even though there were still plenty of krill in the surrounding water.“Food can be difficult to obtain, even if it is not necessarily finished,” Watanabe said. “We found that penguins had to dive progressively deeper and farther to encounter prey, but once prey was encountered, feeding rates remained unchanged. This suggests that prey access, not just prey abundance, can shape foraging patterns of predators. Because repeated diving activity is concentrated near breeding colonies, local prey displacement can accumulate over time, contributing to functional prey depletion, Where prey remains present but becomes increasingly less accessible.“
How chin strap Penguins hunt at twilight
This dependence on prey access is also supported by recent research on other penguin species. In the Scotia Sea, scientists tracked 45 breeding chinstrap penguins from two colonies on Monroe and Powell Islands during 2022 and 2023.By combining tracking information with underwater acoustic surveys of krill, researchers found that chinstrap penguins plan their daily hunts around the vertical movements of their prey.Antarctic krill move up and down in the water every day. During daylight hours, they stay in deep water to avoid predators that hunt. At night, they come closer to the surface to eat small algae.Chinstrap penguins take advantage of this behavior by doing most of their hunting during dawn and dusk. As the krill begin to move upriver, penguins travel farther away from the coast to prey on these dense groups.From an evolutionary perspective, this allows the penguins to obtain more energy while exerting less effort. Catching krill near the surface requires much less energy than diving deep into cold water during the day.When chinstrap penguins hunted during daylight, they stayed closer to the colony and dived deeper to reach krill. This creates an energy trade-off, as adult birds must balance the effort of diving deep with the need to return quickly and feed their hungry young.
The goal is flock, not biomass
The chinstrap study also found that penguins do not always hunt where the total amount of krill is highest. Instead, they often choose areas with low krill numbers if the prey is at easy diving depths.This suggests that finding a single, easy-to-catch flock of krill is more important to hunting penguins than the total amount of krill spread throughout the water. This strategy also helps chinstrap penguins avoid direct competition with Adélie and gentoo penguins, which hunt primarily in the middle of the day.Understanding these detailed hunting behaviors is becoming increasingly important as climate change, recovering whale populations, and human activity are reshaping the Southern Ocean. Warming ocean temperatures and shrinking sea ice threaten krill breeding sites, while commercial fishing fleets harvest krill in the same areas where penguins search for food.For conservationists, it is essential to understand how and when penguins reach their prey. If warming seas drive krill deeper into the water or fishing activity scatters them across the ocean, penguins may struggle to find enough food, even if abundant amounts of krill still remain in the water.