World Krill Day falls on August 11. When discussing Antarctic ecosystems, Antarctic krill (Euphausia superba) is the undisputed keystone species—a vital food source for many whales, penguins, seals, and even seabirds.

Many people assume that as long as there are enough krill in the sea, animals like penguins and whales won't face food shortages. However, recent scientific discoveries challenge this view: for penguins, the quantity of food matters, but its accessibility is equally crucial in determining whether they can eat their fill. Beneath the icy waters of Antarctica, a hidden battle for survival plays out constantly.

6,000 Dives: Unveiling the 'Cat-and-Mouse Game' Under the Ice

Ecologists have long studied a phenomenon known as "Ashmole's halo," where a region of relatively scarce food forms around seabird breeding colonies. In the past, it was assumed that seabirds simply ate all the food near their homes. But is that really the case? A research team tracked a group of Adélie penguins in a sea-ice-covered bay, which could only forage by diving through fixed ice holes. By attaching loggers to the penguins, scientists analyzed over 6,000 dives from 23 birds and discovered an unexpected pattern: as penguins repeatedly entered the water in the same area, they had to dive deeper, swim farther, and spend more time to find krill. However, once the penguins located krill, their hunting efficiency remained unchanged—the amount of krill caught per unit of time did not decrease. In simpler terms, the increased cost came not from the act of hunting itself, but from the process of finding krill. This means that the krill near the colony didn't significantly decrease; instead, in response to continuous predation pressure, the krill likely moved to deeper waters or dispersed their groups to reduce predation risk. This phenomenon is known in ecology as "functional prey depletion." The prey still exists, but it becomes increasingly difficult for the penguins to access.

Penguin Survival Strategy: Ease of Capture, Not Just Quantity

This emphasis on prey accessibility is even more evident in another penguin species—the chinstrap penguin. Antarctic krill have a famous behavior: diel vertical migration. During the day, they dive deep to avoid predators, and at night, they rise to the surface to feed on microalgae. A recent study in the Scotia Sea found that clever chinstrap penguins have shifted their primary foraging time to dawn and dusk. When krill move upward in the water column, the penguins strike. From an energy expenditure perspective, this is a cost-saving strategy—if they dived deep during the day, they would not only burn more energy but also have less time to return and feed their chicks. More intriguingly, chinstrap penguins don't always head for the densest krill swarms. They prefer those at convenient diving depths that are easier to catch. After all, for a penguin that needs to frequently return to the nest to feed its young, a large swarm far away is less valuable than a small, easily accessible one nearby.

How to Interpret These Antarctic Studies?

Together, these two studies offer a new perspective on conserving Antarctic resources: assessing the health of the Antarctic ecosystem cannot merely ask "how many krill are there?" but also "can predators still access these krill?" With global climate change, rising sea temperatures and shrinking sea ice are altering krill distribution patterns. Meanwhile, the Antarctic krill fishery overlaps spatially with the foraging grounds of some predators. Although the current krill fishery is strictly managed, reducing potential ecological competition between fishing activities and predators remains a key focus of international management and scientific research. These studies remind us that when evaluating the health of the Antarctic ecosystem, focusing solely on total krill biomass may be insufficient, which could lead us to underestimate the current pressures on the ecosystem. Whether prey is easily accessible to predators can also impact the foraging efficiency and breeding success of species like penguins. When environmental changes or human activities alter the spatial distribution or aggregation characteristics of krill, even if overall resources are abundant, animals dependent on krill may face higher foraging costs.

Facing the complex Southern Ocean ecosystem and multiple external pressures, conservation measures need to adopt a whole-ecosystem approach and implement precautionary measures even with limited scientific evidence. This is why establishing a network of marine protected areas in Antarctica and enforcing precautionary fisheries management measures are so urgent. Protecting Antarctic krill is not just about ensuring "there are still krill in the sea," but also about ensuring that during critical ecological periods and in key areas—such as when penguins are raising chicks or whales are feeding—these krill can continue to function ecologically, serving as a food resource that predators can "catch and eat sufficiently." This embodies the concept of ecosystem-based management, which focuses not only on the quantity of a single species but also on its spatial distribution, the relationships between species, and the sustainable, healthy functioning of the entire ecosystem. Protecting Antarctic krill has never been about safeguarding one small, inconspicuous crustacean, but about preserving the stability of the entire Southern Ocean food web. Only when penguins, whales, seals, and seabirds can all consistently access krill that are "catchable and sufficient" can this most remote ocean on Earth continue to sustain its vibrant and unique life.