Models Shed Light on Complex Behavior of Krill Swarms

Antarctic krill, a shrimp-like crustacean, may be small but they pack a big punch. As the most abundant single animal species on Earth, krill provide the basis of the diet for everything in the Southern Ocean from seabirds to whales.

Their cognitively advanced ability to school in dense, tight swarms — billions strong and miles long in some cases — makes Antarctic krill unusual among invertebrates. That propensity also makes these tiny creatures a filling meal for large predators and an easy target for fishing nets.

Scientists at Bigelow Laboratory have been using lab experiments and models to illuminate the surprisingly complex swimming and feeding patterns of krill, including how, why, and when they school. Understanding those behaviors, and how they may shift in a changing ocean, is essential for predicting the future of the Southern Ocean’s vibrant and lucrative marine ecosystem.

“The behavior of these krill is still grotesquely understudied, even though it’s one of the largest fisheries in the world,” said Senior Research Scientist David Fields. “With this work, we’re seeing in real-time how krill use smell, vision, and mechanical cues to shape their behavior, which will help us understand what impact environmental changes might have.”

Fields and Postdoctoral Scientist Nicole Hellessey are working with colleagues at the Georgia Institute of Technology, conducting experiments at Palmer Station in Antarctica. They’re trying to quantify how the animals behave in an artificial channel called a flume as the researchers adjust the levels of light and the flow of water. They’ve also been adding small amounts of chlorophyll and penguin guano to mimic the presence, respectively, of the krill’s food and one of their primary predators.

“Behavior is the first sign of change in animals we can see and measure, long before physiological or genetic change,” Hellessey said. “Understanding how krill will respond under different conditions in the lab gives us the ability to predict behaviors in the wild where such experiments are impossible.”

In 2019, the team published research showing how krill alter their swimming speed and direction in response to the scent of food and even slight changes in the flow rate. And recently, they completed a study showing how krill are similarly sensitive to predator smells, taking on tighter swimming paths and even eating less when there’s penguin guano in the water. Their clear avoidance of predators might explain why swarms are rarely seen near penguin colony nesting sites.

Meanwhile, the researchers are trying to understand whether these behaviors differ if the water is flowing horizontally or vertically — as it does in rich upwellings along the continent’s coast. They’re also scaling up the experiments from individuals to hundreds of krill in a single flume.

But, as Fields points out, the conditions and smells krill encounter are changing in the Southern Ocean as some penguin species decline and chlorophyll levels and currents shift in response to melting sea ice. To that end, Hellessey is working with Senior Research Scientist Nick Record to develop computer models, based on their experimental results, that can predict how those environmental changes might alter krill behavior.

“We can now explore how krill habitat is shifting in response to changes in algal blooms and how changing currents will affect krill dispersal and distribution,” Hellessy said. "This is all important information for conservation and fisheries management in Antarctica.”

The work to date has been supported by the National Science Foundation’s Polar and Biological Oceanography programs. As the current round of funding wraps up, the scientists are finding ways to take the research into new exciting directions. That includes observing how behaviors change depending on the season and studying whether other species of krill, like those in the Gulf of Maine, display similar behavior even when they don’t have the capacity to school.

“Schooling is clearly part of the Antarctic krill survival strategy, but it takes a critical number for them to do it, and if you deplete the populations, you could reduce them to concentrations that aren’t enough for them to start schooling,” Fields said. “That would totally change how these animals respond to their environment, which could have huge implications for the whole ecosystem.”

Photo Captions:

Photo 1: Postdoctoral Scientist Nicole Hellessey during experiments at Palmer Station in Antarctica (Credit: David Fields)

Photo 2: An Antarctic krill under a microscope (Credit: David Fields)

Photo 3: Senior Research Scientist David Fields doing experiments at Palmer Station (Credit: Nicole Hellessey)