Off the frigid coast of Norway, a gruesome spectacle unfolded that scientists had only imagined. In the early hours of a February morning, millions of capelin, small Arctic fish, came together to spawn. But instead of peacefully laying their eggs, they found themselves at the center of a predatory onslaught. Within hours, the swarming capelin had become a feast for their relentless pursuers — Atlantic cod.

The dramatic encounter, reported by researchers from MIT in the USA and Norway, marks the largest recorded instance of marine predation. Using advanced acoustic imaging technology, scientists watched as capelin formed a massive shoal stretching over ten kilometers. In response, cod converged to form their own enormous group, devouring more than 10 million capelin in just a few hours.

“We’re seeing predator-prey interaction on a huge scale,” said Nicholas Makris, an MIT professor and lead author of the study. “It’s a coherent battle of survival.”

A Battle in the Deep

The research team’s observations were made possible by a sonic imaging technique known as the Ocean Acoustic Waveguide Remote Sensing (OAWRS) system. This technology, deployed during a cruise in the Barents Sea in 2014, allowed scientists to detect fish movements over vast areas, capturing interactions that typically go unseen.

“Fish have swim bladders that resonate like bells,” Makris explained. The team used this knowledge to differentiate between species: the low, sonorous notes of cod and the high-pitched chimes of capelin. This “multispectral” approach enabled the researchers to track the movements of millions of fish in real-time.

As the sun rose on February 27th, capelin, which had been scattered along Norway’s coast, began gathering into a single dense wave-like formation. “What we’re finding is capelin have this critical density,” Makris said. “Once close enough, they align their movement, forming a massive, coherent shoal.”

However, this strategy proved a double-edged sword. The dense capelin shoal attracted the attention of nearby cod, which quickly organized into their own coordinated attack. In a matter of hours, 2.5 million cod consumed nearly half the capelin shoal.

The event, though dramatic, is unlikely to devastate the capelin population, as this particular shoal represented a mere fraction — about 0.1% — of the species that spawn in the region. But as the Arctic ice continues to retreat due to climate change, capelin will face longer migrations to reach their spawning grounds, leaving them more vulnerable to such large-scale predation events.

Implications for Marine Ecosystems

This study sheds new light on the delicate balance of marine ecosystems. Capelin are a keystone species. They play a crucial role in sustaining the Atlantic cod population. However, rapid climate shifts and human impacts could push this balance to a tipping point.

“In our work, we see that natural catastrophic predation events can shift the local predator-prey balance in just hours,” Makris noted. “If climate and anthropogenic stresses reduce these ecological hotspots, such events could lead to dramatic consequences for dependent species.”

Looking ahead, Makris and his colleagues hope to deploy OAWRS technology to monitor the behavior of other fish species. The goal is to understand these interactions before it’s too late. “When a population is on the verge of collapse, you often see that one last shoal,” Makris emphasized. “And when that last big, dense group is gone, there’s a collapse.”

The findings were reported in the journal Nature Communications Biology.