In just a few hours, over 10 million fish were consumed, marking the largest recorded marine predation event in history.

Romanian media reported on April 4 that, off the cold coast of Norway, a scene of devastation only scientists had previously been able to imagine was unfolding. One morning in February 2024, millions of small Arctic fish—capelin gathered to spawn. But instead of laying their eggs calmly, they became the focal point of a ferocious assault by predators. Within just a few hours, the swarming capelin became the feast of their relentless pursuers—Atlantic cod.

Researchers from MIT and Norway report that this dramatic encounter marks the largest marine predation event on record. Using advanced acoustic imaging technology, the scientists observed capelin forming a massive school stretching more than 10 kilometers. In response, the cod also gathered into their own huge group, eating more than 10 million capelin within just a few hours.

“We’re witnessing an interaction between a large-scale predator and prey,” said Nicholas Makris, a professor at MIT and the study’s lead author. “This is an intense fight for survival.”

The research team’s observations were enabled by an acoustic imaging technology called an Ocean Sound Waveguide Remote Sensing system. Deployed during an expedition in the Barents Sea in 2014, the system allowed scientists to detect fish activity across a large area and capture interactions that are usually difficult to observe.

Makris explained: “Fish have swim bladders, which can resonate like bells.” The research team used this knowledge to distinguish between different species: the cod produce low, loud sounds, while the capelin emit high-pitched bell-like tones. This “multispectral” approach enables researchers to track the activity of millions of fish in real time.

As the sun rose on February 27, the capelin that had been scattered along the Norwegian coast began to gather, forming a dense, wave-like school. Makris said: “We found that capelin have a critical density. Once the distance is close enough, they adjust their movement directions and form a massive, orderly school.”

However, this strategy is a double-edged sword. Dense capelin schools attract the attention of nearby cod, and the cod quickly organize themselves and launch a coordinated, synchronized attack. Within just a few hours, 2.5 million cod ate nearly half of the capelin school.

Although this event is astonishing, it is unlikely to deal a devastating blow to the capelin population, because this specific school accounts for only about 0.1% of the total capelin eggs laid in the region. But as climate change continues to shrink Arctic ice sheets, capelin will face longer migration distances to reach their spawning grounds, making them more vulnerable to such large-scale predation events.

This study offers new insights into the delicate balance of marine ecosystems. Capelin is a key species, playing a crucial role in maintaining Atlantic cod populations. However, rapid climate change and the impacts of human activity may push this balance to the breaking point.

Makris noted: “In our research, we found that naturally occurring catastrophic predation events can shift the local predator-prey balance within just a few hours. If climate and human pressures reduce these ecological hotspots, such events could have serious consequences for the organisms that rely on these species.”

Looking ahead, Makris and his colleagues hope to deploy Ocean Sound Waveguide Remote Sensing system technology to monitor the behavior of other fish species. The goal is to understand these interactions as early as possible. Makris emphasized: “When a population is on the brink of collapse, you often see the last group of fish. And when that final large, dense group of fish disappears, collapse occurs.”