The Predators That Consumed Over 10 Million Fish In Mere Hours

It's difficult to put into perspective just how massive the Earth's oceans are. One way is to consider the fact that, despite humanity's history of studying the planet and all our advanced technology, we've only mapped around 26% of the seafloor. It's also good to remember that we understand so little of what has been mapped that we're still stumbling upon paradigm shifters like the discovery of dark oxygen and finding life in strange places in the Pacific Ocean. 

Another way to conceive of the ocean's vastness is by looking at the behavior of the biology that calls it home. In October 2024, a group of Norwegian oceanographers and researchers from MIT did just that, publishing a paper in Nature Communications Biology that details the largest predation event ever documented on Earth. In the Barents Sea off the coast of Norway, scientists observed Atlantic cod consuming approximately 10.6 million anchovy-sized capelin fish in just four hours.

The remarkable discovery, made near Finnmark, Norway, reveals previously unknown complexities about how oceanic predators and prey interact on such a large scale and showcases how anthropogenic climate change could have huge ramifications for marine ecosystems. 

Though the paper was published in October 2024, the team's data actually come from observations made on February 27, 2014. As dawn started to flood the atmosphere with light that day, scattered schools of capelin, previously spread thinly through the water, suddenly began forming dense, organized shoals as they descended into the depths. As this occurred, the individual capelin fish shifted to group behavior, adapting to the average speed and movement of other fish they sensed around them. This ultimately led to the formation of a single shoal stretching miles in length consisting of around 23 million fish.

Such behavior had never been observed in capelin fish before, but it's thought the activity helps fish conserve energy by moving as a group. However, the accumulation of such biomass didn't just get the researchers' attention; it piqued the interest of nearby Atlantic cod, whose own shoal soon swelled to nearly 2.5 million in number. A few hours and over 10 million eaten capelin later, the shoals dispersed. "It's the first time seeing predator-prey interaction on a huge scale," said Nicholas Makris, a professor of mechanical and ocean engineering at MIT and one of the study's authors in a statement published by MIT News.

The reason the team's paper took ten years to publish was due to an advancement in imaging technology that allowed them to revisit the data they'd recorded that morning in 2014. To obtain the original recordings, researchers employed a unique sonar imaging system called Ocean Acoustic Waveguide Remote Sensing (OAWRS) to send sound waves into the ocean in all directions. Previously unable to tell the difference between species, they later applied a novel "multispectral" technique to distinguish this variable based on the resonance of each species' swim bladders.

This massive predation event represents more than just an impressive display of nature's power and scale; it raises important questions about how vulnerable fish populations in the Barents Sea are responding to warming global temperatures.

Generally, large-scale predation events don't represent a significant threat to healthy fish populations. But as climate change reduces the size of the Arctic ice sheet the capelin depend on for spawning, these massive predation events could end up having dire consequences for both species and the broader ecologies of which they are a part. 

For now, though, the capelin fish in the Barents Sea are doing fine. And, to help us put the scale of the ocean and its ecosystems into perspective even more, the 10 million capelin that the Atlantic cod consumed that day in 2014 make up a mere 0.1% of the overall population in the region. But that's far from the only reason to be awed by the planet's oceans and the species that inhabit them; a quick look at the past reveals 12 prehistoric sea animals that are pure nightmare fuel.