On a seemingly typical morning off Norway’s coast, millions of small fish called capelin began to flock in the ocean. Soon, their numbers surged to an incredible 23 million, creating a shoal that extended over six miles. Nearby, predators like the Atlantic cod quickly took notice.

In just a few hours, marine scientists utilizing sonar imaging technology observed an extraordinary sight: a gigantic gathering of cod feasting on more than 10 million capelin. This represented the largest predation event ever documented in the ocean.

“It’s the first occasion we’ve observed predator-prey dynamics on such an immense scale, and it’s a clear struggle for survival,” stated Nicholas Makris, a mechanical and ocean engineering professor at MIT and one of the study’s contributors, in a university release.

This pioneering research, carried out in the Barents Sea, was recently published in the peer-reviewed journal *Nature Communications Biology*. While the observations originated in February 2014, advancements in techniques have enabled scientists to differentiate clearly between cod and capelin, providing new insights into the phenomenon.

To humans, this predation may appear remarkable or even brutal. Yet in nature, such events are typical. In the deep sea, where numerous animal groups exist, occurrences like this can greatly affect local populations but generally do not pose a threat to the overall survival of the species. The 2014 capelin shoal constituted merely 0.1 percent of the capelin population in that ocean area.

“Our research indicates that natural catastrophic predation events can alter the local predator-prey equilibrium within hours,” explained Makris. “However, for a healthy population with various spatially distinct centers or ecological hotspots, it’s not a significant concern.”

Nonetheless, as marine ecosystems encounter growing threats and ocean temperatures rise, not every population may endure such dramatic declines. “It has been repeatedly shown that when a population is close to collapsing, there will be one final substantial shoal. Once that last dense group disappears, the population fails,” warned Makris. “Thus, we must comprehend what exists before it vanishes, because the pressures are not in their favor.”

Marine researchers were unable to physically witness this vast and swiftly changing predatory event underwater. Instead, they deployed an acoustic device attached to their vessel to send sound waves into the water. These acoustic signals, routinely used in ocean exploration, bounce off objects like fish, exposing the activity beneath the surface. The specific tool employed, called the Ocean Acoustic Waveguide Remote Sensing (OAWRS) system, enabled the researchers to obtain detailed imagery of the occurrence.

Importantly, the acoustic signals reflected differently off various fish species, allowing the scientists to differentiate between capelin and cod. “Fish possess swim bladders that resonate like bells,” Makris elaborated. “Cod have large swim bladders that resonate at low frequencies, akin to Big Ben, while capelin have small swim bladders that resonate like high piano notes.”

The acoustic maps indicated the following:

– Row (i): Both species are dispersed and moving randomly in the Barents Sea.
– Row (ii): In the early morning, both species form dense shoals that span miles.
– Row (iii): On the left, the surviving capelin shoal is noted, while on the right lies the “vast engulfing cod shoal,” as described by the researchers.

Scientists estimate that the cod consumed over half of the 23-million-strong capelin shoal in a brief period. But what prompted the capelin to form such a large, visible group in the first place? Biologists propose that by swimming together, the capelin conserve energy, utilizing the movement generated by millions of fish swimming in unison.

However, this behavior also attracted about 2.5 million Atlantic cod — a species commonly eaten by humans.

While these dramatic occurrences often evade our notice, cutting-edge technology and expeditions are progressively uncovering the astounding biodiversity and activity hidden beneath the ocean’s surface.