More than 10 million schooling capelin were devoured by cod caught off the Norwegian coast in just a few hours, in what is considered the largest predatory massacre on record.

According to researchers from the Massachusetts Institute of Technology (MIT) in the United States and the Institute of Marine Research in Norway, it is the intense grouping of fish that could have attracted the predators, which belies the saying “safety lies in number.” .

“This is the first time that a large-scale predator-prey interaction has been observed, and it is a coherent battle for survival,” said Nicholas Makris, ocean engineer at MIT.

Makris, along with MIT engineers Shourav Pednekar and Ankita Jain, and Institute of Marine Research behavioral ecologist Olav Rune Godø, observed the dynamics of this massive event by echoing sound waves from the animals’ swim bladders .

The team used a new wide-area multispectral underwater acoustic detection technique to track the specific frequencies of different species, allowing researchers to monitor their interactions over an area of ​​tens of kilometers.

“Cod have large swim bladders that have a low resonance, like a Big Ben bell,” explain Markris. “While capelin has tiny swim bladders that resonate like the highest notes on a piano.”

Capelin (Mallotus villosus) gather in massive schools to save energy as they migrate from the Arctic to Europe each February. This schooling behavior allows them to move away from each other’s currents and move collectively.

“If they are close enough to each other, they can adopt the average speed and direction of the other fish they can detect around them, and can then form a massive, cohesive school.” explain Markris.

But the shallows carry risk.

The newly analyzed data from 2014 captured up to 23 million individual capelin grouped together. In response, 2.5 million predatory Atlantic cod (Gadus morhua) also organized themselves into their own bench, preparing to feast.

“It’s happening on a monstrous scale, and we’re watching a wave of capelin zoom, like a wave around a sports stadium, and they sort of come together to form a defense,” explain Makris.

“This also happens with predators, which come together to attack coherently.”

Fortunately, anchovy-sized capelin number in the billions, so the event the team recorded would have wiped out at most about 0.2 percent of the population. But understanding predator-prey dynamics becomes more important as the number of large schooling fish species declines.

A shocking 97 percent of fish species are migratory are currently threatened with extinction, including highly valued species like Atlantic salmon.

The sound imaging used by Pednekar and his colleagues could help researchers identify fish species on the brink of collapse.

“In our work, we find that catastrophic natural predation events can shift the local balance between predators and prey within hours.” explain Makris.

“This is not a problem for a healthy population with many spatially distributed population centers or ecological hotspots. But as the number of these hotspots declines due to climate and anthropogenic stresses, “The type of ‘catastrophic’ natural predation event we witnessed on a keystone species could lead to dramatic consequences for that species as well as the many species that depend on it.”

The team has previously used similar techniques to explore the population dynamics of cod populations, also in decline. They found that if populations drop below the average number of individuals in a school, it becomes much more difficult for them to recover.

Their research was published in Biology of natural communicationsYes.