Scientists have witnessed what they believe to be the largest ocean predation event ever observed off the coast of Norway. During the annual capelin spawning season, billions of these small Arctic fish gather in massive schools, seeking safety in numbers. But this protective strategy, typically used to deter predators, had the opposite effect when Atlantic cod flocked to feast.

The study, now published in Nature Communications Biologywas led by Professor Nicholas Makris of MIT. The research team used advanced sound imaging to document this natural phenomenon. “What we’re seeing is that capelin has this critical density, which comes from a physical theory, that we’ve now observed in nature,” Makris said.

The team observed individual capelin lining up on top of each other, eventually forming a dense school spanning 6 miles (9.6 km) with an estimated 23 million fish moving as one. This synchronized formation, however, attracted Atlantic cod, forming a counter-bank of 2.5 million fish.

As the cod closed in, the result was a “coherent battle for survival,” as Mr. Makris described it, where predators consumed more than 10 million capelin in a matter of hours.

This interaction played out across miles of ocean, providing a rare glimpse into predator-prey dynamics on a scale that scientists had never observed before. “This is happening on a monstrous scale, and we are watching a wave of capelin zoom, like a wave around a sports stadium,” Mr. Makris said, likening the training to a synchronized survival strategy which, in this case, had no effect. to repel the attackers.

The implications of this event extend beyond a single day. As the Arctic ice sheet retreats due to climate change, capelin could face an even longer migration path, putting them at increased risk of predation. “In our work, we find that catastrophic natural predation events can shift the local predator-prey balance within hours,” Makris said. With fewer ecological hotspots for capelin due to warming oceans, catastrophic events like this could have a lasting impact on the species and the marine ecosystem as a whole.

Capelin is a keystone species that supports seabirds, whales and larger fish. As Makris explains: “It has been shown time and time again that when a population is on the verge of collapse, there is one last school left. And when this last large and dense group disappears, there is a collapse.