Scientists have witnessed what they believe to be the largest oceanic predation event ever observed off the coast of Norway. During the annual capelin spawning season, billions of these small Arctic fish gathered in massive shoals, seeking safety in numbers. But this protective strategy, typically used to deter predators, had the opposite effect when Atlantic cod swarmed in to feast.
The study, now published in Nature Communications Biology, was led by Professor Nicholas Makris from MIT. The research team used advanced sonic imaging to document this natural phenomenon. “What we're finding is that capelin have this critical density, which came out of a physical theory, which we have now observed in the wild,” Mr Makris explained.
The team observed individual capelins aligning with each other, eventually forming a dense shoal stretching over 9.6 km with about 23 million fish moving as one. This synchronised formation, however, attracted Atlantic cod, forming a counter shoal of 2.5 million fish.
As the cod closed in, the result was a “coherent battle of survival,” as Mr Makris described it, where predators consumed over 10 million capelin in a matter of hours.
This interaction unfolded across miles of ocean, providing a rare glimpse into predator-prey dynamics on a scale that scientists hadn't observed before. “This is happening over a monstrous scale, and we're watching a wave of capelin zoom in, like a wave around a sports stadium,” Mr Makris said, comparing the formation to a synchronised survival strategy that, in this case, did little to fend off the attackers.
The implications of this event stretch beyond a single day. As the Arctic ice sheet retreats due to climate change, capelin may face an even longer migration path, putting them at increased risk of predation. “In our work, we are seeing that natural catastrophic predation events can change the local predator-prey balance in a matter of hours,” Mr 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 broader marine ecosystem.
Capelin are a keystone species, supporting seabirds, whales, and larger fish. As Makris explains, “It's been shown time and again that, when a population is on the verge of collapse, you will have that one last shoal. And when that last big, dense group is gone, there's a collapse.”
