Largest predator-prey feast ever seen in the ocean was massive, altering the food chain

In the vast ocean, you might think there’s safety in numbers for fish consider to be “prey” as they seek to elude predators. Recent observations suggest, however, that for some fish, grouping together can actually spell trouble.

During the height of capelin spawning season off the coast of Norway, scientists witnessed a dramatic encounter that challenges our understanding of survival strategies in the sea.

Professor Nicholas Makris, a mechanical and ocean engineering expert at MIT, led the team that captured this remarkable event.

Alongside his colleagues, he observed how the behavior of tiny fish can have big consequences when ocean predators are on the prowl.

Arctic fish called capelin

Every February, billions of capelin — a small Arctic fish about the size of an anchovy — embark on a long journey.

They migrate from the edge of the Arctic ice sheet down to the Norwegian coast to lay their eggs. This migration is not just crucial for their reproduction but also plays a significant role in the Arctic food web.

Capelin are a key food source for many marine animals, including seabirds, whales, and larger fish. Their journey supports the diets of these predators, making capelin an essential link in the ecosystem.

Atlantic cod are ocean predators

Hot on the tails of the capelin are Atlantic cod, their primary predators.

As the cod migrate southward, they take advantage of the spawning season to feed extensively on capelin.

This annual feast is a critical time for cod to build up energy reserves.

It’s a natural cycle: capelin spawn, cod feed, and the balance of the ecosystem is maintained. But what happens when this balance is disrupted?

Sonic imaging sees underwater

To understand these underwater dynamics, Makris and his team used a cutting-edge sonic imaging technique called Ocean Acoustic Waveguide Remote Sensing (OAWRS).

This system sends sound waves deep into the ocean, bouncing off fish and other obstacles, allowing scientists to create real-time maps over vast areas.

Previously, researchers could map fish movements but couldn’t tell which species they were observing.

This time, they applied a new multispectral technique that differentiates species based on how their swim bladders resonate with sound.

“Fish have swim bladders that resonate like bells,” Makris explains. “Cod have large swim bladders that have a low resonance, like a Big Ben bell, whereas capelin have tiny swim bladders that resonate like the highest notes on a piano.”

Massive gathering begins

On February 27, 2014, at the peak of the capelin spawning season, the team witnessed something astonishing.

In the early morning, capelin moved individually in loose clusters along the Norwegian coast. But as the sun rose, they began descending to deeper waters, possibly searching for spawning grounds.

As they dove, the capelin started to group together. Individual fish aligned with their neighbors, and soon, a massive shoal formed. This shoal spanned over six miles and included about 23 million fish moving in unison.

“What we’re finding is capelin have this critical density, which came out of a physical theory, which we have now observed in the wild,” Makris says.

“If they are close enough to each other, they can take on the average speed and direction of other fish that they can sense around them, and can then form a massive and coherent shoal.”

Ocean predators assemble

But the capelin weren’t the only ones gathering. Individual cod began to notice the enormous shoal and started grouping together themselves.

In response to the capelin, a cod shoal formed, amounting to about 2.5 million fish.

Over just a few hours, this shoal of cod overtook the capelin. The predators consumed over 10 million capelin — more than half of the gathered prey — across miles of ocean.

Then, just as quickly as they formed, both shoals dissolved, and the fish scattered.

Predator vs. prey ocean battle

“It’s the first time seeing predator-prey interaction on a huge scale, and it’s a coherent battle of survival,” Makris remarks.

“This is happening over a monstrous scale, and we’re watching a wave of capelin zoom in, like a wave around a sports stadium, and they kind of gather together to form a defense. It’s also happening with the predators, coming together to coherently attack.”

This event is the largest recorded predation event in terms of the number of individuals and the area covered. It offers a rare glimpse into the dynamic interactions that shape marine ecosystems.

Climate change connection

While this one event represents just 0.1 percent of the capelin that spawn in the region, it raises concerns about future impacts.

As climate change causes the Arctic ice sheet to retreat, capelin will have to swim farther to reach their spawning grounds. This extra distance can stress the fish, making them more vulnerable to predation.

“In our work we are seeing that natural catastrophic predation events can change the local predator-prey balance in a matter of hours,” Makris notes.

“That’s not an issue for a healthy population with many spatially distributed population centers or ecological hotspots. But as the number of these hotspots decreases due to climate and human stresses, the kind of natural ‘catastrophic’ predation event we witnessed of a keystone species could lead to dramatic consequences for that species as well as the many species dependent on them.”

Why ocean predator events matter

Understanding these massive interactions is crucial. Capelin are a keystone species, meaning their health directly affects many other species in the ecosystem.

Continuous monitoring at a scale that captures individual fish behavior across thousands of square miles can inform conservation efforts.

“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,” Makris warns.

“So you’ve got to know what’s there before it’s gone, because the pressures are not in their favor.”

Ocean predators, climate change, and the future

Makris and his team, including co-authors Shourav Pednekar and Ankita Jain at MIT, and Olav Rune Godø of the Institute of Marine Research in Norway, hope to use OAWRS to study other species.

By capturing these large-scale dynamics, scientists can better understand and protect the delicate balance of marine life.

So, what does this mean for the future of our oceans? It’s a reminder that even the smallest creatures play a big role.

By paying attention to these intricate relationships, we can make more informed decisions about how to protect marine ecosystems.

Next time you think about the phrase “there’s safety in numbers,” consider the capelin and their journey.

Sometimes, grouping together can create risks, especially when hungry predators are nearby. It’s a delicate balance, one that scientists are only beginning to understand.

The full study was published in the journal Nature Communications Biology.

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