Nature’s largest predation event ever recorded unfolds in Norway’s waters

In a groundbreaking study, researchers have documented the largest predation event ever recorded in the ocean, where over 10 million capelin were consumed by a swarm of cod off the coast of Norway.

This encounter illustrates how fish behavior impacts survival, showing that in the ocean, grouping together can sometimes lead to peril rather than protection.

Capelin migration and spawning

MIT and Norwegian oceanographers explored a vast area of the ocean during the peak spawning season for capelin, a small Arctic fish similar in size to an anchovy. Each February, billions of capelin migrate from the Arctic ice edge to the Norwegian coast to lay their eggs. This region also serves as a critical feeding ground for their primary predator, the Atlantic cod, which migrates southward during this time.

Using advanced acoustic mapping techniques, researchers captured interactions between migrating cod and spawning capelin across a vast expanse. They observed how the capelin began to form a massive shoal, which attracted increasing numbers of cod, ultimately resulting in a swift and devastating predation event.

Largest predation event recorded

The encounter, which lasted just a few hours, resulted in the cod consuming more than half of the gathered capelin, marking it as the largest predation event ever recorded in terms of the number of individuals involved and the area covered. Despite the scale of this event, it represents only a fraction – 0.1% – of the total capelin that spawn in the region.

However, climate change poses a significant threat to capelin populations, as the retreat of the Arctic ice sheet forces these fish to migrate further to spawn, increasing their vulnerability to such predation events.

Understanding predator-prey dynamics

“In our work, we are seeing that natural catastrophic predation events can shift the local predator-prey balance in mere hours,” explained Nicholas Makris, a professor of mechanical and ocean engineering at MIT.

“While this event may not impact a healthy capelin population overall, decreasing ecological hotspots due to climate change could have dramatic consequences for this keystone species and the many species that depend on them.”

Innovative acoustic mapping techniques

For their study, the researchers reanalyzed data collected during a 2014 cruise to the Barents Sea, utilizing the Ocean Acoustic Waveguide Remote Sensing (OAWRS) system. This innovative sonic imaging technique uses sound waves to create comprehensive maps of fish populations and their movements over large areas.

By employing a multispectral technique, the team differentiated between capelin and cod based on the resonance of their swim bladders. This allowed them to visualize fish groups and analyze their movements across tens of kilometers.

New insights into fish behavior

The researchers observed that as capelin descended to spawn, they shifted from individual to group behavior, ultimately forming a massive shoal of about 23 million fish. This coherent migration is thought to help fish conserve energy over large distances.

However, the emergence of this shoal attracted cod, which quickly formed their own group and ultimately devoured 10.5 million capelin over a brief period. Makris noted that such large-scale predation events may be more common in the ocean than previously thought.

Future monitoring of marine life

The research team aims to continue using OAWRS to monitor dynamics among other fish species, emphasizing the importance of understanding population behaviors before significant declines occur.

“It’s crucial to know what exists before it disappears,” Makris concluded, highlighting the urgent need for ongoing monitoring of marine ecosystems in the face of climate change.

This study not only sheds light on predator-prey interactions in the ocean but also underscores the critical role of capelin in maintaining the health of marine ecosystems.