It took place in the early morning hours in a swathe of ocean off the coast of Norway; the largest predation event ever to be recorded by oceanographers. By mid-morning 10 million capelin, almost half the number within the amassed shoal stretching 6 miles long, had been eaten.

The hosts of this ‘unprecedented’ feast? The capelin’s nearby predator, the Atlantic cod.

In findings reported this week in Nature Communications Biology, this particular feeding time has made history as the largest predation event on record. This was the moment a massive shoal of around 23 million capelin, spanning an area of ocean some ‘tens of kilometres long’, became sitting prey for an equally sizeable gathering of predatory cod.

Recorded at the height of spawning season for capelin – a small Arctic fish around the size of an anchovy – during which billions migrate each February from the edge of the Arctic ice sheet southward to the Norwegian coast to lay their eggs, it marks the first time scientists have measured such a process over large timescales.

Norway’s coastline has, at the same time, become a favoured stopover for migrating Atlantic cod.

Norwegian oceanographers and a team of researchers from MIT captured the interaction between the two species using a sonic-based wide-area imaging technique, first observing a grouping of random capelin as it formed into a massive shoal, creating as they did, an “ecological hotspot.” At the same time, the team noticed an individual cod grouping, forming in response, their own “huge shoal”. 

Not long after the swarming began, the cod swiftly overtook the spawning capelin in an event that witnessed the quick consumption of over 10 million fish, a number estimated to be half of the gathered prey. The dramatic encounter is now the largest such predation ever recorded, both in terms of individuals involved and the area over which it all occurred.

This one event is unlikely to weaken the capelin population as a whole, the team writes, as the preyed-upon shoal represents just 0.1% of the capelin that spawn in the region. But, as climate change causes the Arctic ice sheet to retreat, they note, capelin will have to swim farther to spawn, making the species more stressed and vulnerable to natural predation events, like the one observed.

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

“That’s not the issue for a healthy population with many spatially distributed population centres or ecological hotspots. But, as the number of these hotspots decreases due to climate and anthropogenic 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.”

The science behind this particular predation is both significant and illuminating. The study was conducted by analysing data retrieved back in 2014 from the deployment of an Ocean Acoustic Waveguide Remote Sensing (OAWRS) system.

The way this device works is by sending sound waves down into the ocean and out in all directions. These waves travel large distances of up to ten kilometres, bouncing off any obstacles or fish in their path. By picking up and analysing the scattered and reflected sound waves, scientists are able to create an instantaneous map of the ocean over a huge areal extent. 

So, how have scientists been able to distinguish predating cod from snackable capelin?

Researchers have been able to reconstruct maps of individual fish and their movements by using the sound wave technique for quite some time, but attempts to tell one species from another have – until now – been relatively fruitless. However, by applying a new ‘multispectral’ technique to the collected data, researchers involved in this latest study have been able to differentiate between species based purely on the characteristic resonance of their swim bladders.

“Fish have swim bladders that resonate like bells,” said Makris. “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.”

Applying this multispectral technique to OAWRS data collected in the February of 2014 (the peak of the capelin spawning season) Makris and his colleagues were able to build a picture of the historic event. 

So, here’s how it unfolded.

In the early morning hours, their new mapping showed that capelin largely kept to themselves, moving as random individuals, in loose clusters along the Norwegian coastline. As the sun rose and lit the surface waters, the capelin began to descend to darker depths, possibly seeking places along the seafloor to spawn.

As the capelin descended, they began shifting from individual to group behaviour, forming a huge shoal of around 23 million fish that moved in a coordinated wave, spanning over ten kilometres long.

“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,” continued Makris. “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.”

As soon as the capelin shoal formed, however, it attracted increasing numbers of cod, which quickly formed a shoal of their own. Based on the team’s acoustic mapping, this shoal of cod numbered around 2.5 million individuals. It was thus over a few short hours that the cod managed to swiftly consume 10.5 million capelin over tens of kilometres before, eventually, both shoals dissolved and the fish scattered away. 

While this is the first time scientists have been able to document such an event, it’s now suspected that such massive and coordinated predation is a common occurrence in the ocean.

“This is a truly fascinating study that documents complex spatial dynamics linking predators and prey at scales previously unachievable in marine ecosystems,” said George Rose, professor of fisheries at the University of British Columbia, who studies the ecology and productivity of cod in the North Atlantic, though wasn’t involved in this particular study.

“Simultaneous species mapping with the OAWRS system enables insight into fundamental ecological processes with untold potential to enhance current survey methods.”

Makris now hopes to deploy OAWRS in the future to monitor the large-scale dynamics among other species of fish. 

“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 done, there’s a collapse,” said Makris. “So you’ve got to know what’s there before it’s gone, because the pressures are not in their favour.”

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