More Than 10 Million Capelin Devoured by Cod Off Norway

More than 10 million shoaling capelin were eaten by cod gathered off Norway’s coast in only a few hours, in what has been described as the largest recorded predatory slaughter.

The event challenges the notion that there is safety in numbers. Researchers from the Massachusetts Institute of Technology (MIT) in the US and Norway’s Institute of Marine Research suggest that the fish’s exceptionally dense aggregation may itself have drawn in the predators.

"It's the first time seeing predator-prey interaction on a huge scale, and it's a coherent battle of survival," says MIT ocean engineer Nicholas Makris.

Makris, MIT engineers Shourav Pednekar and Ankita Jain, and behavioural ecologist Olav Rune Godø of the Institute of Marine Research documented the dynamics of this vast event by reflecting sound waves from the animals’ swim bladders.

Tracking capelin and cod off Norway

The researchers employed a new wide-area, multispectral underwater acoustic-sensing method. By following frequencies unique to each species, they were able to observe their interactions across an area spanning tens of kilometres.

"Cod have large swim bladders that have a low resonance, like a Big Ben bell," explains Markris. "Whereas capelin have tiny swim bladders that resonate like the highest notes on a piano."

Every February, capelin (Mallotus villosus) migrate from the Arctic towards Europe in enormous shoals, which helps them conserve energy. This shoaling behaviour enables the fish to benefit from one another’s currents and travel as a group.

"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," explains Markris.

A vast predatory event

However, forming shoals also carries a danger. The newly examined 2014 data showed as many as 23 million individual capelin gathering in one place. In turn, 2.5 million predatory Atlantic cod (Gadus morhua) formed a shoal of their own in readiness to feed.

"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," explains Makris.

"It's also happening with the predators, coming together to coherently attack."

Fortunately, anchovy-sized capelin are counted in the billions, meaning the incident recorded by the team could have eliminated no more than about 0.2 percent of the population. Even so, understanding predator-prey relationships is increasingly important as populations of large shoaling fish species decline.

An alarming 97 percent of migratory fish species currently face extinction risk, among them commercially valuable species such as Atlantic salmon.

Why underwater acoustic sensing matters

The sound-imaging approach used by Pednekar and colleagues may allow researchers to spot fish species nearing population collapse.

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

"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 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 team have previously applied comparable techniques to investigate the population dynamics of cod, which are likewise declining. They determined that when populations fall below the average number of individuals in a shoal, recovery becomes far more difficult.

Their study was published in Nature Communications Biology.