In the early hours of the morning, scientists recorded the largest ocean predation event in the waters off the coast of Norway. Thanks to new ocean-mapping methods, they witnessed 2.5 million Atlantic cod feasting on 23 million capelin.
The shoal of capelin stretched six miles long. By mid-morning, about 10 million capelin had been devoured. The feeding time has made history as the largest ocean predation event ever documented.
Capelin are small Arctic fish around the size of an anchovy. Each February, billions of them migrate from the edge of the Arctic ice sheet to the Norwegian coast to lay their eggs.
This makes Norway’s coastline a preferred spot for migrating Atlantic cod, the capelin’s predator, to stop and rest. The record-breaking event occurred at the height of capelin’s spawning season.
A team of Norwegian oceanographers and researchers from MIT used a sonic-based imaging technique to capture the interaction between the two species.
They observed a group of random capelins that quickly morphed into a massive shoal. At the same time, an individual group of cod began forming their own huge shoal. Soon enough, the cod overtook the capelin and consumed over 10 million fish, about half of the gathered prey.
It is unlikely that this one predation event will weaken the overall capelin population, as the victims represented only 0.1 percent of the capelin that spawned in the region.
But as climate change melts the Arctic ice sheet, capelins will have to swim farther to lay their eggs. It will make the species more susceptible to natural predation events.
“In our work, we are seeing that natural catastrophic predation events can change the local predator-prey balance in a matter of hours,” Nicholas Makris, professor of mechanical and ocean engineering at MIT, said.
Travel Faery – stock.adobe.com – illustrative purposes only, not the actual cod
“That’s not the 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.”
In the study, the researchers analyzed data from 2014 that was collected using an Ocean Acoustic Waveguide Remote Sensing (OAWRS) system.
The device sends sound waves down into the ocean. The waves travel long distances and bounce off any creatures or obstacles in their path. By examining the sound waves, scientists can produce a map of the ocean over a vast area.
However, looking at sound waves does not allow researchers to tell the difference between species. The team applied a new technique to the data to help them differentiate between species based on their swim bladders.
“Fish have swim bladders that resonate like bells,” Makris said. “Cod has large swim bladders that have a low resonance—like a Big Ben bell—whereas capelin has tiny swim bladders that resonate like the highest notes on a piano.”
It was how the researchers were able to capture the historic event. In the future, they hope to deploy OAWRS again to track the interactions between other fish.
The study was published in Communications Biology.
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