In the Barents Sea north of the North Cape, a research team recorded the largest predation event ever observed using a new acoustic mapping technique: 2.5 million Atlantic cod descended upon a shoal of capelin nearly ten times their size, reducing its numbers by almost half within just a few hours.
Billions of capelin (Mallotus villosus) migrate each February from the Arctic ice edge southward to the Norwegian coast to spawn. The Atlantic cod (Gadus morhua) are well aware of this, and for them, the small capelin – about the size of anchovies – make a welcome meal on their journey towards the Lofoten Islands.
The scale of such a predation event was recently revealed by the research of a team from the Massachusetts Institute of Technology (MIT) and the Norwegian Institute of Marine Research. In the study published in Nature Communications Biology on October 29, the researchers describe the almost unimaginable event:
A massive number of capelin, initially swimming individually or in small groups, gathered early in the morning into a massive shoal of approximately 23 million individuals, stretching across dozens of kilometers. For the nearby cod, this ecological “hotspot” was an invitation to a feast. The cod, which can grow up to 1.5 meters in length and weigh up to 50 kilograms, formed their own enormous shoal in response.
Using new large-scale acoustic imaging technology that maps marine environments over tens of thousands of square kilometers, the researchers observed how around 2.5 million cod consumed 10.5 million capelin within just a few hours, nearly halving the shoal. This is the largest predation event ever recorded in the ocean, both in terms of the number of animals and the size of the area.
“It’s the first time seeing predator-prey interaction on a huge scale, and it’s a coherent battle of survival,” Nicholas Makris, professor of mechanical and ocean engineering at MIT and senior author of the study, described in an institute’s press release. “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.”
Although millions of capelin ended up in the cods’ stomachs, the researchers do not believe that a predation event of this size will weaken the capelin population as a whole. In their study, they note that the targeted shoal only accounts for 0.1 percent of the capelin spawning in the region.
“In our work we are seeing that natural catastrophic predation events can change the local predator-prey balance in a matter of hours,” says 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.”
Global warming is causing both capelin and cod to travel increasingly greater distances between their feeding grounds, located in the ice edge zone that is steadily shifting northward, and their spawning grounds along the Norwegian coast.
A dramatic reduction in the overall capelin population would have devastating consequences for the ecosystem in the Barents Sea, because in addition to cod, numerous other species depend on capelin as prey, including marine mammals and seabirds.
The data for the current study was collected back in February 2014 during a research expedition off the coast of Norway. Using a specialized acoustic imaging technique called the Ocean Acoustic Waveguide Remote Sensing (OAWRS) system, which sends sound waves in all directions into the ocean and captures them with an array of acoustic receivers, the team created large-scale maps of the ocean.
The new, “multispectral” analysis of the data allowed the researchers to differentiate between the species and map the movements of the individual species over a large area. “Fish have swim bladders that resonate like bells,” explains 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.”
In their study, the researchers emphasize that large-scale monitoring of fish populations and research into the interactions between different species are essential for making sustainable decisions in fisheries management.
“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,” says Makris. “So you’ve got to know what’s there before it’s gone, because the pressures are not in their favor.”
Julia Hager, Polar Journal AG
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