A new study details how, in just a few hours, more than 2 million Atlantic cod consumed 10 million tiny capelin. Scientists say the feeding frenzy is the largest on record, both in terms of the number of fish involved and the area covered.
Each winter Atlantic cod feed on spawning capelin along the Norwegian coast. In February 2014, using acoustic imaging, researchers discovered a massive group of fish near Finnmark, in northern Norway, but at the time, they could not distinguish between cod and capelin. For the new study, researchers reanalyzed the acoustic data using a new technique for identifying fish based on the sound of their swim bladders, the air-filled sacs that keep fish buoyant.
“Fish have swim bladders that resonate like bells,” said study coauthor Nicholas Makris, of MIT. “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.”
The analysis revealed 23 million capelin grouping together in a shoal that stretched for miles. The shoal drew the attention of 2.5 million cod, which formed their own massive group in response and ate up 10.5 million of the assembled capelin. The findings were published in Communications Biology.
The feast did little to diminish spawning capelin, which number in the billions, but Makris said that warming could stress capelin to the point where such massive feeding frenzies could have “dramatic consequences” for the species. Some cod and capelin are already seeking out cooler waters.
“It’s been shown time and again that, when a population is on the verge of collapse, you will have that one last shoal,” Makris said. “And when that last big, dense group is gone, there’s a collapse.”
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