Each year capelins, which are a small fish, migrate up the Norwegian coast in order to spawn. Naturally, larger fish such as cod follow them and eat them along the way.
One way that many fish, including capelins, try to protect themselves is by forming a shoal, which is just a large school of the fish that move together as one. This can help to confuse the predators or even scare them off.
Of course, if the predator fish know that it is just a massive number of prey, it can also attract them in. That is what happened in 2014, and it was captured by a sonar system called the Ocean Acoustic Waveguide Remote Sensing (OAWRS) system. This system is run by Nicholas Makris of MIT and his colleagues.
While lots of information was gathered from it back then, it took advancements to the systems to be able to identify exactly what was going on during this one event, which was recently determined to be the largest feeding frenzy ever recorded. An estimated 10 million capelins were eaten in just a few hours.
Makris released a statement about the event, saying:
“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. 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.”
During this event, the shoal was attacked by another shoal, but this one was made up of 2.5 million cod. The results were not good for the capelin. Makris went on to say:
“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.”
While this is an incredible event that the scientists were lucky to be able to observe, they do not believe it is a threat to the long term health of the capelin species in the area. Feeding frenzies of this size are likely rare, but large feeding events are a normal part of the ecosystem as the capelins come together to spawn each year.
A study that looked at this event was published in the journal Communications Biology.
It is hard to imagine so many fish being eaten so fast.
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