It seems to me that if they spread out from each other, so a predator could only eat one in every bite, any single fish is safer because it’s less likely to be that single one, whereas if they’re huddled together and the predator can gobble ten at a time, the likelihood for any single fish being eaten in a bite is 10x higher.
I understand if the optics of a school are designed to look big and maybe scare off predators, but it’s always been phrased to me as if the “game theory” for a single fish makes it safer to pack in with others.
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It’s not usually so much scaring a predator as confusing it. A predator has to be very big to eat 10 of them at a time; usually, they’re trying to single out a target to eat and once the school starts swarming and circling the predator, it becomes very hard for them to pick out a single time in the school. Imagine that you’re in a very crowded concert or a mall or something, and you’re trying to reach a single, specific person in the crowd.
As for game theory, the survival of one fish is higher if it’s harder to spot them in a crowd, and it’s harder for them to get spotted as the target if they’re not alone. Plus, the crowd makes it easier to breed and to find food of their own.
Ever seen a person in one of money booths where the fans turn on and blow the dollar bills around everywhere?
How easy is it to grab all the money?
It’s like that.
If you’re in a street fight against 2 dudes, do you think you’re more likely to get hit if there is 1 of you, or 10 of you? Who gets hit is all about statistical probability.
Suppose you are a predator fish trying to eat a fish in a school. You match your approach to intersect its path and are reacting to its changes in direction. You are faster so you are going to catch it.
But before you can bite it you lose track of it, confusing it for a different one. This other fish is already moving in a different direction, dodging a different way. There is no time to react because they started that movement before you even focused on them. By switching your attention to a different fish it is like your prey instantly changed its movement, something you just can’t follow.
Normally of course you are unlikely to lose track of a single fish in this way, but if you clump a bunch of fish into a chaotic ball then it is very likely.
Each of the prey fish can focus on avoiding the predator, but the predator can’t keep track of any specific fish to follow its dodge. This creates an imbalance that reduces the rate of success of the predator.
The only defence for the small fish is to swirl around in a large ball to confuse the attackers. The bigger the ball, and the faster it swirls, the harder it is to pick off an individual fish.
There was a wildlife programme on a while ago that showed a ball of small fish (sardines?) trying to escape attacks from seals and barracuda. The barracuda just swam into the ball as fast as possible, snatching anything that got in their way. The seals worked together to scatter the prey into much smaller groups outside of the main ball, keeping them separate and thus making it easier to pick off individual fish.
The answers about confusing the predator are great, but it’s really a more grim story than that. It’s open water. There is no cover. The larger fish are faster than you. The only choice you have is hope they eat your friends first and get full. The end. It’s not great being a grazer with no cover. That’s why coral reefs and trees are so good for biodiversity.
It depends on what predator is attacking a school of fish. However, on balance, it’s safer to be in a dense ball, for all the reasons the other answers gave – most predators can’t eat multiple fish at once, so being 1 in 10,000 gives you much better odds of survival than being 1 in 1.
The exception to that is large filter-feeding predators, such as baleen whales or whale sharks. These predators have truly enormous mouths and can essentially do the equivalent of net fishing, which allows them to just ignore all the benefits of schooling fish and devour the whole school at once.
However, these large predators are sufficiently rare in the ocean (even before humans began artificially decreasing their numbers) that schooling is still the better strategy overall.
The fish are all shiny and they camouflage against the school so that you can’t tell where they are.
Imagine trying to find one solitary zebra out in the open. Pretty easy, right? Now imagine trying to single out one zebra in a herd. You can’t, because all the stripes blend together and obscure the outline of any individual. A lion can’t kill or eat a whole herd, it’s looking for one zebra to single out.
So it’s a good defense against any predator that is about the right size to eat one fish. It’s less good against a predator that could eat half a school. But you have to play the percentages. A lot of marine predation is a slightly larger fish eating a slightly smaller fish.
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