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 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.
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