Safety in Numbers: There's a Reason Freshwater Fish Form Schools
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The single most important benefit that fish gain from schooling is thought to be the reduction of predation risk. Schooling works in a number of ways to confound hunters, but the net effect -- survival of the school members -- has been shown again and again.
Can a predator sneak up on a school?
To hunt successfully, most predators need the element of surprise.They must approach sufficiently close to their quarry so that when they attack, their victim has no opportunity to flee. But as we have seen, the sensory abilities of all the individuals in a school combine to provide excellent long-range detection. As a result the school is likely to be aware of the approaching danger; because of its "many eyes," even before the predator itself has detected the school. With the advantage of this early warning, the fish in the school draw close together and behave in a much more uniform fashion.
What is meant by the "confusion effect"?
Although some predators attack blindly, rushing at the school in the hope of snagging a fish at random, this approach rarely works effectively. Instead, hunters usually try to select a victim before attacking, picking out an individual and chasing it down. This causes grouping fish to select their schoolmates with care, strongly preferring to associate with conspecifics of the same size and color to produce schools of near identical fish.
Faced with such a mass of like-for-like fish, the predator is overloaded with choice and becomes "confused." This often has the effect of delaying an attack or causing the predator to abandon it altogether. However, if one school member has chosen its schoolmates poorly and is somehow different from the other school members, say, larger than them, the predator can often overcome the confusion effect and catch out the odd one. Therefore, schooling fish that are in any way different are at huge risk of being picked out, which is why they are all so similar in terms of their color patterns, body shape, and even behavior.
How else might fish in a school survive against predators?
One important benefit of being in a school is known as "attack dilution." Most predators are only capable of capturing one victim at a time. Therefore, even if a predator makes a successful attack a fish in a school of 100 others has only a 1% chance of being unlucky. In this way, the risk of being the chosen one is diluted among the members of the school. It means that in high-risk situations it pays to be in the largest school possible.
Convict cichlid parents exploit these mathematical probabilities in a remarkably clever way. In the wild they "kidnap" the fry of other pairs and add them to their own brood, thus diluting the risk to their own offspring.To make this even more effective, studies have shown that they preferentially kidnap fry that are slightly smaller than their own and therefore less skilled at evading attack. If a predator does attack the brood, the foster fry suffer a disproportionately greater amount of risk leaving the pair's own young safer than might otherwise be the case and without adding any substantial extra guarding costs to the parental fish.