A red-bellied piranha in open water is prey as well as predator. Caimans, river dolphins, large catfish and fish-eating birds all take them, and a fish in a shoal is harder for a hunter to single out and quicker to be warned.
When researchers manipulated perceived predation risk, piranhas responded by tightening their groups, and fish held in larger shoals showed calmer breathing and less alarm behaviour than fish held alone or in pairs. The feeding that gave the species its reputation is opportunistic: fins, scales, insects, seeds and carrion make up much of the diet, and mass attacks are associated with confined dry-season pools where fish are crowded and food is scarce.
What remains harder to measure is how much shoal size varies with local predator density across a whole floodplain season, since most careful observations come from captive or enclosure settings rather than from continuous field tracking of wild groups.