When a shoal of starving piranhas spot a large school of fish, they charge right into it. After that they can swallow smaller fish whole. Piranhas will feed on just about anything dead or alive. They could eat a cow in a matter of hours unlike most fish it would take years. The reason that piranhas are so good at hunting is because they have a keen sense of smell and are very smart.
Some states prohibit the importation of wild piranhas.
However, many fisherman like piranhas because they give a good fight and are good to eat. They swim through the water in groups called shoals. They spend most of their time hunting for small fish. So they would dump them out into nearby lakes or rivers. That would cause a problem, because most of the other fish were eaten.