Quick, everyone out of the lake. A fisherman on Lake Winnebago caught a carnivorous tropical fish that was (mis)identified as a piranha. One of the commenters claims it is actually a pacu, which is a similar (but larger) species. I don't know if johnsam is an icthyologist or just a bullshitter, though, so take it with a grain of salt.
Presumably, some jackass dumped the fish into Lake Winnebago, since the DNR says they can't survive our winters (so there couldn't be a breeding population in the lake).
Still, it makes one reluctant to swim in Wisconsin's lakes. Another commenter claims that piranhas don't really attack people, but that's what folks used to say about otters.