I saw an episode of River Monsters the other night- he was fishing in Papa/New Guinea I think, and caught a big fish he first thought was a piranah, it turned out to be pacu - they look very similar, the pacu doesn't have the same type of teeth like his flesh eating cousin. A villager on the epi said the equivalent of their DNR stocked the Amazonian fish as a way to increase the number and type of fish in this lake they were fishing.
Personally, I'd rather have the "fighting fish" than piranah or whatnot. I had a few when I was a kid and had a tank. I also had some neat albino cats, a couple gold fish, and a fish that I swear looked like a pumpkinseed, but they called it something else. THen my sister decided to clean my tank one day while I was at school. She put all my fish in zip lock bags, w/o aeration. And forgot about them. Killed EVERY one of them, including the tank sucker fish...

I had something like 30 fish in my tank. I can't imagine what people are thinking when they decide to dump those fish in a river or lake or pond... if you don't want them, and can't give them away - flush 'em down the toilet - of course, if you live in portland, drop the fish in boiling water first or something to kill them before doing so.