Piranhas are found almost every year in the Great Lakes. They are red-bellied piranhas that are found, which are sold in pet stores. It's easy to guess how they got into the Great Lakes. Somebody decides it's getting too big and it's eating everything else in the tank so let's get rid of it.
I call this the “free Willy syndrome”, because people like dumping their fish—their goldfish and so on—into the wild. I think maybe it eases their conscience.
Every pond, natural or otherwise, on the Island of Montreal has had or now has goldfish in it. Tens of thousands of fish are estimated to have been dumped into the St. Lawrence River every year through this particular vector.
This is the reason we emphasize public education. This is an example of something involving individuals, which when multiplied by large populations creates this huge opportunity for many species to get in.
Piranhas will probably never become established in the Great Lakes, unless winter becomes far less severe than it is now. That is possible, but not in the immediate horizon.
There are other species that have been established that way. The piranhas are a signal that it's happening when we see them every year. I like bringing them up because people are surprised to hear about it.