It's illegal to dump or transfer live fish from one body of water to another in Canada, but people do it anyway. They probably think that's more humane than flushing the fish, but the practice disrupts the entire aquatic environment.

They're cute in an aquarium, but out in the wild the constraints of living space and food no longer exist.

"Some of these species can get really big," says an Alberta Environment and Parks aquatic invasive species specialist. "The biggest one we've caught is the size of a dinner plate."

Read More