I saw a fully grown ornamental koi in a stream that feeds into lake michigan once...funny thing was the carp was trying to swim upstream unsucessfully.
I actually picked it up, and sent it into the lake. I doubt that he survived, he was in fairly rough condition.
I realize they are invasive, and that the color would breed out in a couple generations...but for some reason I still turned that fish around.
Never saw it again, and have killed hundreds of carp since...doubt that it made a net gain on me and my spear.