Many snakehead species are from NE Asia (Siberia) so even a Michigan winter wouldn't be a problem for them. Hopefully no one has done anything stoopid like release them in the area; hope springs eternal.
Well, as a matter of fact, a couple of Asian markets around here were recently busted for importing snakeheads for culinary purposes. It's probably only a matter of time before some genius releases them into the wild. Hopefully any ponds or small lakes that they are released into will freeze over, and they will drown.