Snakeheads (at least, the species that are kept in captivity by aquarium hobbyists in the US) are tropical fish and have a "comfort zone" of temperature between roughly 70 and 85 degrees; with sustained temperatures below 70 the immune system of the fish begins to fail, making it susceptible to parasite infestations (typically attacking the gills of most fish first, where the most oxygen is present, though in the case of snakeheads, being air-breathers, I can't say whether or not this is the case). The colder the temps and the longer the exposure, the faster the fish dies. It takes many, many generations for a species to be able to adapt to new environmental conditions, as fundamental changes in morphology may be required to cope. That being the case, unless these invading species are able to locate the required water temps throughout the colder months, as well as a source of food, they're extremely unlikely to survive long, particularly now that the cold weather has begun to settle in across the upper states. Addditionally, the fact that the fish must surface to breathe means that any "invaded" bodies of water that freeze completely over are essentially safe, since the fish aren't likely to develop the ability to breathe through gills over the period of a few weeks.
Snakeheads are indigenous to China and Madagascar, as well as other areas throughout Southeast Asia; I'd be worried about the fish if I lived in the southern-most states of the US, but probably no where north of the latitude of Tampa unless the local water has a hot spring or power plant effluent draining in.
Oh, and the reason that there have been few, if any, reports stating that the fish will die in cold weather is that traditionally, it's the procedure of the twits in the news media to blow everything out of proportion before they know all the facts; this is one case where ichthyologists and aquarium hobbyists all over the US were laughing at the supposed "experts" in the fish and wildlife divisions. While I support the claim that these fish can cause significant damage to a fishery by outcompeting native gamefish species for prey (as well as directly eating the gamefish, themselves), I also believe that the fish that have invaded the waters of the Northern US will be dead very shortly.
Last edited by flyfisha1; 10-10-2003 at 07:17 PM.