I can see most people agree that warm water fish arent going to be found in a cold water habitat and vice versu. If you notice, as you move down stream from a coldwater to cool water then warm you find fish species changing. Its not that the introduced trout are out competeting smallmouth or anyother fish populations in tailwaters, its just that habitat regimes for species differ. As for the snail darter, competition among fish was not the problem that was addressed, it was the dam itself. Snail darter young are palagic, high releases mean that the fry are washed further downstream, reducing young of year within their niche. Im a wildlife and fisheries student myself, and actually today we had a lecture from Ed Scott from TVA who ran stream surveys and is still deeply fascinated with the snail darter. The question of what is the snail darters main predatory fish came up, and it actually wasn't a game fish at all, he belives its sculpin, theres also a problem associated with (star grass?). It blankets the substrate, becoming to dense for proper habitat.
~If our father had had his say, nobody who did not know how to catch a fish would be allowed to disgrace a fish by catching him.~