Really that low?! I dont know to much about the fishes natural habitat, but when I was growning out channel catfish I kept the at 86 and with my p bass even a little higher than that. I guess it would make sense though becuase of its closesness to the cool ocean waters....
Brackish water is generally colder than the lake waters because, like catfishluver said, they are mixing with the cool sea waters.