Dr. Willis and I have taken redear sunfish from a well oxygenated, high water quality pond with water temps in the upper-thirties, and placed them in a tank with the exact same water, and good oxygen (the other fish lived), and watched the redears roll over before our eyes because the water temps were in the low-thirties. I think there's a magic number (36 perhaps?) where it doesn't matter how good the water quality is, the redear will perish. Also keep in mind that there may be differences between redears in northern latitudes and southern latitudes that have to do with short term evolution..i.e. redears that have had a few generations to allow slightly more cold-tolerant individuals to breed with other slightly more cold-tolerant individuals...that would lead to populations that can survive more stressful wintertime events.
