Jim, my thoughts are that RES might winter kill in much the same manner that gizzard shad do, it's the sudden drop in water temperature, usually 5 degrees or more dropping temps below 39 degrees that gets them. They simply can't acclimate to the temperature change quick enough and the fish oils start to solidify.
http://www.in-fisherman.com/biology/science-of-shad-winterkill/
