Each species of fish has a water temp range it does well in and grows. Each species also has a range of temps (high end and low end) called upper and lower lethal temps that causes stress and if extreme for a sufficient period then death (morts). The more stress a fish encounters the more likely it dies. No fish I know of (there probably are some) can survive a total ice out (no water just ice top to bottom). When ice occurs the warmest water 39 F is on the pond bottom. There is within in each species some local adaptation over time thus the suggestion to buy local adapted fish if you are pushing the species range limits. RES biology based on genetics have a warmer lower lethal temp limit than BG for example. I hope that was the question you asked. If you mean instead how does the cold kill them I can post that as well - it has to do with cellular metabolism and imbalance.

Last edited by ewest; 11/21/09 09:03 PM.