Mark, I don't mean to disturb you, if I have I apologize. You are the man in the field seeing this first hand. I am from Florida and know as much about snow and ice as well....This discussion is not pointless. I find it very interesting so hang in there if you will please.
I am confused about one thing. These mud ponds do not carry water thru the winter because they are drained? If they were not drained for example and there was 3 feet of water that did not freeze solid the koi could live in this condition. If the ponds froze solid the koi would expire most certainly. Now back to the 3 feet of water example. The part that I am getting lost is the weight of the tons of snow on top of the water with fish living.
Technically speaking we need to address this situation. Here is where I get messed up. Water can be compressed but very little even at high pressures. For practical purposes water is considered an incompressible fluid. Density does not change with pressure.
If there was 6 feet of snow on top of the 3 feet depth of water, it would be equivalent plus or minus to 6 feet of water. So the fish would experience the added pressure as if it was 9 feet in the water column instead of 3 feet. That is not going to crush our friends I don't believe but hey, Iam from Florida so what do I know?