Seefrdo,
The mud ponds of Niigata are not like lakes in northern Michigan which have depth well below the frost line. These mud ponds are sculpted into the sides of the mountains. with less depth and in a more severe climate. The scenerio of the ice and snow "pushing the water out" is a dramitization of what "could happen" if these ponds were left filled through the winter. Of course the breeders in this area drain these ponds during harvest (October) and allow the snow to refill them and as such it really isn't an issue.
The real issue is that koi are not left our under the ice through the winter by the breeders in Niigata for the koi's benefit and their own (breeders) pocket books.
Steve