I'm getting into this discussion late and have been a lurker, but I feel
I must comment about fish survival under ice. It all has to do with
OXYGEN, rarely being frozen solid. Any body of water which freezes over
and does not allow for production of or free oxygen exchange from the
air will have low oxygen content resulting in a fish kill. Here in
northern Wisconsin, we have frequent winter kill in some of our lakes.
When does this happen? It happens when we have early ice cover followed
by thick blankets of snow retarding or even eliminating light
penetration. (The ice doesn't even need to be very thick.) Without light
the algae (still alive and functioning) are unable to produce oxygen and
the organic matter in the water/sediment causes an oxygen demand on the
water due to microbial action. The net result is winter kill. How can
we prevent it? Get oxygen into the water. How? Naturally it occurs
when flowing water enters the lake carrying with it some oxygen. Also,
spring holes may keep the ice from forming. What can we do? Keep the
lake open by aerating (very commonly done here) either using compressed
air or some mechanical devise powered by electricity or by air (wind).
I pass a lake not too far from my house which has had periodic freeze
outs for the past several decades. Two years ago, an aerator was
installed to keep the lake open and to aerate the water. Since then we
have seen no fish kills and the newly stocked populations of game fish
are thriving. Now in our area we commonly have 24 inches + of ice on
most of our lakes, so shallow artificial ponds would be expected to
freeze solid, or nearly so. Without question this would kill the fish.
So in our area we would expect to need to heat the water to keep it open
and unfrozen. In fact, most of the people I know who have garden ponds
remove the fish and sensitive aquatic plants and over-winter them in the
house and don't worry about keeping the water unfrozen.

My $0.02.

Bill

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Bill and Nancy Weiler
Tony, Wisconsin
http://home.centurytel.net/spinandfish/spinandfish