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The trout either manage to find a pocket of water with somehow acceptable DO levels OR the extremely low water temps allow them to survive the low DO conditions


I think it probably isn't black and white but rather gray instead. In other words, I think both these contribute to the winter survival of your trout.

It occurs to me that you do not take DO measurements over the diffuser. Seems a potentially life-threatening situation if the ice were thin there. The temperature of the water is near freezing at a temperature where the potential for saturation is at its greatest. This means that the absorption of oxygen that you are pumping into the lake it is at its most efficient. If enough oxygen is absorbed to meet the lowered demand then the aeration creates a refuge where your fish are concentrated. Its not enough to add DO to the whole lake but sufficient to supply the oxygen demand of the fish in the refuge that the aerator may create.

It isn't what we don't know that gives us trouble, it's what we know that ain't so - Will Rogers