IME, 32 degree water kills fish. Generally, fish seek the warmest water when there is ice cover. This is a little above 39 F unless the water is receiving additional cooling by aeration or something like that.

The energetics of LMB have been experimentally verified down to about 50 F. The energy consumption required to maintain weight at 50 F is about 1/5 that required at 80. Extrapolating, the energy consumption at 40F is about 1/10 the amount needed at 80 F.

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