Was there ice on the pond? Turtles and crayfish hibernate in the bottom mud. If there was a low O2 event and the pond had ice on it, O2 disappears from the lowest water (i.e. the bottom) first. If that happened and they were in hibernation, they wouldn't live.

How cold does the water get during the winter?

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).