It is because, only the top layer of the lake or river freezes. And as we know that the density of ice is lower than the water, so it comes at the top surface. As the thermal conductivity of ice is very less so the heat is not conducted by the lower water (in which fish is living). So it makes fish to survive even in icy water because heat does not flow from water to the atmosphere and the water remains warm.
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shubham.krishnan