Where do fish like when the ice is open?

The river is frozen, and the fish are underwater, but the temperature under the water is still constant, and there is no freezing ~ to be exact, it is warm under the water, which is suitable for fish to survive. In winter, the river is covered with a thick layer of ice, just like covering the river with a thick quilt. As long as the depth is enough, no matter how cold it is outside, the bottom of the ice can be kept at about 4℃, and we fish can swim around without it. In other words, the outside temperature is different from the underwater temperature.