The water under the ice is not simply 4 degrees Celsius, but from the ice layer down to 0 to 4 degrees, gradually freezing down, and the water under it gradually cools down. If it is cold for a long time and the water is not too deep, it can be frozen to the end.

Fish can survive as long as they are not frozen to the bottom of the water. But if the thawed water is too little, the fish will die of lack of oxygen, and the phenomenon of dead fish can be seen every year when the ice melts.