How do fishes survive in the coldest of winter?

When ice water freezes to become solid ice, its volume increases and its density is lowered. That’s why ice floats on water. If water were most dense at 0°C, it would settle to the bottom of a pond or lake. Water at 0°C, however, is less dense and floats at the surface. The water of a pond or lake freezes from the surface to the downward direction. The temperature at the bottom of an ice covered pond is 4°C, which is very much warm for the fishes that live there. More importantly, it is not possible to cover very deep ponds with ice even in the coldest of winters. This is because all the water must be cooled to 4°C before lower temperatures can be reached. For deep water, the winter is not long enough to reduce an entire pond to 4°C. This is because water has a very high specific heat capacity and poor ability to conduct heat. That is why the temperature in a cold region remains at a constant 4°C year-round.