How the fishes can survive in the extreme winter, when ponds and lakes are frozen?

Answer

When it is exterme cold, the temperature of water in ponds and lakes starts falling. On getting colder, whater becomes denser and it goes down. To replace it, the warmer water form below rises up. However, it happens, so till the temperature of water at the bottom of the pond beomes `4^(@)C`. once ice is form on water surface, it floats due to low density and as ice is bad conductor of heat, the rate of loss of heat decreases with increasing thickness. So temperature of water at lower levels remain at `4^(@)C` for sufficiently long suration and that is why fishes can survive.