burrowsy

Fish are cold blooded creatures and have a very slow motabolism. Some sunligth does penetrate the ice though and because this ice forms from freshwater ( the salt is squeezed out on formation ) there's very little impurity and so some sunlight can get through allowing plant life / algae to survive also.

Water is an exception to most liquids when you talk about expansion on heating. Water present below an icy surface is always warmer than above. So the fish live!