It's very important that there's at least some part of your pond that isn't completely covered by ice during the winter. Allowing contact between the air and the surface of your pond allows toxic gases to escape the water and oxygen to enter it from the air. A de-icer can be used to accomplish this, but sometimes a heater is used in its place.
If you see fish up near the surface of your water by the hole you've created, they're braving the cold temperatures in order to get gasps of oxygen. This means you need to provide more aeration to your pond through the addition of holes or the use of an aerator.
