A winter fish kill can occur in any pond. Even if an aerator or heater is in place, fish still experience different levels of shock throughout the winter. Snow can pile up on top of the ice which will reduce sunlight penetration and cause oxygen levels to decline. An aerator will add some oxygen to the water, but it will not do any good if all of the oxygen is used up by the organisms and fish in the pond. A heater will attempt to keep the water temperature up, but changing water temperatures from warm to cold to warm will send the fish into shock, causing them to die. If you see many dead fish in your pond, you should first assess whether a total fish kill or a partial fish kill has occurred.
The first step is to try to catch some fish, especially bluegills. Also, try to record the species and sizes of the fish that have died. This should be done in the early spring. If you have been fishing for hours and have not caught anything, a total fish kill has likely occurred. If few bluegills are caught, but no bass, a partial kill has occurred.
If some bluegills survived, they may not need re-stocked. Bluegills produce large numbers of offspring and will easily rebound. In a partial kill, bass, catfish, and grass carp are often the species that require re-stocking.