As the ice melts away, pond owners are likely to find something they did not want to see: dead fish. The problem was not the cold winter temperatures; the die off was from a lack of dissolved oxygen in the water.
The saddest part of all is that the largest fish in the pond are the most likely to be affected. Larger fish require more oxygen per pound of body weight, and are the most likely to starve for oxygen.
Why does this happen? Most of the oxygen in pond water is produced through photosynthesis by aquatic plants. When the surface freezes, photosynthesis continues if the ice is clear enough to let light through. But when snow covers the ice and blocks the sunlight, photosynthesis stops.
If light is blocked long enough, aquatic plants begin to die and, instead of producing oxygen, the plants use oxygen as they decompose. Gradually, the oxygen in the water is depleted and fish, bacteria, and other aquatic creatures suffocate. In some ponds, especially shallow ponds with high nutrient levels, this can happen in just a few days, but kills occur even in large, deep ponds when nutrient levels are too high.
Even if only some of the fish die, a winterkill can permanently upset the balance of fish species in a pond. Large fish eat a lot of small fish. Losing just a few large fish in a small pond can affect the pond for years. Sometimes the only way to return a pond to a balance of species that will provide good fishing is to kill off all the fish and re-stock the pond.
If your pond experienced a severe winter die off, there are a few things you can do to reduce the chances of it happening again. Aerating the pond through the winter can help. So can controlling excessive aquatic vegetation during the growing season and removing heavy layers of snow from at least a third of a pond’s surface. If you try to remove snow, be especially cautious of thin ice.
If a pond seems especially prone to winterkill, it might need to be drained and deepened. If excessive nutrients are causing too much vegetative growth, try to find and eliminate the nutrient source. Another alternative for very shallow ponds is to let the fish die off and manage the pond as wetland habitat for wildlife other than fish.
Jeff Burbrink is an Extension educator in agriculture and natural resources. Write to him at 17746 E. C.R. 34, Goshen, IN 46528; call 533-0554; or fax 533-0254.