Ponds Need Winter Care
Even in the worst Ohio winters, fish aren't likely to freeze in a well-built pond because they can retreat to deep water when the surface freezes. Still, each year many pond owners find dead fish when their ponds thaw. The problem isn't the cold; it's a lack of dissolved oxygen.
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 die and, instead of producing oxygen, they use it 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. For instance, if too many largemouth bass are killed, less desirable species might become over-populated because the bass aren't eating them. 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.
A better option is to prevent winterkill in the first place. Aerating the water 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.