Photo: Bryan Swistock, Penn State University
Fish kills in Pennsylvania ponds are rare. However, they do happen, and some years are worse than others. Fish kills are caused by several factors but the most common are weather, pond vegetation, and the interaction of the two.
Fish, frogs, and turtles have adaptive features to accommodate the less friendly environment in winter. Their body temperature falls with the water temperature decreasing their respiration rate and energy needs. Frogs and turtles burrow into the mud at the bottom of the pond and hibernate there. They are able to do this by breathing through their skin. A completely snow covered pond can cause "winter kill," the death of fish, frogs, and turtles.
Oxygen enters ponds by moving from the air into the water aided by wind, or from photosynthesis by the plants in the pond. Ice cover blocks oxygen movement from the air into the water and oxygen produced by algae and plants become the only source. Thin, clear ice allows oxygen production by plants and algae since enough sunlight can filter through the ice to allow photosynthesis. If the ice remains into mid-winter or thickens slightly, oxygen levels will begin to decrease because each day pond plants and animals use oxygen, even in winter, as the animals breath and dead plant material decays.
When ice cover continues into late winter and thickens, or worse yet, is covered by snow, problems develop. Sunlight reaching the plants and algae is reduced, and so is the amount of oxygen produced. Since metabolism and decomposition continue, oxygen levels begin to drop.
The more water volume there is in a pond, the less likely the pond will experience winterkill. A one-acre pond that is 6 feet deep is less likely to have winterkill than one that is 3 feet deep. The amount of decaying organic matter on the bottom of the pond is critical. A pond bottom covered with leaves and dead pond plants is more likely to experience winterkill than a pond without such materials. Ponds with a very dense aquatic plant community in summer are most susceptible to winterkill during harsh winters.
The pounds and numbers of fish in the pond during winter influences oxygen decrease under the ice. Even though fish metabolism has slowed during winter, they still need oxygen. A pond that contains many fish will consume oxygen faster than a pond with fewer fish.
So what can a pond owner do?
Hand plowing lanes across a pond to clear the snow from the ice allows sunlight to reach the plants. One winter management strategy is to keep about 30% of the ice free of extended snow cover. Be sure that the ice is safe for the method of snow removal proposed. An alternative is to use a diffuser type aerator to add oxygen and keep a small area free of ice.
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