PREBLE COUNTY — On Feb. 2, Pennsylvania’s Punxsutawney Phil popped out of the ground and saw his shadow, thereby predicting six more weeks of winter. Two weeks into February, I’d say he’s been right so far. Unless you are going ice fishing, the fish population in your pond might be the last thing on your mind right now. But when you combine the recent cold and snow with the fact that many ponds came into winter having lower-than-usual water levels, it could set up conditions leading to a winter fish kill.
First, a winter fish kill primer. Most of the fish kills in Ohio result from a loss of dissolved oxygen (DO) in the water. Fish respiration requires oxygen, which they obtain from the water. Decomposition going on in the pond requires oxygen as well. Fish metabolism slows down in the colder months, as does the decomposition process, leading to less overall demand for oxygen.
Through photosynthesis, aquatic plants continue to add DO to the water even in the winter, though at a much slower rate than during the warmer months. Wind action also adds DO to open water. Lastly, cold water holds DO better than warm water.
If a pond does not freeze over in the winter, both the wind action and photosynthesis continue to add DO to the water. At this point, these processes are adding enough DO to make up for what oxygen is being used all along by metabolism and decomposition in the pond. When a pond freezes over, this brings an end to the diffusion of DO into the water from wind action. But all is not lost. If the pond should freeze over with clear ice, photosynthesis will continue to add DO to the pond, potentially enough to maintain oxygen levels in the pond.
The trouble comes when either the ice is opaque (not translucent), or if the ice becomes snow-covered. Four inches or more of snow cover on top of ice on a pond can drastically reduce sunlight penetration into the water. Now DO production is limited or totally cut off, while the consumption of oxygen continues. When DO levels become critical, fish first become stressed, then if levels fall too low, a fish kill occurs.
Typically, our fall rains refill ponds where the water level has dropped during the summer. As we well know, this past year has been anything but typical. The weather was strange, too. Many ponds in the area were not full going into winter and are still well below normal levels. Less water in a pond means less DO in the pond. The heavy snow covering the ice on ponds means decreased DO production. If you have a pond that had an excessive amount of vegetation going into winter, the decaying plants will also lead to increased oxygen usage.
To sum it up, if you have a shallow pond with too much aquatic vegetation going into winter, you have an increased likelihood for a winter fish kill.
Now for the good news. Two things can be done to prevent winter fish kills, neither of which sounds like a walk in the park. An aerator can be installed in a shallow portion of the pond to maintain or erode a hole in the ice, allowing for some surface agitation and oxygen diffusion. It’s a little late for that now. Another option is to shovel the snow from 1/4 to 1/3 of the pond’s surface, but do make sure it’s safe to walk on before venturing out. Clearly, shoveling snow from your pond is best suited for small ponds. Either pick aeration or shoveling, but not both. Aerated ice is not safe to walk on. A third option would be to roll the dice and see what happens when the thaw comes. If nothing else, you have some ideas on how to manage you pond prior to next winter.
The Preble Soil and Water Conservation District is here to answer your pond questions no matter what the season. You can reach us at (937) 456-5159, or visit our website at www.prebleswcd.org. Thanks to OSU Extension for information contained in this article.