Minimize Fish Kill in Extreme Winter Weather

Dead fish in open water along the ice after a winter fish kill

Winter Fish Kill: How to Prevent It in Your Pond

A winter fish kill happens when a pond runs low on dissolved oxygen under the ice. It is most common after long stretches of snow-covered ice. We saw it across the Midwest in 2013, and we don’t want a repeat.

Why Fish Die Under the Ice

Ice alone is rarely the problem. Sunlight still passes through clear ice, so algae and aquatic plants keep making oxygen. Snow changes that. A few inches of snow blocks most of the light, so plants stop producing oxygen. Meanwhile, fish, bacteria, and decaying leaves and plants keep using it up. As a result, shallow ponds with a lot of muck on the bottom are at the highest risk.

Warning Signs

Dead fish often stay hidden under the ice until it melts. However, a few clues point to trouble: weeks of snow cover, a rotten-egg smell from holes in the ice, or fish crowding near open water.

How to Prevent Winter Fish Kill

  1. Clear snow from the ice. After each snowfall, shovel or blow snow off a large part of the pond so sunlight can reach the water. Only go out on ice you know is thick enough to hold you.
  2. If your pond is at risk, run your aeration system. Whether you need winter aeration depends on your muck, how long the ice stays, the snow on top, and your fish population (see our winter aeration FAQ). If you run it, start before ice forms and place diffusers in shallower water rather than the deepest spot, so you don’t mix away the warmer bottom water where fish rest. Aeration keeps a patch of water open, so mark it, keep people and pets off thin ice, and don’t clear snow near the open water. If you don’t have aeration yet, install an electric or solar aeration system before the pond freezes.
  3. Cut the muck before freeze-up. Less muck means less oxygen demand under the ice. Treating with MuckEater and PondFit bacteria during the open-water season lowers that load.

Plan Ahead

The best time to prevent a winter fish kill is before the pond freezes. See our guide to getting your pond winter-ready, or contact us about aeration. Good luck, and here’s hoping for an early spring!