Fish, koi overwinter at bottom

Q. How are fish in backyard ponds surviving the winter?

Cindi from Youngstown

A. Water is relaxing to the mind and body and thus makes a lovely addition to your home garden. Many gardeners create fish and koi ponds to supply their need for water features. But when the weather turns cold, as it does in Northeastern Ohio, how should home gardeners protect their finned friends?

Fish survive in frozen ponds due to a process called pond stratification. Water reaches its maximum density at 39 degrees Fahrenheit. In the winter, the denser water sinks to the bottom of the pond, while water on top of this layer cools to below 39 degrees and can freeze. Fish survive in the warmer water toward the bottom of the pond.

However, the oxygen can become depleted if there is too much competition for it by having too many fish or too much organic material decaying on the bottom of the pond. It also can be depleted by ice covering the pond for extensive periods which blocks the exchange of gases with the atmosphere. When these events occur, the lack of dissolved oxygen can lead to “winter kill” of your fish.

How can you prevent this? Some pond owners remove their fish from the pond in the fall and overwinter them in tubs or aquariums. This can be stressful for the fish as they are captured and the water is adjusted to suit them. Also, many people don’t have space to accommodate their fish through the winter.

If you plan to leave your fish in the pond, there are several things you should do. Limit the number of fish in your pond so they have access to enough oxygen. Also, remove dead leaves from the bottom to prevent excessive oxygen depletion and a buildup of toxic gases which may harm your fish. If your pond will freeze over for more than a few days, provide your pond with a de-icer to melt a hole in the ice and allow for an exchange of gases with the atmosphere. For an in-depth discussion about these devices, read “Koi and Goldfish Pond Winter Care” by Jeff Rugg, University of Illinois Extension at http://go.osu.edu/koi.

With a little planning and care, your ornamental fish can overwinter safely and be healthy residents of your pond as you enjoy the return to warmer weather in your backyard garden.

Today’s answer is provided by OSU Extension Master Gardener Volunteer, Sheila Cubick. Call the office hotline at 330-533-5538 to submit your questions. Clinic hours vary this time of year due to the winter season.