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Overwintering Fish in Backyard Ponds

There are several secrets to successfully overwinter fish outdoors.

By Stephen M. Meyer

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Q. I plan on constructing two goldfish ponds and two koi ponds later this year. We live on the Oregon coast, so I hope not to have to move the fish indoors during winter. I am told that if I build my pond 6 to 8 feet deep the fish will survive all winter outdoors. Any comments?

A. Over-wintering koi and goldfish outdoors is easy if there is little chance of surface ice forming for more than a few days. Just maintain the pond as usual, keep the filter running and do not feed until water temperatures stabilize above 55 degrees Fahrenheit.

Assuming you do get extensive surface ice, the secret to over-wintering goldfish and koi outdoors is to 1) have plenty of free water per fish available under the ice and 2) keep an opening in the ice to allow some gas exchange.

Depth is certainly the key to maintaining free water below the ice. A depth of 6 to 8 feet is good for your area. The entire pond need not be that deep — two-thirds might suffice — but, more is always better.

I know many readers have not had problems, but one bad cold snap every few years or so and there go all your fish! Considering that goldfish and koi in outdoor ponds should live 20 years or so, winter extremes are worth worrying about.

For example, I have over-wintered pool comets in ponds about 2 feet deep for about eight years. All have made it through without problems, until last year. Ice came a week or two early and left a week or two late. All the fish died. The same thing happened in hundreds of ponds across the state. Everyone lost prize goldfish. Koi are even more sensitive to the vagaries of winter than goldfish.

A light fish load, which is about one goldfish per 10 square feet of pond surface area, or one 12-inch koi per 50 square feet of surface area, is equally important. And this assumes no winter feeding.

Given these guidelines, you can determine the minimum size of your pond based on the number of fish you would like to have. Suppose, for example, you want to raise eight koi. The pond should have a surface area of at least 400 square feet and be 6 feet deep. This amounts to 2400 cubic feet of water or 18,000 gallons.

For those in the pond community who are about to overreact, keep in mind that these guidelines are for "limited-risk" management. You can certainly try a smaller pond, or more fish, but you increase risks and problems.

Use one or more floating, thermostatically controlled de-icers to keep some surface area ice free. Gas exchange is crucial for successful over-wintering. Leaves and debris on the pond floor will not matter if gases can move in and out of the water column.

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