Protect Pond Fish

Learn ways to keep your fish safe during harsh winter weather.
By Craig Adams, DVM

Even in areas with mild winters, it pays for owners to understand weather patterns and anticipate the extremes. (Courtesy of Dave Jones)

In western Washington, where I live and practice veterinary medicine, unless you live in the foothills, winters generally stay mild. We occasionally see snow and frozen pond surfaces for a short time each year. We do not experience the honest winters seen throughout much of the country.

Regardless of where you live, it pays to understand the weather patterns and what kind of extremes to anticipate. Often the extremes are what harm pond fish.

Fish are poikilothermic, or cold-blooded, which is only partially true. “Poikilo” actually means variable, making it more accurate to call fish variable-blooded, but “cold-blooded” is the term we use. For the most part, it is descriptive enough.

Fish such as tuna and some sharks are warmer than the surrounding water because of the heat generated through muscle activity. Some arctic fish stay warmer than the surrounding water by utilizing antifreezelike molecules in their circulation. Most fish, however, match the water temperature, even if the water reaches 80 or 90 degrees “cold.” What happens to koi and goldfish then in the winter?

Because koi and goldfish are cold-blooded, as the water temperatures decrease, their internal temperatures mirror the cooling. Their activity levels also reduce accordingly. At very low temperatures, koi generally find a peaceful and protected location, often toward the pond bottom, and enter into a hibernationlike lethargy.

If water circulation stops, the bottom usually becomes the warmest part of the pond. As most substances get colder, they shrink and condense. Water does the same thing, and the cooler, denser water sinks to the bottom until it reaches about 39 degrees Fahrenheit (4 degrees Celsius). As it continues to cool, the water thins and ultimately floats when it turns to ice. Therefore, in an ice-covered pond without circulation, the fish tend to hide in the warmest water on the bottom. Vigorous circulation from features such as waterfalls and streams can cool the water even more and actually make it colder than freezing.

This does not usually pose a problem where I live in Washington, but in many parts of the country, pond owners should turn off the circulation in the winter. This also keeps overflowing urn fountains or bamboo deer knocker from turning into a large monolith of ice as the level of your pond mysteriously lowers. Those in colder climates can try a subsurface winter return that circulates through the filtration.

Most pond oxygen occurs from exchange at the surface. As surface ice forms, it limits the gaseous exchange. Oxygen stops entering the pond, and other gases that formed in the substrate cannot leave the pond. Some gases can build up to toxic levels, a rare concern, for well-designed ponds.

Because koi demand little oxygen in cold water, which holds more dissolved oxygen than warmer water, low oxygen usually does not pose a problem. It still is worthwhile to keep a portion of the surface ice-free. A small heater can accomplish this. Manufacturers offer several styles for use in stock tanks. Some people use warm water from a teakettle each morning. Avoid chipping ice with shovels or axes, because the percussion transmitted through the water can stress your fish.

If the pond’s depth prevents it from freezing solid, some owners let nature take its course. Many breeding-facility grow-out ponds do just that, and the fish overwinter just fine. Naturally, if your pond is shallow enough to freeze solid, you will have to arrange accommodations for your fish in the garage or basement during the winter. This is also recommended for the high-end fish and the ones with sentimental value in your collection.

Feeding Freezing Fish

Experts recommend reducing the amount of food offered to fish in the winter. (Courtesy of Anastasia Thrift)

As the temperatures get below 60 degrees, experts recommend feeding pond fish lower-protein diets, such as a spring or fall formula. The limited protein reduces the amount of ammonia the fish produce. This also helps in cooler water because filter bacteria that break down the ammonia also slow down as the water gets cooler. You also can feed smaller amounts of typical diet when the temperatures reach the 50s. Sometimes the fish beg for food at low temperatures, especially during a warm day or two. Resist the temptation. Feeding them at low temperatures might kill your fish with kindness.

The cooling trend poses a bigger problem than the absolute lowest temperature of a pond. Pond fish can tolerate very warm water and very cold water, provided they can gradually acclimate to it. In the Pacific Northwest, as in many other parts of the country, we see wide temperature fluctuations in the in the fall and spring. It might hit the upper 60s one day, drop into the 30s overnight and remain there the next day, with occasional jumps back into the 60s. These changes can stress our fish, and stress leads to disease.

The word “stress” gets thrown around quite a bit. There is good stress (getting married) and bad stress (losing your job), daily stress (work) and annual stress (tax time). We cannot escape stress, so should we ignore it, especially in our fish?

On a physiological level, stress causes the adrenal glands to produce cortico-steroids, similar to an animal preparing for the fight or flight response. Elevated corticosteroid levels also tend to hinder the immune system. For example, many diseases caused by a hyperactive immune system, such as Crohn’s disease or lupus in humans, are treated with corticosteroids such as prednisone to suppress the overactive immune system.

Therefore, the warm weather you enjoy in an un-usually sunny day in October might suppress your fish’s immune system. This can lead to disease outbreaks, especially diseases associated with opportunistic pathogens such as protozoa, flukes or bacteria and fungi. During fluctuating temperatures, you can use a heater to allow for gradual cooling of the water without the daily ups and downs. Some people keep the pond heated all winter. Another approach is to supply good nutrition during the warm months and excellent water quality year-round in a large volume of water. Larger ponds resist temperature fluctuations better than smaller ponds. Healthy fish tend to overwinter very well, provided no disasters crop up.

During the winter storms of 2006 in Washington, I saw several disasters, some that were avoidable. One gentleman discovered a suddenly empty pond after the power came back on after a storm.

His system consists of two ponds, one higher than the other, connected by a waterfall. When the power went out, much of the water in the upper pond flowed into the lower pond and drained through his overflow pipe. Once the power returned, the lower pond emptied when the displaced water was all pumped into the upper pond. Easily avert this situation with appropriate pond design, such as making the lower pond larger than the upper.

Another family was out of town during the storm. They returned to find tree branches and sawdust were all around their pond. A large tree had fallen directly into the pond. A well-meaning neighbor had put on his waders and trudged through the pond, chainsaw in hand, and cleaned up the problem, or so he thought. I was called out as multiple fish began to die. All of the fish I examined had heavy parasite infestations, likely associated with the stress of the tree coming down and the subsequent activity from the neighbor. Avoidable? Not really, unless the fish had been spending the winter in the garage or greenhouse, which is not a bad idea, and often is done with valuable fish. Clearing overhanging branches and dead limbs can help to keep them out of your pond in a storm.

A short time later, I examined the fish in a very advanced hobbyist’s pond. Several large trees had come down around the couple’s home. They related how terrifying the storm had been. They huddled around the seldom-used wood stove for a week, frustrated to live without power for so long. They resorted to cooking over the tiny stove and roughing it without television or the Internet.

When I saw the fish thriving in their steaming, well-heated pond, I asked how the fish handled the lengthy power outage. Their reply was quick and matter-of-fact: “We have a generator for the pond, of course.” <HOME>

Dr. Craig Adams owns and operates Aquatic Veterinary Services in western Washington, and his interest extends to all fish as evidenced by his published articles in popular aquarium and pond magazines. Adams can be reached at aquaticvet@wavecable.com. This article originally appeared in the Winter 2007 issue of Ponds Magazine.

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