Overwintering Koi and Goldfish in Backyard Ponds

Generally speaking Koi (carp) and goldfish are very "hardy" fish that can tolerate lower oxygen levels and overall poorer water quality than most other types of fish. This means in most enviroments they can survive through frigid winter conditions. Even so, some consideration must given to ensure thier survival through freezing conditions in smaller "backyard" ponds.

The first consideration is the geographic location of the pond and how much and how often does ice typically form on the surface. If you are not sure a general guide would be the USDA Hardiness Growing Zones. These are based on minimum annual temperatures and range from Zone 4 (coldest) to Zone 9 (warmest) in the lower 48 states. Generally Zones 7 and higher rarely see surface ice, Zones 5 and lower will usually have good amounts, and Zone 6 can go either way. This is a generalization and of course these zone locations will continue to change with current trends in climate change.

The reason the amount of ice is important is because this is a large factor in nearly all winter fish deaths. Obviously fish cannot survive freezing in ice, but this is only possible in the shallowest ponds in the coldest climates. Normally in most ponds there is a layer of unfrozen water that will remain under the ice. FYI Many people will swear thier pond fish "froze solid" only to thaw and begin swimming in the spring. This is simply impossible. It is likely the pond appeared to be frozen to the bottom and the fish remained visibly motionless appearing to be frozen stiff. If they begin to swim with the thaw then there was at least some unfrozen water under the ice. The ice usually does not have a direct effect on the fish. The direct effect of the ice is on the pond. Specifically on the ponds ability capture oxygen and expel carbon dioxide. Oxygen in the water is used by the fish and most living organisms in the pond. This is replenished simply by the water directly contacting the air, at the pond's surface. Moving (pumping) the water through a waterfall, fountain, etc. increases the oxygenation or areation of the water by simply creating more direct contact with the air. Conversly carbon dioxide created by the water life must be released from the water and may only be accomplished the same way, by the water contacting the air. This means when the pond's surface is completely covered with ice this "exchange" of gases ceases completely. If these conditions persist long enough it is possible for fish to perish from lack of oxygen. Other factors that increase the potential for fish loss in these conditions include, accumulated organic debris (especially deciduous tree leaves), shallow water less than 2 ft deep, snow cover on the ice, and large fish loads (populations). A combination of these factors left unchecked can lead to low oxygen levels, low water quality, and eventually fish fatalities.

Historically, experts have given the following advice to help ensure survival of ornamental pond fish in the winter. First remove any organic debris from the pond that can be netted, vacuumed, etc. A netting cover in the fall minimizes debris collection. When ice begins to form on the surface turn off your pump(s) and place an adaquate de-icer in the water and plug it in. The reason for stopping recirculation of the water is to allow the pond water to naturally "stratify" into temperature layers. This meant the water at the bottom would always be slightly warmer than the frozen water at the surface, creating a more "comfortable" environment for the cold-blooded fish. So a de-icer is used to maintain an opening in the ice coverage. Check your de-icer periodically to be certain it is working by keeping an area or "hole" ice free on the surface. Turn the pumps/ filters back on in the spring once the threat of a prolonged freeze has passed. This advice will and has worked nearly without fail for many years. The pond water is typically oxygen rich since most living organisms are using minimal amounts due to the low temperatures. The fish remain generally motionless using minimal amounts of oxygen, and any size hole in the ice will allow enough gas exchange to prevent problems.

Recently however some pond owners/ experts, specifically koi enthusiasts have pointed out that these methods may not be enought to sustain large populations of large koi (up to 30 lbs or more each) in freezing climates. Clearly the overall oxygen demand can be much greater for a pond full of these large fish. Larger fish also burn more energy than smaller fish in cold water due to the extra weight they must carry. This means they will need to eat, and even if they are not "fed" by the pond owner they will consume material, organisms, etc., in the pond. This will result in more organic (fish) waste which can only be removed or detoxified through filtration. When considering these factors and the fact that fish like this are usually named family pets, extra caution/ care should be used when overwintering these types of ponds in freezing climates. Pumps, recirculating water, and filtration should all be continued 365 days/ year if possible in ponds with many large koi. Koi experts look at it this way. The emphasis is to create an environment for the fish that is as "stable" as possible. This makes for healthy, "comfortable" fish. So even though the recirculating water minimalizes the stratification of temperature layers, it more importantly slows water temperature swings affected by changing air temperatures. These weather conditions particularly in late winter/ early spring can create unstable water conditions. Dramatic, constant water temperature swings negatively affect aerobic bacteria colonies in the pond needed to breakdown increasing levels of organic waste. The resulting lower water quality can negatively affect fish health. Moving water allows/ promotes filtration for maintaining good water quality. The moving water obviously prevents complete ice coverage and aerates the pond.

There is one other strategy that is sucsessful. The use of an air pump with some type of weighted diffuser/ airstone. The electric pump forces compressed air through thin tubing to be released as "bubbles" from underwater. This will keep an area ice free and recirculate/aerate the water. This method works alone or can be used combined with the first two methods.

Some may consider actually heating the pond to prevent freezing, such as people do with swimming pools. The fact is heating water requires a tremendous amount of energy. The costs for even minimally heating water outdoors overwinter exceeds the practical budget of most households.

There are some things to further consider. All of these methods require electical power, so allows have a plan for power outtages. This is especially true when recirculating water to filters/ points outside of the pond. Check to make sure pipes drain and/ or be ready to act to manually drain water where needed. Consider creating a weather-proof structure to house large out of pond pumps, filters, etc. and winterizing pipe lines/ valves. In ponds where de-icers are used it is best to simply wait for the power to return. Otherwise a rubber child's ball can be placed in the hole to allow temporary periodic removal. Normal outtages rarely last more than a few days not long enough to cause trouble. Snow can also create negative pond conditions. If allowed to build up and remain on any ice coverage it blocks light penetration into the water. This may create dying algae/ bacterias that would lower oxygen levels. Any snow that can be safely removed will improve conditions. Conversly any existing ice should never be forcibly cracked or broken as the acute sound waves this produces in the water can cause harm to the fish.

In summary try to keep the pond fairly clean prior to winter, do not allow the pond to completely ice over, safely remove any snow build-up, do not feed fish through the winter and check the pond regularly. Return the pond to its normal operation as promptly as possible in the spring. If you are still not sure which method may be right for you consult your local pond expert.