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The suddenness with which a low oxygen fish kill happens can leave pond owners in shock. Fish that were healthy and active one day are dead the next. A little effort put into preventing fish kills can be very worthwhile.


The first line of defense against summer fish kills is working to reduce factors which lead to low oxygen levels. In summer, the most common cause of low oxygen involves the excessive growth and die-off of algae. It is normal and healthy for pond water to have a green color due to the presence of microscopic algae. However, when excess phosphorous and other fertilizers run-off into a pond, the growth of algae skyrockets, leading to die-off of algae. As the dead algae decays, it rapidly uses up the pond’s scarce supply of oxygen and the fish, especially the bigger fish, have a hard time getting enough oxygen to survive.


Continuous aeration systems are not recommended as the first line of defense against low oxygen problems. They have their place but can be costly or of little to no benefit if they are undersized for the pond you have. To be effective, multiple units are usually needed, and must be correctly installed and maintained. All such systems require maintenance and eventually replacement. Calculate the cost of running electricity to the pond and the electricity itself before committing to an aeration system. An experienced aeration equipment provider is needed to insure proper installation and application.


Head off a summer fish kill by following these steps:


On a sunny day, lower a secchi disk or other white object into the pond and measure the depth at which it disappears. If it disappears at less than 12 inches, your pond is at a high risk. If it disappears at between 12 and 18 inches, your pond is at intermediate risk. If it disappears at 18 or more inches or your pond is muddy, then you are at low risk. Repeat this check throughout the growing season.


To reduce the risk of a fish kill, you should always be aware of any nutrients that might eventually wash down into the pond and do everything you can to reduce the amount of nutrients reaching the pond. Phosphorus is generally the main culprit in causing large algae blooms and we can reduce the risk by trying to reduce phosphorous runoff from lawns, livestock and other sources. If you absolutely cannot reduce excess phosphorous runoff into the pond, then explore the pros and cons of an aquatic dye, like Aquashade, to reduce algal abundance by reducing light levels.


If the pond is at high risk, have a means of emergency aeration on hand and ready to go. Keep a pump set, boat motor, brush hog or other means of getting oxygen into the water on short notice. Anything that can be done to break the water into droplets will allow it to absorb oxygen and create a small refuge in which fish may survive until the algal bloom is reestablished and can once again produce oxygen.

Check your pond at dawn for signs of fish mouthing or “piping” at the surface. This indicates that they are stressed by lack of oxygen. On an average day oxygen in a pond will be lowest at dawn.


A change in color of the pond and perhaps formation of a surface scum or film often indicates an algal die-off. If you experience either sign and observe fish strongly piping at the surface, begin aerating and discuss the situation with a fisheries biologist. In some cases, up to two weeks of continuous emergency aeration may be needed for algal populations to recover and once again produce oxygen.


Understanding how algal die-off fish kills happen, doing regular pond checks and being an active pond manager will greatly reduce your chance of a disastrous fish kill.

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