According to the Missouri Department of Conservation, several conditions can cause summer fish die-offs in ponds.
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Overheated Missourians can rest in the shade during blistering weather. Fish in ponds aren’t so lucky.
Fish and plants need oxygen in the water to survive. According to the Missouri Department of Conservation, several conditions can cause summer fish die-offs in ponds. Ultimately, though, it is lack of oxygen that does the killing.
The Conservation Department recommends every pond be at least 8 feet deep. The more water there is, the more oxygen there is for fish, and even ponds that once were deep enough can become death traps for fish.
Rainwater running off hillsides carries sediment into ponds. Over time, this sediment raises pond bottoms. Owners should check their ponds’ depth every year, especially if the surrounding land is prone to soil erosion.
Another pond problem that can contribute to summer fish kills is overabundant aquatic vegetation. Aquatic plants are a necessary part of ponds and lakes. They keep the water oxygenated, provide food, cover and nesting sites for fish and stabilize the shoreline and pond bottom. But there is a limit to the amount and kind of plants that are desirable in ponds.
Excess vegetation is a symptom of an underlying problem-excess nutrients in the water. Anything that puts phosphorus or nitrogen in pond water can trigger excessive algal growth. Common sources of nutrients include wild or domestic waterfowl droppings, runoff of fertilizer from lawns, crop fields or livestock feedlots or malfunctioning septic tanks or sewage lagoons.
To reduce the effects of nutrient runoff, pond owners can establish buffer zones of unfertilized land 100 feet or wider around ponds. Repairing faulty sewage systems or diverting animal feedlot runoff away from the pond may be the solution in other cases. You can provide a nutrient and sediment trap by creating a wetland between the pond and the source of nutrients. It’s also a good idea to check for leaks in sewage lines and lagoons.
Rooted plants are a healthy addition to a pond as long as they don’t cover more than 10 to 15 percent of the pond’s surface. Algae are another story. Often called seaweed or moss, this free-floating slop can interfere with fishing and swimming. Some types of blue-green algae produce toxins that can harm fish, waterfowl and other animals. But this isn’t the most common or the most serious problem caused by algae.
Like all plants, algae need oxygen to live. Plants normally produce more oxygen than they use, but on cloudy days they may use more than they produce. Worse, after several overcast days algae may die off, tying up more oxygen as it rots. When this happens, the amount of oxygen in pond water can fall to levels that are harmful or even fatal for fish.
Oxygen levels usually are lowest just before sunrise. If you see fish gulping air at the surface, you can provide temporary relief by spraying water across the surface of the pond with a high-pressure hose nozzle or by mixing air into the pond surface with an outboard boat motor.
If there is enough algae in your pond to interfere with fishing or swimming, you probably ought to take action to limit its growth. Chemical treatment is effective and harmless to fish if done properly. Copper sulfate or other copper-based herbicides are the most common types for algae control and are usually available at farm supply stores.
Oxygen depletion can be a problem as the treated vegetation dies and decomposes; it is best to treat the pond when daytime temperatures are below 80 degrees. Only one-quarter to one-third of the vegetation should be treated at one time.
Mats of algae also can be removed by hand with a rake or other tool from small areas such as swimming sites. Algae removed from the pond should be dumped below the pond’s dam to ensure that nutrients don’t run back into the pond as the algae decompose.
More information about controlling aquatic vegetation and other pond management advice is available in the Aquaguide series of publications, available from Conservation Department regional offices in Cape Girardeau, Clinton, Columbia, Kansas City, Kirksville, Springfield, St. Charles, St. Joseph, Sullivan and West Plains.