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Every summer a few farm pond owners experience fish kills in their ponds. While some of the fish kills can be attributed to chemical contamination, most are due to either an algae die-off or a physical process called thermal destratification or pond turnover. Both problems result in decreases in dissolved oxygen levels in farm ponds. When the dissolved oxygen levels drop too low, the fish will either die directly from asphyxiation or days later due to disease outbreaks caused by stress associated with low dissolved oxygen. Cause of Fish Kills Green plantonic (microscopic) algae are essential to oxygen production in farm ponds. Farm ponds normally have a faint green color, and the water should be clear enough to see around 2.5-3 feet vertically into the pond water. When there are enough nutrients present in the water, algae "blooms" occur and give the water a rich green or "pea-soup" color. Water clarity decreases as a result of these alga blooms to the point that your hand is not visible in as little as six inches of water. If these alga blooms are followed by periods of cloudy weather, there will be a potential of an alga die-off. Temperature determines the amount of dissolved gases (oxygen, carbon dioxide, nitrogen, etc.) in the water. The cooler the water the more soluble the gas. Water has a high-heat capacity and unique density qualities. Water has its maximum density at 39.4 F. In spring, water temperatures are nearly equal at all pond depths. As a result, nutrients, dissolved gases, and fish wastes are evenly mixed throughout the pond. As the air temperature increases in the summer, the surface water becomes warmer and lighter while the cooler-denser water forms a layer underneath. Circulation of the colder bottom water is prevented because of the different densities between the two stratified layers of water. Dissolved oxygen levels decrease in the bottom layer since photosynthesis and contact with the air is reduced. The already low-oxygen levels are further reduced through decomposition of waste products, which settle to the pond bottom. Summer stratification is a greater problem for fish in deeper farm ponds. Stratification may last for several weeks. This condition may develop into a major fish kill when sudden summer rains occur. These rains will cool the warmer upper layer of water enough to allow it to mix with the oxygen poor layer below. Decomposing materials in the oxygen-poor layer are again mixed evenly throughout the pond, resulting in an overall reduction in the dissolved oxygen level. Fish previously able to avoid the oxygen depleted layer are now susceptible to low-dissolved oxygen syndrome and possibly death. Warning Signs Warning signs of potential "problem" ponds include heavy algae blooms which result in a "pea soup" look of the pond water. Heavy algae blooms coupled with extended periods of cloudy weather or thunderstorms are all that is needed to trigger oxygen depletions. Cloudy weather can lead to algae die-offs while thunderstorms can cause pond turnover. An early warning sign of impeding problems is a color change in the water from a rich green to a tea color. Another more acute signal will be fish swimming lethargically at the surface of water. The fish may appear to be "gasping" for air. By the time you see the fish at the surface it is usually too late for the farm pond owner. The fish are stressed to the point that some, if not most, of the fish will die. At best only the large fish will die. At worse all of the fish will die. Prevention There is no way to avoid pond turn over or algae die-offs. Both are natural processes in farm ponds. There are, however, a few precautions a farm pond owner can take to reduce the risks. First, follow recommended fish stocking densities. As a rule, farm ponds can support around 300 pound of fish per acre. Second, excessive nutrients in ponds promote aquatic plant growth. By eliminating unnecessary nutrients the farm pond owner lowers the risk of excessive algae populations. Fish feeds contains nutrients which promote algae growth. Even though it is enjoyable to watch fish eat, feeding does increase the risk of excessive algae buildup. Finally, proper control of aquatic plants lowers the risks of a summer fish kill.
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