During late summer or early fall, there are always a few farm ponds that experience fish kills. Most fish kills are due to either an algae die-off or a physical process called thermal de-stratification 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 stresses of low dissolved oxygen (DO). Ponds significantly vary in nutrient levels, depths, size, fish populations, plant diversity and many other physical features.
This is why one person may have a fish kill while a neighboring pond owner may not have any problems. DO is the single most important chemical characteristic of a farm ponds.
Several factors can cause low DO. Green 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 30-36 inches vertically into the water.
When there are enough nutrients present in the water, "algae blooms" occur presenting 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 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 with unique density characteristics like having a 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 does not occur due to 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 (respiration) 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.
There is now way to prevent a pond turnover. Minimizing the addition of nutrients and controlling excessive plant growth in the spring is the best prevention of DO problems. Aerators do very little to truly add DO to pond water. The windmill
aerators are a very poor investment as wind velocities sufficient to turn windmills during the dog days of summer when ponds are most at risk are rare. When fish kills do occur, it is not necessary to remove all dead fish as in the end there is no addition of organic material being added since the fish were already in the pond. If one desires they may wish to remove dead fish to hasten the minimization of the unsightly mess and odor while using care to not cause pollution or other health problems. Rarely are all fish killed within the pond and the larger fish requiring more oxygen are typically first or most likely to die. After a fish kill, a new balance of fish species and growth characteristics will be established.
You can contact the local Purdue Extension Office by calling 829-5020 Ext. 14 in Owen County or 448-9041 in Clay County for more information or publication copies regarding this week's column topic or to RSVP for upcoming events. It is always best to call first to assure items are ready when you arrive and to RSVP for programs.
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Sept. 26 Pen Space Request Due for IBEP winter evaluation
Sept. 27 Indiana Goat Assoc. Member Meeting, Quincy, 10 a.m.
Oct. 8-9 Indiana Flower Growers Assoc. Annual Conference
Oct. 9 Indiana Cooperative Summit, Danville, 8:30 a.m.
Oct. 11 Adventures in Gardening, Danville 8:30 a.m.
Oct. 14 Ribeye Blood Drive, Spencer, 3-6 p.m.
Oct. 18 IBEP Bull & Bred Heifer Sale, Springville, 2 p.m.