Dissolved Oxygen for Fish Production 1
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High Water Temperature
Warm water is much less capable of holding oxygen gas in solution than cool water. For example, water that is 90 F can only hold 7.4 mg/L DO at saturation, whereas water that is 45 F can hold 11.9 mg/L DO at saturation. This physical phenomenon puts the fish in double jeopardy because at high water temperatures their metabolic rates increase, hence their physiologic demand for oxygen increases.

Cloudy, Still Weather

Muggy, overcast summer days often precipitate oxygen depletions. During cloudy weather, the intensity of light reaching surface waters is greatly diminished, resulting in a marked decrease in oxygen production from photosynthesis. Oxygen consumption, however, remains unchanged. This results in a net loss of oxygen over each 24-hour period. This loss of oxygen from decreased production is confounded by still, muggy, humid weather common on overcast summer days. Oxygen transfer (from the atmosphere into the water) is minimal because there is little or no wind/wave action. The net result over a period of several days is oxygen depletion and, often, fish kills.

Stratification/Pond Turnover
During hot weather, surface waters warm up more rapidly than deeper waters. As the difference in temperature increases between warm surface water and cool bottom water, a thermocline develops. A thermocline is an area of rapid temperature change that acts as a physical barrier between warm water at the surface (epilimnion) and cold water at the bottom (hypolimnion). When a thermocline is present there is no mixing of surface and deep layers of water. Because photosynthesis and oxygen production only occur near the surface, water in the deep layer becomes devoid of oxygen and develops an oxygen demand. The thermocline can be broken by heavy wind and cold rain, common during summer thunderstorms. When the thermocline breaks down, the oxygen-rich surface waters mix with oxygen-deficient bottom waters. If the oxygen demand is sufficient, all DO present will rapidly be removed from the water column, resulting in severe oxygen depletion and a fish kill.

How to Determine If Low DO Is the Cause of a Fish Kill
All fish die at approximately the same time (often during the night or in the pre-dawn hours).

Large fish may be affected more than small fish.

Moribund fish may be seen at the surface "gasping" for oxygen (this is called "piping").

Some species may die with their back arched, gills flared and mouth open. This is most commonly seen in hybrid striped bass and, occasionally, in catfish.

The weather immediately prior to the fish kill may have been hot, still and overcast. A severe thunderstorm may have occurred immediately prior to the fish kill.

An oxygen depletion event severe enough to result in significant fish mortality is often observed in water with heavy populations of algae or aquatic plants.

What To Do if Low DO is Suspected as the Cause of a Fish Kill
The most important thing to do if fish are dying from low DO is to turn on an aerator. If emergency aeration is not available, little can be done to help the fish. To confirm the problem, oxygen levels should be tested while the fish kill is in progress. Some county extension agents are equipped with water testing equipment. In addition, biologists with the Florida Game and Freshwater Fish Commission or an IFAS Aquaculture Extension Specialist may be available to assist.

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