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Dissolved Oxygen for Fish Production 1
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Preventing Oxygen Depletion
An oxygen depletion event can be predicted and, therefore, prevented by monitoring dissolved oxygen levels in a pond. The most efficient tool for measuring DO is an electronic oxygen meter. These instruments are available through most aquaculture supply companies at a variety of prices. Chemical test kits are also available. These are more troublesome to run, but are accurate and do not require as great an investment by pond owners.

Commercial catfish farms often hire night oxygen crews to monitor the DO concentration in each pond at two-hour intervals through the night. This is the surest way of avoiding a fish kill caused by low DO. Aeration systems can be turned on if oxygen levels drop below a certain concentration (usually 2-4 mg/L) depending on the fish species.

Monitoring oxygen throughout the night is impractical for recreational pond owners and part-time fish farmers. For these people it is easier to "predict" an oxygen depletion by measuring DO levels in the late afternoon (5-6 p.m.) and late evening (8-10 p.m.). The decline in DO during the night can be predicted by graphing DO concentration against time on standard graph paper ( Figure 2 ). If the projected concentration of DO is below 4 mg/L before 7 a.m. emergency aeration is recommended.


Figure 2: Estimation of potential for dissolved oxygen depletion.



If equipment to test DO concentration (meter or test kit) is not available, the following observations and conditions can be used to anticipate oxygen depletion:

Fish swim at or near the surface gulping air (piping).

Fish suddenly stop feeding.

There is a rapid change in water color to brown, black or gray, signifying loss of an algal bloom.

A putrid odor arises from the water.

There has been an extended period of hot cloudy weather.

There is a heavy summer wind and a rainstorm.

Emergency aeration should be applied whenever fish show signs of oxygen depletion or when dissolved oxygen drops below 4 mg/L.

Many recreational pond owners purchase aerators and place them on electric timers. Proper use of the timer should have the aerator turn on during the late evening (10 p.m. to midnight) and turn off after daylight (7-8 a.m.). Using an aerator is not a complete substitute for monitoring DO concentrations and an oxygen depletion event resulting in a fish kill may still occur. However, use of an aerator is recommended and will prevent many problems.


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