Oxygen Depletion Causing Fish Kills


A grower called me to look at a pond in the middle of a cotton field where fish had been dead a few days. There are a mixture of bass and bream that have died. I asked which fish died first, and he said the larger fish. Fish kills can often be associated with chemical runoff or oxygen depletion. When larger fish die first, it is generally the result of some kind of oxygen depletion, since lack of oxygen would affect larger fish first. UGA Extension Aquaculture Specialist Dr. Gary Burtle said there have been several fish kills related to low oxygen in south Georgia in the past week. With rain comes cloudy weather and low oxygen has been a problem recently. Below is what Dr. Burtle has to say:

FishKill (3)“The usual reason for fish death after rains is a partial oxygen depletion caused by the influx of water which has no algae in it or a high concentration of bacteria in it.  When algae are replaced by bacteria, the water turns brown in color and oxygen concentration drops. Bass are larger than bream, so may be affected first. The oxygen stress may cause the fish to become susceptible to disease, so fish deaths may occur for several days after the partial oxygen depletion.

When the pond begins to ‘green up’ again, oxygen concentrations will increase and fish deaths stop. Until oxygen concentration returns to normal levels, low oxygen each morning or during cloudy weather may cause more fish stress and death. The solution is to install an aerator and operate during cloudy weather, which seems to be more prevalent in Georgia in the late summer and fall.”

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