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In the recent weeks I have received numerous calls about fish dying in ponds. This sudden and seemingly inexplicable event usually leads people to suspect some sort of contaminant (ie chemical). However, this is very rarely the case. Most sudden fish kills from June to October are the result of oxygen depletion. Oxygen depletion can occur as the result of pond mismanagement (ie overstocking, over fertilization, overfeeding, etc) or from turnover. Most cases I have observed are the result of a turnover event to an already overstocked pond.

Ponds tend to stratify into three layers during the summer months (figure 1). The upper layer is the richest in oxygen because it contains algae which produce oxygen and is also open to influxes of oxygen from the atmosphere. The middle layer where the top and bottom layers mix/meet. The middle layer is relatively thin and is where a temperature change will occur between the upper and lower layers. The bottom layer is very cool, stagnant, and contains little oxygen.

l0233-01fig. 1

(image credit-G. Lewis, UGA)

Turnover occurs when these layers mix. The result is much lower oxygen levels throughout the pond. Turnover can occur as a result of  heavy rains, strong winds blowing for long periods in one direction, and rapid cooling in the fall.

A sure sign of an oxygen depletion problem is the presence of fish gulping air at the surface of the pond early in the morning when oxygen levels are at their lowest. Aeration is the only way to prevent or reduce fish deaths from oxygen depletion. Starting aeration once fish death is noticed will reduce the number of subsequent deaths. However, if you have the ability to aerate, it is best to begin when the following conditions exist in order to prevent an oxygen depletion problem.

  • After a heavy rain;
  • During periods of strong winds;
  • During periods of calm, cloudy days;
  • During the fall when air temperatures are rapidly cooling;
  • After chemical treatment of aquatic weeds.

 

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