Oxygen Depletion

Oxygen depletion occurs when too much vegetation dies at one time and the bacteria that are breaking down the dead vegetation take the dissolved oxygen out of the water and suffocate your fish. A fish kill from oxygen depletion can happen in a number of different ways.

Winter Kill is a condition where your pond or lake has excessive vegetation going into the winter, the ice forms and you get a heavy snow cover for an extended period of time blocking out the light, the vegetation dies and the bacteria takes the oxygen out of the water and the fish die.

Summer Kill is a condition where all the vegetation dies during the summer, this condition is not as common as winter kill, but can happen. During the warmer months of the summer the pond or lake water is much warmer, the warmer the water is, the less dissolved oxygen it can hold, so when the vegetation dies during the summer there is a good chance that oxygen depletion can occur.

When chemically treating aquatic vegetation oxygen depletion is the major concern, if the chemicals used are registered for aquatic use and are used at the proper application rate, the chance of killing fish due to chemical toxicity is very slim. On the other hand if you kill to much vegetation at one time the chance for a fish kill due to oxygen depletion is very likely.

There are a number of ways to avoid oxygen depletion, if you are chemically treating and the pond or lake has excessive weeds or algae, treat one half of the pond, wait 7 - 10 days and then treat the other half, this will give the half that has been treated time to recover from its loss of oxygen before treating the other half. Most fish kills due to oxygen depletion come from treating algae, since it is only recommended that weeds be removed from areas you wish to use for swimming or fishing, partial treatments on weeds seldom cause oxygen problems. If you are treating either filamentous or planktonic where it is necessary to do a total algae treatment, this is where you can run into trouble with oxygen depletion..

Aeration is very effective in eliminating oxygen depletion problems. See our section on Aeration.

Another good way to avoid oxygen depletion is to treat your algae before it gets to out of hand.

....