Unexplained Fish Kill in Pond

Thank you Fishead and Alice,, Will DO!

Plow jockey is right. It sounds like you had a classic low oxygen level fish kill. That is why the big fish are dieing and the little fish seem fine. They do not need as much oxygen. And ther is something you can do to prevent it. It is call an areator. There are many different reasons why you could of had a low oxygen level. Algea bloom, more and bigger fish, too many fish, and pond turn over. An areator can help everyone of these problems. Pond turn over often happens during the dead of summer. The water on the top heats up and the water on the bottom gets cold and becomes oxygen deficient (sp). A couple of things can cause it to turn over a hard rain introduces cold water to the top of the water which can push this warm water down mixing with the cold oxygen deficient water on the bottom. This uses up a lot of the oxygen and causes a fish kill.

Another thing that could of happend. Is as a pond ages the fish get bigger and you get more of them. This puts more nuetrients into the pond which causes the algea to bloom more. a 1/3 of an acre pond can only hold so many fish once you get to that point the pond will correct itself with a fish kill. An areator will help both of these situations. But, you can still only have so many fish in a pond even with areation. Areation will help pond turn over by keeping all the water oxygenated. Keep the cold wate on the bottom moving that way it will not become oxygen depleted.

Sorry for the long post and I hope it makes sense to you. If you have any questions send me a pm or go to pondboss.com. They can explain it better than I can.

Scoop the duckweed up with a pool rake (small caliber net) and add around anything that needs mulching.
That's what I've been doing with the lake algae for many years now.

Grass carp should control the duckweed if there enough of them.

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As an Ag Environmental Sp. I have investigated many fish kills and from your description I am certain that low Dissolved Oxygen is the problem. The largest fish usually die first and you seeing them lathargic in the shallows is another indicator. It is not uncommon to see them at the surface "piping" at such times of distress. Why is a little more difficult to say. If an algea bloom was to blame it is usually indicated by a few days of blue-green or green water color followed by a change to coffee colored water as the algea die off. The decaying algea tie up the available oxygen. An actual "turnover" in a small body of water is unlikely. In any body of water there are layers or strata within the water seperated by temperature. Different layers have different DO levels and in large bodies there is sometimes a lot of dead water (low DO). When the conditions, such as a sustained wind over a period of days, cause the water to turn over it mixes the strata together dilluting the available DO and stirs up organic matter from the bottom. The organic matter in suspention as well as increased possibility for an algea bloom can cause significant die off from low DO. Your problem is more probably a water quality problem or overcrowding issue. I would recommend seeking assistance from the Cooperative exstention Specialist. For short term relief mechanical agitation will give you some help. Even backing a bushhog just far to stir the water will help

I think you nailed it, Bryan! They are "piping" at the surface, Been running the gas powered pump every afternoon to help oxygenate. Going to get a pool net to scoop leftover duckweed which is now only about 1/5th what it was.. no more fish have died. aereator still in the works. Thank you all for your input..

The smaller the pond, the worse the problem.

One of my uncles raised fish for a living. For about ten years he had a fish out pond... he built a rig to put on the back of his tractor... jury rigged from a bush-hog, that aerated the water.

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