I have had no less than three inquiries this week about massive fish die-offs in local ponds. Pond owners have been alarmed by a shore lined with dead fish. There are always questions about fish diseases or potential toxins, but the cause is a lack of oxygen resulting, oddly, from our recent cool fronts and resulting rain. 

These fish die-offs are the consequence of a pond “turning over”. Each year, pond owners experience massive die-offs of their largest fish from a pond that has decreased oxygen from a turnover.

A common way for ponds to turn over is for a thunderstorm to cause the waters to mix. A turnover event occurs when the upper layer of water cools down from a cold front. A cool snap or a thunderstorm with wind and hard rain can cool the warm surface water making it heavy enough to sink and mix with the oxygen-deficient bottom layer. The net result is a dilution of the oxygen and an increase in the demand for oxygen from decaying organic matter stirred up from the bottom. 

The most serious pond problem during a turn-over is oxygen depletion. Here’s how many of these turn-overs happen. During hot summer weather, surface water becomes less dense as it absorbs heat and floats over a cooler, denser layer of water that is very low in oxygen. 

All the oxygen in a typical pond is produced by aquatic plants, algae, rain splashing, or wind action is in the upper, warmer layer.  The two layers may not mix for weeks at a time, especially in deep ponds. Meanwhile, all the oxygen is depleted in the lower, cooler layer by the biological and chemical activities that take place there as a result of decomposition.

When a turn-over occurs, the surface of the pond often becomes streaked with gray, black, or brown colors. The color of the water may eventually become totally brown, gray, black, or even milky. A distinct foul smell from the rotting organic matter (that was previously on the bottom) may also be noticeable. 

Using surface aerators can help resolve low-oxygen situations. An aerator like the ones used on catfish farms can provide a refuge. In deep ponds, it will not aerate the whole pond or de-stratify the pond’s temperature and oxygen layering, but it will save fish. Fountains rarely will provide enough aeration of surface waters to help a severe depletion.

These turnover events cause the most catastrophic fish kills in ponds. Fundamentally, there are two options: increase the oxygen level or reduce the demand for oxygen by reducing the number of fish. A rule of thumb for ponds is to have no more than 1,000 lbs. of fish per surface acre. Most ponds in our part of the world are much less than one acre.  If yours is only a quarter acre, then keeping more than 250 lbs. (following the rule) is risking massive losses.  

Just how many fish does one have in their pond can be difficult to tell. If you have recently stocked your pond with fish, you may be able to determine the total pounds of fish in a pond. However, on older ponds where fish have been stocked for years, perhaps decades, the natural reproduction of fish populations could make this determination nearly impossible.  Unlike a pasture with livestock grazing, we simply cannot visually determine the stocking rate. 

One can reduce the fish numbers by… just going fishing and cooking what you catch!  Do not toss back anything. If it is too small to eat, throw it over the pond dam and for other wildlife to eat.  By doing this, you will lower the fish population and lessen the level of oxygen needed to keep everything alive. Don’t think of catching 10-20 fish. Think more about removing at least a hundred pounds worth to make a significant impact. 

The other practice to consider is the installation of an aerator that can be run at night and during extended periods of cloudy days. An aerator is good insurance against fish kills and is a piece of equipment that commercial fish producers need to have. That area of oxygenated water near the aerator will help the fish survive until the pond recovers from a low oxygen period. Fountains and other attractive options are used by folks who have no intentions of raising fish commercially but want to add to the beauty of their pond. 

Choosing the right type of aerator is important. In an urgent situation where the pond has been turned over, a surface aerator needs to be used to correct the problem. The attractive fountains can be of help but don’t do enough in severe oxygen depletion situations.  A bottom-style ‘bubbling’ aerator is recommended at the start of the year to keep the entire water column mixed so fish can utilize the entire pond throughout the year.

Finally, check your ponds early in the morning as oxygen levels will be at their lowest at sunrise. If you notice slow, sluggish fish swimming at the surface, it is time to go fishing and remove the excess demand.