Summer Thunderstorms and Fish Die-Offs Ponds


It is completely possible that your pond may have had a fish die-off recently as a result up the stormy weather that we have experienced. The most common cause of a massive fish die off in a pond is a lack of oxygen. The result is almost always a shoreline of dead fish.

One way this lack of oxygen can occur is from a pond “turning over”. A common way for ponds to turn over is for a thunderstorm to cause the waters to mix.

Each year, pond owners experience massive die-offs of their largest fish from a pond that has decreased in oxygen from a turn-over. With the weather that we have been experiencing over the past few days some pond owners may be close to experiencing this.

The most serious pond problems 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. All the oxygen is produced 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 used up in the lower, cooler layer by the biological and chemical activities that take place there.

When the sun heats pond water, typically about six to eight feet deep in Texas, the top layer becomes less dense. So, you have warm, less dense water sitting there, and the cool bottom layer becomes devoid of oxygen. A turnover event occurs when the upper layer of water cools down from the cool thunderstorm and the water with no oxygen mixes with the top layer.

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. Fish will naturally go to find better oxygen levels at the surface and will not be able to find it.

When a turn-over occurs, the surface of the pond often becomes streaked with gray, black or brown. The color of the water may eventually become totally brown, gray, black or even milky. A distinct foul smell may also be noticeable.

These turn-over events cause the most catastrophic fish kills in ponds. Fundamentally, there are two options: increase the oxygen level or reduce demand for oxygen by reducing the number of fish.

First, consider your stocking rate. 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 cook 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.

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. A bottom-style aerator is recommended because it mixes the entire water column so fish can utilize the entire pond. They cost a little more but are more reliable than surface aerators.

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 or, perhaps, purchase that beautiful fountain for which you have always been longing.