By Mike Isbell
University of Georgia

There were dead catfish out there all right. A big one floated near the overflow pipe at the dam. Jack, who had called me earlier that morning, had already removed several others he'd found floating in the water and tossed them out on the grass.

"What do you think is killing the fish?" Jack asked as we stood near the edge of the pond.

But before I told him what I suspected, I asked him a few questions. His answers gave me clues to what might have happened in the pond.

The first clue was that the dead fish were all large fish -- no small ones were dying. They had all died within the past two days. The sky had been overcast for several days. And it had rained a really cold, hard rain for a long time right before they began to die.

Forces at work

Just looking at that pond from the surface, it's hard to imagine the dynamic world below. But in this mysterious, watery world, biological, chemical and physical forces are at work.

We all know that fish require oxygen. Oxygen dissolves in water as it mixes at the surface, and it's produced in the water by photosynthesis of aquatic plants.

In almost every pond, oxygen levels will change daily. The highest level occurs in the mid to late afternoon and the lowest in the hours just before sunrise.

A healthy algae bloom and aquatic plant populations will produce enough dissolved oxygen to support life in a pond throughout a 24-hour period.

Layers of water

The coolest water in the pond, having the greatest density, remains in a layer near the bottom. The warmer water, being less dense, moves to the surface. Swim to the bottom and you'll feel the temperature difference.

Actually, there are three layers of water. Almost no oxygen is produced in the cold-water layers near the bottom, because there's no light for the aquatic plants.

If you've stayed with me so far, I'll tell you what I think happened in Jack's pond.

The overcast days, without the bright sunlight, resulted in less oxygen produced in the water. But fish don't die just because it's cloudy. We have cloudy days all the time. Something else happened.

The culprit

Remember, Jack said it rained really hard about two days earlier. And rain water is cold. The surface water, now suddenly cold and dense, begins to sink, which forces the warmer, bottom water to the surface. And the bottom water is low in oxygen.

Ponds can "turn over" during the summer following heavy rains. This "turnover" mixes the water and can cause the entire pond to become oxygen-starved. When that happens, fish will sometimes begin to die, and usually the largest ones die first.

This is what I suspect happened.

Luckily, Jack has a mechanical aerator in the pond to agitate the water and mix oxygen back in the water. Running it at night until the pond stabilizes will help reduce the severity of the "water turnover."

(Mike Isbell is the Heard County Extension Coordinator with the University of Georgia College of Agricultural and Environmental Sciences.)