OVERTON, Texas – “My fish are dying! What do I do?"
Texas Cooperative Extension agents throughout East Texas are hearing this lament from pond owners early this year. Fish kills in small ponds are most often caused by oxygen depletion, a condition usually occurring July and August, when hot, cloudy weather is the rule of the day.
Most often, oxygen depletion occurs when water levels are low during drought. This year, water levels are high from heavy spring rains, and pond owners are being caught unprepared, said Billy Higginbotham, Extension wildlife and fisheries specialist.
"Summer came on all at once, with cloudy weather and temperatures in the high 80s and low 90s. People with high fish populations are seeing early die-offs," Higginbotham said.
"Oxygen depletion problems account for about 85 percent of all fish die-offs in Texas farm ponds," he said.
Furthermore, catfish ponds are those most likely to be affected. This is because catfish ponds are more likely to be intensively managed and stocked with too many pounds of fish per surface area of water.
"Pond owners need to think in terms of pounds of fish; not in numbers. You can have a lot of small fish and not have oxygen depletion problems," he said.
Higginbotham recommends pond owners follow a simple set of steps to determine pounds of fish per surface area of pond.
"If the total pounds of fish exceeds 1,000 per surface acre -- that's only 100 pounds of fish in a tenth-acre pond – then your pond is a prime candidate to suffer an oxygen depletion problem before the summer is over," Higginbotham said. "Having a deep pond does not make up for a lack of surface area. In fact, it might hurt since deep ponds tend to stratify during early summer. The deeper levels may become devoid of oxygen."
The first step is to determine if your pond is at risk. Begin by estimating the surface area of a pond in acres. It's a process that sounds simple, but in practice Higginbotham has found most pond owners over-estimate the size of their ponds by two or three times. An acre consists of 43,560 square feet.
The next step is to determine the pounds of fish in the pond. Usually, the pond owner knows how many catfish were originally stocked in the pond and has a pretty good idea how many fish have been taken since stocking, Higginbotham said. By catching a few fish and weighing them, the pond owner can closely estimate the total pounds of fish currently in the pond.
Under sunny conditions and moderate temperatures, aquatic plants, mostly single-celled algae, produce enough oxygen to maintain oxygen levels in ponds. Wind, or the lack of it, is also a factor as it helps to aerate the water.
Summer conditions slow down these processes in a number of ways, Higginbotham said. Warm water holds less oxygen than cool water. As the water temperature rises, a fish, being a cold-blooded animal, experiences a rise in its metabolic rate, increasing its need for oxygen at the same time less is available. Cloudy days slow down photosynthesis, making even less oxygen available to the fish. If the pond is too heavily stocked, fish can run into oxygen debt even in cooler weather. Hot weather can even bring about oxygen debt in moderately stocked ponds.
Often the pond owner won't need to determine pounds of fish per acre to diagnose oxygen depletion. Oxygen-starved fish can be seen gasping at the surface or swimming weakly to the edge of the pond. Oxygen depletion will affect all sizes and species of fish to various degrees.
"But often the largest fish present are the first to be affected," Higginbotham said. Because photosynthesis shuts down during the night, fish showing symptoms of oxygen depletion will be most obvious during early and mid-morning hours. Pond owners who have a motor-equipped boat can easily and cheaply counteract oxygen depletion, Higginbotham said. Just back the trailer into shallow water and leave the motor running in gear until the fish recover. The submerged prop will stir water enough to increase oxygen levels. If no trailer is available, Higginbotham recommended, lodging the boat against a stump or in deep water against the bank.
Simply cruising around the pond in the boat won't help much, he said. Cruising means the prop is pushing the boat, not the water, resulting in considerably less oxygen absorption. Pumps can also be used to increase oxygen, but the intake should be set two or three feet beneath the surface.
Higginbotham cautioned that using boats and pumps to increase oxygen levels are only temporary solutions.
"If the real problem is too many fish present; it's time to go fishing and significantly lower the fish population," he said.
Of the estimated 1 million farm ponds in Texas, the majority are in East Texas, representing an estimated half million surface acres, according to Higginbotham. For recreational value alone, these ponds are conservatively estimated to have a value of $125 per surface acre annually.
"In addition, properly managed ponds can produce 1,000 pounds of catfish per acre annually," Higginbotham said.
Robert Burns is a writer for Texas A&M University.