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My pond fish are dying so what can I do about it?

By Randy Reeves
June 8, 2010 at 2:13 a.m.


The Extension Office here in Harrison County has been hearing this lament from pond owners early this year. Fish kills in small ponds are most often caused by oxygen depletion, a condition which usually occurs during July and August, when hot, cloudy weather is the rule of the day.

Farm ponds started out the year with high water levels. With the lack of normal rainfall amounts, many local ponds and lakes are starting to see water levels drop which will only serve to aggravate oxygen depletions problems.

"Then summer came on all at once, with cloudy weather and temperatures in the high 80s and low 90s. People whose pond conditions and stocking levels were on the edge are now seeing die-offs early," said Dr. Billy Higginbotham, wildlife and fisheries specialist with the Texas Agricultural Extension Service.

"Oxygen depletion problems account for about 85 percent of all fish die-offs in Texas farm ponds," Higginbotham said. Ninety-nine percent of the time, this problem is due to ponds being over-stocked.

Furthermore, catfish ponds are those most likely to suffer because they are more likely to be intensively managed and stocked with high numbers of fish. Higginbotham recommended 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 one-tenth-acre pond - then your pond is a prime candidate to suffer an oxygen depletion problem before the summer is over," he said.

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 which sounds simple but in practice, Higginbotham has found, most pond owners over-estimate the size of their ponds by two or three times what it is.

If the pond is rectangular, the simplest way to determine its size is to measure the length and width in feet, then multiply one measurement by the other to get surface area in square feet. Divide this number by 44,000 to get the approximate area in acres. A pond 200 feet wide by 200 feet will have 40,000 square feet of surface area, or about one acre.

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. By catching a few fish and weighing them, an owner can estimate the total pounds of fish in the pond.

Under sunny conditions and moderate temperatures, aquatic plants, mostly single-celled algae, produce enough oxygen as a byproduct of photosynthesis 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. 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 oxygen 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. Normally, this time of year, May temperatures are around the mid 80's for a high, this year, we have already seen temperatures in the mid 90's.

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. 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 move enough water about to cause oxygen levels to increase. If a pond owner doesn't have a trailer, Higginbotham suggested, lodge the boat against a stump or in deep water against the bank.

Just cruising around the pond in the boat won't help much, Higginbotham 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 within two or three feet below the pond 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.

An estimated 840,000 small ponds pepper the Texas landscape. East Texas has about 300,000 ponds, 80 percent of which have been stocked with fish.

For further information on Extension programs, call us at 903-935-8413, or visit us on the web at http://harrison-tx.tamu.edu.

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