OVERTON – Despite recent rains in East Texas, as far as pond fish are concerned, it's still a drought, according to a Texas Cooperative Extension expert.
Farm pond levels remain low, and when water levels recede, fish become stressed and may die, said Dr. Billy Higginbotham, Extension fisheries and wildlife specialist here.
Fish stress and die-offs in small ponds are often caused by oxygen depletion, a condition that occurs when hot, cloudy weather is the rule of the day. Most years that occurs in July and August.
"Oxygen depletion problems account for about 85 percent of all fish die-offs in Texas farm ponds," Higginbotham said.
There's more at stake than a lot of uncomfortable fish. Texas has an estimated 1 million farm ponds, most of which are in the eastern part of the state. Those million ponds represent 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, he said.
Under sunny conditions and moderate temperatures, aquatic plants, mostly single-celled algae, produce enough oxygen to maintain oxygen levels in ponds.
Summer conditions can upset this balance of oxygen production and use in a number of ways, Higginbotham said. First, warm water holds less oxygen than cool water. And as water temperatures climb, fish, being cold-blooded animals, experience a rise in their metabolic rates, which increases their need for oxygen.
At the same time, less oxygen may become available. Cloudy days slow down photosynthesis – a process dependent upon solar energy – which results in aquatic plants making less oxygen available to the fish. Also, as the temperature of water rises, it absorbs less oxygen. Extremely hot weather can even cause oxygen debt in moderately to highly stocked ponds. Add drought and its concomitant low water levels, and fish in even under-stocked ponds can be at risk, Higginbotham said.
After a drought, more than just a few good rains are needed to bring pond levels back to normal. Heavy rains for several days are needed to significantly raise water levels, he said.
And this just hasn't happened, according to the weather records kept at the Texas A&M University System Agricultural Research and Extension Center at Overton. The center has collected weather records with automated weather stations since 1968.
July was the seventh-wettest month in nearly 40 years, said Indre Pemberton, research associate with the Texas Agricultural Experiment Station. Historically, rainfall for July has averaged 2.63 inches; the total rainfall from January through July has averaged 27.39 inches.
This July, rainfall averaged 4.11 inches; but the total for the year so far is 20.97 inches. "So we are 6.42 inches behind for the year," she said.
As fish die-offs happen in even in wet years, Higginbotham recommends pond owners begin to address such problems by determining the stocking rate.
Begin by estimating the surface area of a pond in acres. 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.
Next, determine the pounds of fish in the pond. If the pond owner knows how many fish were originally stocked in the pond, he or she may have a 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.
"Once the surface area in acres is calculated, make sure that you are not carrying in excess of 1,000 pounds of fish per surface acre in the summer months," he said. "This fish density is seldom achieved except for heavily stocked catfish ponds that are supplementally fed on a regular basis. For a one-quarter surface acre pond, that's only 250 pounds of fish total. This poundage should be figured on the basis of the pond's surface area during its annual low water level, not when it is brimming full in the winter or spring."
Drought or not, a pond owner doesn'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. Oxygen depletion affects all sizes and species of fish to differing degrees, he said.
"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, he recommends 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, because 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 2 or 3 feet beneath the surface.
However, Higginbotham cautioned, 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 – or pray for rain," he said.