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Drought puts heat on ponds
By Robert Burns
Texas AgriLife Extension Service
OVERTON -- “You know it’s dry when your fish have ticks,” jokes Dr. Billy Higginbotham, Texas AgriLife Extension wildlife and fisheries specialist. But all jokes aside, it’s a serious matter as water levels drop for the owners of the more than a million private water impoundments in Texas. Channel catfish, bluegill, and largemouth bass must survive both a shrinking habitat and dropping water-oxygen levels.
Dropping oxygen levels can be a problem during a hot Texas summer even when there’s normal rainfall. But there’s nothing like a drought to highlight a poorly constructed pond and magnify the potential for fish kills, Higginbotham said.
“It’s hot and dry even by Texas standards, but the ponds that show the problem first and foremost are those that have either very small watersheds or those ponds that were built on marginal soil,” he said.
A small watershed means there is not a large enough area surrounding the pond for sufficient runoff to maintain water levels, even during years with average rainfall. To hold that runoff, the soil the pond is situated in should contain enough clay.
“These are important construction concepts for landowners to remember whether they are watering livestock, or if fish are an important recreation use of that pond,” Higginbotham said. “Fish remain a primary concern to many owners of small ponds.”
But even if a pond is well-constructed and its water level has only dropped a foot or two, it still behooves pond owners to pay attention to fish management and water oxygen levels, Higginbotham said.
“In any case, we want to avoid trying to carry more than a thousand pounds of fish per surface acre during the warm months,” he said. “As water temperatures increase, the ability of that water to hold oxygen decreases, so typically, the hot summer months are when oxygen depletions are most likely to occur.”
Also, as water temperature rises, the metabolic rate of fish, being cold-blooded animals, will increase, and with that increase comes a need for more oxygen. But under sunny conditions and moderate temperatures, aquatic plants -- mostly single-celled algae -- will produce enough oxygen to somewhat offset the low oxygen concentration levels of warm water.
Cloudy days have been rare during the 2011 drought, Higginbotham noted, but when skies are overcast, photosynthesis is decreased and oxygen levels drop further.
“Furthermore, small ponds that are intensively managed for high-standing crops of catfish at or above 1,000 pounds per surface acre are going to be among the first ponds to experience oxygen shortages as water levels drop,” Higginbotham said.
He added that he has found that many pond owners over-estimate the surface acreage of their ponds by a factor of two or three.
Once the approximate surface area is determined, 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 have been removed since stocking,” he said. “By catching a few fish and weighing them, an owner can estimate the total pounds of fish in the pond.”
Pond owners who suspect low oxygen concentrations should monitor their ponds closely, even if their stocking levels are 1,000 pounds per acre or less.
“Visit the pond shortly after daybreak,” Higginbotham said. “If fish are crowded up at the surface at first light, that’s a pretty good indication that you’ve got low-oxygen levels.”
If this is the case, pond owners should either immediately harvest fish to reduce the stocking level or aerate that water. Aerating the water with a pump or boat motor will help a pond owner “get over the hump.”
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