With the ongoing drought, many ponds in Texas are nearly dry. Some are barely more than mud wallows. Even those with water levels down only a foot or two still need to focus on fish management and water oxygen levels.
Ponds dry out prematurely not only due to the drought, but also because of lack of sufficient clay content in the liner and a limited drainage area, according to Dr. Billy Higginbotham, Texas AgriLife Extension Service wildlife and fisheries specialist.
“You know it’s dry when your fish have ticks,” Higginbotham joked.
All joking aside, it’s a serious matter as water levels drop for the owners of the more than a million private water impoundments in Texas, Higginbotham said. 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, he 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, Higginbotham explained. 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,” he said. “Fish remain a primary concern to many owners of small ponds, and there are more than a million private impoundments found statewide.”
Obviously, Higgibotham said, when ponds are not much more than mud wallows, there’s little that can be done about saving fish.
“Certainly, when pond levels reach this point, I hope they have done something about fish populations already, hopefully harvested them,” he said.
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 added.
“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.
A thousand pounds of fish per surface acre of water is a good rule of thumb for a number of reasons.
“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,” Higginbotham said.
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, but when skies are overcast, photosynthesis is decreased and oxygen levels drop further.
“If the pond is too heavily stocked, fish can run into an oxygen debt,” he said. “Hot weather can even bring about oxygen debt in moderately stocked ponds.
“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 continued.
He said he has found that many pond owners over-estimate the surface acreage of their ponds by a factor of two or three.
There are simple methods to determine the size of a pond, he said. If the pond is more or less 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.
As an example, a relatively square pond measuring about 200 feet on each side will have 40,000 square feet of surface area, or about one acre.
Calculating a round pond’s size is a little more complicated. Divide the distance across the pond by two, square the result, then multiply that number by 3.14.
Once the approximate surface area is determined, the next step is to determine the pounds of fish in the pond, Higgibotham said.
“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, Higginbotham said.
“Visit the pond shortly after daybreak,” he 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, Higginbotham said.
“Aerate with a pump or boat motor just to get over the hump and through the immediate emergency in order to raise the oxygen levels, so you don’t lose an entire pond of fish.”