Humans and their domestic pets aren't alone in being at risk from triple-digit summer heat, according a Texas Cooperative Extension wildlife and fisheries specialist.
"When summer temperatures climb, I start getting calls saying 'my fish are dying," said Dr. Billy Higginbotham.
July was one of the wettest months on record, and most of the estimated 1 million farm ponds in Texas are full to overflowing, he said.
But though high water levels lessen the risk, they're no guarantee against large-scale fish die-offs from oxygen depletion this time of year, Higginbotham said.
However, pond owners can take preventive measures if they are aware of the danger of oxygen depletion and understand why it happens, he said.
Though fish live in water, they still need oxygen to live. They just take the oxygen from the water with gills instead of getting it from the air with lungs as other animals do. The oxygen in water comes from two main sources, Higginbotham said.
Aquatic plants, mostly single-celled algae, produce enough oxygen as a byproduct of photosynthesis to maintain oxygen levels in ponds. Wind also helps to aerate the water with oxygen from the atmosphere.
Hot weather throws these natural processes out of whack in two ways, he said. First, warm water holds less oxygen than cool water. Secondly, because fish are cold-blooded animals, their metabolic rate rises with warmer water temperatures. Just as a car running at higher speeds uses more gasoline, a fish's metabolism that's revved up by the heat calls for more oxygen.
"So hot weather means the fish need more oxygen at the same time less oxygen may be available," Higginbotham said.
And, if cloud cover accompanies the hot weather, the problem is worsened. With less sunlight, photosynthesis is slowed and aquatic plants produce less oxygen.
As long as the stocking rates of fish are relatively low and pond volumes are up, pond owners may not see fish die-offs.
But if ponds are home to too many pounds of fish, then it's almost a sure bet there will be die-offs, he said.
"Because catfish ponds are most likely to be over-stocked and intensively managed, that's where we see the most fish die-offs from oxygen depletion in hot weather, "Higginbotham said. "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 recommends simple steps to determine pounds of fish per surface area of pond. Begin by estimating the pond's surface area in acres. 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 these numbers to get surface area in square feet. Divide this number by 44,000, which is roughly the number of square feet in an acre. For example, a pond 200-feet wide by 200-feet long will have 40,000 square feet of surface area, or about 1 acre.
Next, determine the pounds of fish in the pond. One way to do this is by knowing how many fish were originally stocked, and how many fish have been taken since stocking. Another way is to catch a few fish and weigh them.
"Take the difference between the number stocked and removed, and multiply by the average weight of the fish caught to estimate the poundage," he said.
More often, the pond owner won't need to do anything to diagnose oxygen depletion. Oxygen-starved fish will be seen gasping at the surface or swimming weakly to the edge of the pond. Oxygen depletion affects the larger fish first, Higginbotham said.
Because photosynthesis shuts down during the night, oxygen levels will lowest at daybreak. Gasping fish will first be noticed in the early to mid-morning hours, he said.
By the time that happens, it's too late to do much about reducing the stocking rate, but pond owners who have a boat with an outboard motor can easily remedy the problem. But just cruising around in the pond won't do much good. Much better results will be realized, he said, by backing the boat trailer into shallow water until the outboard's propeller is submerged. Then leave the motor running in gear, churning the water, until the fish recover.
If they don't have a boat trailer, Higginbotham recommended lodging the boat against a stump or the bank over deep water.
Pumps can also be used to increase oxygen, but their intakes should be set within 2 or 3 feet below the pond surface.
"Pulling water from the near pond bottom only compounds the problem because that water layer is already low in oxygen," Higginbotham said.
Using either a boat or a pump is only a temporary solution, he said.
"If the real problem is too many fish present, it's time to go fishing and significantly lower the fish population," he said.