Substantially increasing oxygen levels in catfish ponds may ease pond management, but it won't necessarily increase production. A two-year study by researchers at the National Warmwater Aquaculture Center in Stoneville, Miss., found no benefit to maintaining a dissolved-oxygen concentration above 2.5 parts per million. When oxygen levels decrease below 1.5 parts per million, however, production-related problems can begin.
“Increased aeration will allow us to maintain a higher minimum dissolved-oxygen concentration, which in turn will increase voluntary feed consumption and growth, allowing us produce food fish from fingerlings in as little as one season,” says Les Torrans, with the USDA Agricultural Research Service's Catfish Genetics Research Unit in Stoneville, Miss.
“Fish in the study's high-oxygen treatment consumed 37,296 pounds of feed per acre, compared to 20,482 pounds per acre in the low-oxygen treatment. This greater food consumption can mean the difference between fingerlings reaching market size in one season, instead of having sub-marketable fish at the end of the growing season.”
When the catfish industry was still in its infancy, the general rule of thumb was to limit feeding rates to no more than 30 pounds per acre per day. At that maximum feeding rate, oxygen provided by wind action and photosynthesis of the algal bloom was normally sufficient to meet the oxygen demand of the fish, the bloom and the sediment, without a need for aeration.
It was rare for the dissolved-oxygen concentration in a pond to fall to unsafe levels. However, that was also when catfish production yielded little more than 1,000 pounds of fish per acre, and daily per-acre feeding rates stayed below 30 pounds.
As production and feeding rates have increased, so too has the amount of oxygen needed to sustain the fish swimming in those ponds.
Today, most commercial catfish operations use permanently-installed electric paddlewheel aerators and maintain additional tractor-powered units for emergency use. Feeding rates have also increased dramatically, and producers routinely feed 100 to 120 pounds of fish feed per acre per day to produce an upwards of 4,000 pounds of catfish per acre.
“Due to the high total pond oxygen demand resulting from our high feeding rates, dissolved-oxygen concentration often falls below one part per million by morning, no matter when aeration is started. The primarily goal of most managers is to keep the fish alive through the night and minimize visible stress as much as possible,” Torrans says. “Our assumption has always been that if catfish aren't ‘up’ or crowded around the paddlewheels, they aren't stressed.”
To determine the truth of that statement, Torrans equipped six quarter-acre ponds each with three one-half-horsepower paddlewheel aerators, and one one-half-horsepower circulator.
“The large aeration capacity allowed us to maintain the dissolved oxygen at predetermined concentrations, and presumably allowed us to stock and feed at rates much higher than the industry standard. This could provide insight into what other water quality parameters might degrade if industry feeding rates continue to increase,” he says.
A commercial system was used to control aeration and to continuously monitor and record dissolved-oxygen concentration, aerator status and water temperature. Catfish in the study were fed a commercial floating feed daily to apparent satiation, and well water was added only to compensate for evaporation and seepage. Ponds were harvested at the end of each growing season.
Aeration began in the high-oxygen control treatment when the dissolved-oxygen concentration dropped below 5 parts per million. In the low-oxygen treatment, aeration began when the concentration dropped below 2.5 parts per million in the 2001 study, and 1.5 parts per million in the 2002 study. The second and third aerators in each pond started if the dissolved-oxygen concentration continued to fall, according to Torrans.
“Allowing the dissolved-oxygen concentration to decrease below 2.5 parts per million before beginning aeration had little impact. Feed consumption was 6.3 percent less than the high-oxygen treatment, but the net production in the two treatments was similar,” he says.
“However, there is a big difference between maintaining the minimum daily dissolved-oxygen concentration near 2.5 parts per million, as was done in this study, and maintaining the dissolved oxygen at a constant 2.5 parts per million. If the oxygen concentration was not temporary and did not increase after sunrise due to photosynthesis, a constant oxygen concentration at that level would likely have had a major negative impact on the fish.”
The study found that delaying aeration until the dissolved-oxygen concentration dropped to 1.5 parts per million did have a major impact on production. “Although visible signs of oxygen stress were never observed in the low oxygen treatment, feed consumption was reduced by 45 percent, and average fish weight in the low oxygen treatment was 30 percent less than the control. Net production was also cut in half,” he says.
Torrans adds, “It appears that at least in our small research ponds, feed can be increased in proportion to available aeration far above levels currently used in the industry.”
Since dissolved-oxygen concentrations vary greatly within a pond, Torrans says it is possible to measure adequate oxygen levels at one end of the pond and have fish belly-up at the other end. To avoid this problem, he recommends starting one aerator at a higher dissolved-oxygen concentration, which provides a margin of safety and mixes the pond. “Fish will orient to the current established by the first aerator and can easily locate and approach the source of aeration and the zone of best water quality as oxygen levels decrease further and other aerators are turned on,” he says.
Torrans cautions, “Results from these small research ponds should not be extrapolated directly to large commercial ponds that have very different mixing characteristics. However, it does appear that both feeding and production rates in large ponds can still be increased above current levels with further increases in aeration.”