Dissolved oxygen management for aquaculture ponds
November 23, 2015
By John Nickum
One of the fundamental ironies in aquaculture is the perception by many observers that pond systems are relatively primitive and simple; certainly not as advanced as tank and raceway systems. Perhaps this perception is founded in the historical development of aquaculture; pond systems preceded tanks and raceways by centuries. Also, tanks and raceways require sophisticated pumping and electrical systems, as well as formulated feeds specifically designed for the nutritional needs of each species. The perception that all one needs to do to rear fish in ponds is to dig some ponds, fill them with water, add fish, and provide supplemental feeding is very misguided. Such a perception is wrong.
Although pond management for fish production is often a matter of intuition and artful maneuvering, it must be based on science… science that evolves continuously… if the farm is to survive in an intensely competitive marketplace. Really effective pond management for fish production requires managing a pond ecosystem; as well as, the fish stocked in it. Drs. Claude Boyd and Craig Tucker have written several books dating back to the late 1970s that discuss pond water quality and the problems in managing it for fish production. Their writings provide a foundation for new studies at the research centers in Mississippi, Arkansas, and Alabama.
Dr. Eugene (Les) Torrans, a scientist at the Warmwater Aquaculture Research Unit (WARU) in Stoneville, Mississippi, has rightfully earned the title “The Pond Fish Doctor”. Les has been rearing fish in ponds and conducting scientific research on the factors that promote efficient, economical production for more than 25 years. In addition to studies in their research ponds, he and his colleagues, Brian Ott and Brian Bosworth take their findings and new strategies out to commercial farm production ponds across southern United States for collaborative testing. Increased feed prices and competition from imported catfish products force US catfish farmers to demand farm tested results before adopting new ideas. More rapid growth and increased feed efficiency can be the difference between profitable catfish farming and returning the land to soybeans. Implementing the recommendations of Les and his colleagues has enabled one farmer to triple his production per acre and produce a marketable product in one year instead of two.
Several factors are involved with these dramatic improvements, but the primary factors appear to be changes in pond design, greater use of channel catfish-blue catfish hybrids (♀ channel catfish Ictalurus punctatus X ♂ blue catfish I. furcatus), and, new techniques for managing dissolved oxygen concentrations. Split-pond designs have proven to be effective and efficient, while eliminating some of the pond management problems inherent in large ponds. Hybrid catfish provide the faster growth rates typical of channel catfish, while retaining the greater tolerance for lower dissolved oxygen concentrations displayed by blue catfish. However, new information about the effects of low oxygen concentrations on feed consumption and growth rates may be the greatest contributor to faster, more efficient growth and greater profits.
Traditional dissolved oxygen management for catfish has been based on the assumption that fish suffering from low oxygen stress would come to surface and “pipe” or crowd around aerators as they sought waters with higher dissolved oxygen concentrations. If fish were not seen at the surface, “all was well”. Application of this understanding translated into oxygen management strategies that were “less than scientific”. In the early days of catfish farming, B.M.A. (before modern aeration), farmers arose early and simply checked the ponds at sunup. If there were fish at or near the surface, the farmers would slam their car/truck door to spook fish near the surface and get them to dive down. Given the fact that deeper waters generally had even lower concentrations of dissolved oxygen, the only probable benefit from the noise was to make the fish less vulnerable to bird depredation.
If problems occurred regularly, the next step was to reduce or stop feeding for a few days, so as to reduce oxygen demand. Running well water over “splash boards” and into ponds could provide some aeration and a little cooling. As a last resort, makeshift aerators could be deployed. Some of these early attempts at pond aeration were quite ingenious, including outboard motors, Crisafulli pumps, and tractor powered bush hogs; unfortunately, they were generally ineffective. An often told, but not verified, story claims that one farmer used a helicopter for emergency aeration.
As catfish farming grew from 1970 to 2000 more effective and efficient approaches for pond aeration were the focus of research at both aquaculture research centers and farm machine sheds. Generally, some form of a paddlewheel aerator was the answer, with the biggest question being how to power the aerator. Over the 20 years after 1980, electric, floating, 10-hp paddlewheel aerators became the norm, based on a “formula” calling for 1-2 hp per surface acre of water. However, the industry’s understanding of oxygen requirements in the late 1990s was largely unchanged from the 1960s. Experience, combined with intuition suggested to Dr. Torrans that research on the effects of low, but non-lethal dissolved oxygen concentrations in the pond water would provide information leading to better management of oxygen conditions in catfish ponds.
Results from field studies at WARU and collaborative studies on commercial farms, have demonstrated the need for oxygen concentrations much higher than previously believed. Catfish may survive oxygen concentrations under 1.0 ppm, but the research results showed that when dissolved oxygen is maintained above 3.0 ppm, catfish eat more than twice as much feed and grow twice as fast as fish exposed to lower levels of oxygen.
New guidelines that recommended more intensive aeration technologies were based on these results. The first farmers to adopt the new aeration guidelines technology often saw feed efficiency improve by 25% of more, and were able to stay in business while many others were quitting catfish farming. They served as an inspiration to others. Given recent sharp increases in feed prices, feed efficiency and rapid growth made the difference.
The farmers who have remained in business and profited, have done so largely through the understanding and application of oxygen management provided by Les Torrans and his colleagues. Common modern practices, such as higher aeration rates and use of new, intensive “split-pond” systems, are based on the efforts of these scientists. When asked, “What’s the limit on catfish production in open production systems?” Les replied, “I don’t think anyone knows yet, but in a recent Southern Regional Aquaculture Center (SRAC)-funded project that looked at intensive production on commercial farms, we observed many ponds/split-ponds with production of 15,000-20,000 lbs/acre and feed conversion ratios better than 2:1.” “Not bad.”
— John Nickum
Effects of dissolved oxygen on catfish production
Field studies led by Dr. Torrans and colleagues reared hybrid catfish (♀ Channel Catfish Ictalurus punctatus X ♂ Blue Catfish I. furcatus) over two years as single-batch crops under two different dissolved oxygen (DO) regimes each year; a high-DO (control) treatment in which the minimum daily DO was maintained above 3.8 ppm during the growing season (June –September), and a low-DO (test) treatment in which the minimum DO was maintained at 1.6 or 1.3 ppm in the two years. Fish were fed to apparent satiation daily with a 32% protein commercial feed and clean harvested each year.
Results showed that dissolved oxygen concentration significantly impacted gross and net production, final fish weight and mean fish weight gain due to reduced feed intake when fish were maintained at reduced DO concentrations both years.
Feed intake was reduced in the low-DO treatment by 26.6% in year 1 (1.6 ppm minimum DO), and 29.2% in year 2 (1.3 ppm minimum DO). Food conversion ratios averaged 1.83 overall and were not affected by minimum DO.
Fish in the high-DO treatment gained an average of 44% more weight (an average of 1.50 lbs/fish gain in the high-DO treatments, compared to 1.04 lbs/fish gain in the low-DO treatments).
Initial weight at stocking also had a significant impact on weight gain. Fish stocked at 0.16 lbs average weight (year 2) gained 37% more weight than fish stocked at 0.11 lbs average weight (year 1), even though the stocking rate of the larger fish was 50% higher.
Hybrid catfish appear to be less affected by low dissolved oxygen levels than either parent species with hybrids responding better (less reduction in feed intake at reduced DO) than reported values for Channel Catfish and similarly to reported values for Blue Catfish.
The research project is published as: Torrans, Les; Ott, Brian; & Bosworth, Brian. 2015. Effects of Minimum Daily Dissolved Oxygen Concentration on Production Performance of Hybrid Female Channel Catfish × Male Blue Catfish. North American Journal of Aquaculture, Vol. 77(4), pages 485 – 490.