Water supplies for catfish ponds are usually of good initial quality. However, once the water is used for culture, its quality deteriorates. This deterioration of environmental conditions is ultimately traceable to the use of feed. Despite the use of high quality feeds and careful feeding practices, relatively little of the nutrient value of feed is converted to catfish flesh. The remaining nutrients derived from fish wastes stimulate excessive phytoplankton growth. High rates of phytoplankton metabolism cause pronounced diurnal fluctuations in dissolved oxygen concentrations, dissolved carbon dioxide concentrations, and pH. Such fluctuations cause stress in fish resulting in reduced fish growth rates, poor feed conversion, and reduced resistance to disease. In extreme instances, such as depletion of dissolved oxygen, fish may be unable to adapt and will die.
Literally hundreds of environmental variables may affect fish health and survival, but fortunately only a few are important in commercial catfish culture. Because their concentrations may change rapidly, substances affected by biological activity (dissolved oxygen, carbon dioxide, ammonia, and nitrite) are the most important aspects of water quality and its management in catfish pond aquaculture.
The development of environment-related off-flavors is another important aspect of water quality management. Off-flavor is unlike the previously listed water quality variables because it does not pose a direct threat to fish health. Rather, it affects the acceptability of fish for processing, which causes delays in harvesting. As such, it increases the cost of production and exposes fish to additional risk of loss to diseases or predators.
Important Water Quality Variables
Dissolved Oxygen: The supply of dissolved oxygen often becomes limiting to catfish because the combined respiration of fish, phytoplankton, and mud-dwelling organisms exerts a tremendous demand for oxygen. At high phytoplankton biomass levels (which is the typical condition in catfish ponds during summer), oxygen production by algae is insufficient to meet the respiratory demand of the pond community and a daily oxygen deficit develops. If this deficit is not offset by artificial aeration, dissolved oxygen levels will drop very low and fish will die.
The key to successful management is early identification of those ponds that may require supplemental mechanical aeration to keep fish alive. Aeration is initiated when dissolved oxygen concentrations fall to a level considered critical (usually around 3 to 4 mg/L). Under current production practices, nearly every catfish pond has dissolved oxygen concentrations less than 2 mg/L at dawn during mid-summer. The duration of low dissolved oxygen concentrations at night usually ranges from 3 to 6 hours/day during mid-summer. Aeration is continued until past dawn when measurements indicate that dissolved oxygen concentrations are increasing as a result of photosynthetic activity.
Carbon dioxide: High rates of respiration in ponds with abundant plankton and high densities of fish result in rapid loss of dissolved oxygen and accumulation of carbon dioxide over the nighttime hours during summer months. Dissolved carbon dioxide concentrations of 5 to 10 mg/L are common on summer mornings in catfish ponds and appear to be well tolerated by channel catfish. They can survive in waters containing up to at least 60 mg/L dissolved carbon dioxide provided dissolved oxygen concentrations are high. Higher concentrations may cause death but chronic problems are rare because daytime uptake in photosynthesis normally serves to remove all the carbon dioxide that is produced in overnight respiration.
Ammonia: Ammonia is the major nitrogenous waste product excreted by fish. The fact that culture is possible at high feeding rates indicates that transformations and losses of nitrogen act to reduce ammonia concentrations. Additionally, as ammonia begins to accumulate, fish respond with reduced appetite, leading to lower rates of ammonia excretion and reduced ammonia concentrations in the water. As such, there are very few documented cases of acute ammonia intoxication in commercial channel catfish ponds. However, ammonia levels can be used to predict the onset of possible nitrite accumulations.
Nitrite: Nitrite is an intermediate product in nitrification, which is a common, bacteria-mediated transformation of ammonia to nitrate in soils and water. Nitrite accumulates to significant levels in ponds only when ammonia concentrations are relatively high and some factor causes the rate of ammonia oxidation to nitrite to exceed the rate of nitrite oxidation to nitrate. Accumulation of nitrite is undesirable because it can be toxic to fish at relatively low concentrations.
Nitrite toxicosis caused large losses of catfish in the early days of the industry, but losses are now very rare. An inexpensive and convenient prophylactic treatment using common salt has been developed and monitoring programs are easy to implement. As such, losses to nitrite toxicosis will result only when the farm manager is negligent in instituting the proper management plan.
When channel catfish are fed a grain-based diet and raised in clean water, they have a characteristic mild flavor. Pond-raised catfish may, however, develop flavors that can be disagreeable. All catfish processing plants sample fish for flavor quality before processing as a quality control measure. Fish are sampled several times over the weeks before a projected harvest date and if any of the samples have undesirable flavors, the fish will not be accepted for processing.
Most off-flavors in pond-raised catfish are caused by odorous compounds absorbed by fish from the water. Most off-flavors in pond-raised catfish are caused by naturally occurring organic compounds produced by aquatic bacteria or algae. These microorganisms synthesize and release compounds into the water, where they are absorbed through the gills, skin, or gastrointestinal tract of fish
Managing off-flavors can be divided into two general approaches: purging the compound by moving fish to a "clean" environment or using algicides to kill odorous aquatic bacteria or algae. Many farmers choose a more passive approach, however, and simply wait to harvest fish when they are on-flavor. This approach works to some degree because the composition of pond phytoplankton communities constantly changes. When community composition changes and the odor-producing species disappears, off-flavors produced by aquatic bacteria or algae will be purged from the flesh and flavor will improve. However, it is impossible to predict how long the odor-producing microorganisms will remain in the pond. They may disappear in a week or may persist for months.