Water Quality

Water quality management is a key ingredient in a successful fish operation. Most periods of poor growth, disease and parasite outbreaks, and fish kills can be traced to water quality problems. Water quality management is undoubtedly one of the most difficult problems facing the fish farmer. Water quality problems are even more difficult to predict and to manage.

Oxygen

Oxygen stress is the most frequently encountered water quality problem in cage culture of fish. The concentration and availability of dissolved oxygen (DO) are critical to the health and survival of caged fish.

Critical dissolved oxygen levels will vary depending on species being reared and with interactions with other water quality parameters; e.g., carbon dioxide, ammonia, and nitrite. In general, warm water species such as catfish and tilapia need a dissolved oxygen concentration of 4 mg/l DO (or ppm) or greater to maintain good health and feed conversion. Healthy warm water fish can tolerate 1 mg/l DO for short periods of time but will die if exposure is prolonged. Prolonged exposure to 1.5 mg/l DO causes tissue damage, and any prolonged exposure to low dissolved oxygen levels will halt growth and increase the incidence of secondary diseases, apparently by reducing the fishes’ ability to resist infection. Many parasites, diseases, and chemical agents can damage the gill filaments affecting oxygen transport across the gills. This can cause the fish to behave as though the dissolved oxygen concentration is low, when in reality the cause is a disease problem.

The concentration of dissolved oxygen in any body of water varies over time and is affected by physical, biological, and chemical factors. Physical controllers of dissolved oxygen are temperature, atmospheric pressure, and salinity. As temperature and salinity increase, and as atmospheric pressure decreases, the solubility of oxygen will decrease. Temperature is an important physical controller of dissolved oxygen. As water temperature increases 10°F the amount of oxygen that will dissolve in water decreases by approximately 10 percent. The physical transfer of oxygen between the atmosphere and water occurs across the water surface when dissolved oxygen concentrations are above or below saturation. The rate of this transfer is regulated by turbulence across the water surface.

Biological factors that affect dissolved oxygen are plant photosynthesis (both phytoplanktonic and macrophytic) and plant and animal respiration (fish, invertebrates, bacteria, etc.). Most of the oxygen in aquaculture ponds is produced by plant photosynthesis during sunlight hours. Planktonic algae (phytoplankton) usually produce the bulk of this oxygen. High densities of aquatic macrophytes (rooted underwater plants) usually reduce phytoplankton growth and water circulation and, therefore, can cause dissolved oxygen problems in cage production ponds.

Plant and animal respiration are the most important oxygen reducing processes in aquaculture ponds. Fish must compete with all other living organisms for the ponds’ available dissolved oxygen. This is particularly acute at night when plants in the pond are also consuming oxygen through the process of respiration. In most aquaculture ponds nighttime phytoplankton respiration is the major consumer of oxygen. Respiration rates are temperature driven in cold-blooded animals (i.e., fish) and plants, increasing oxygen consumption with rising temperatures. Total plant and fish biomass is also usually greatest during warm weather and high light intensity conditions. For all of these reasons, summer nights, with high water temperatures and respiration and low wind turbulence, bring most oxygen problems.

In cage culture situations, low dissolved oxygen is particularly acute because the fish are crowded into such small areas. Most fish kills, disease outbreaks, and poor growth in cage situations are directly or indirectly due to low dissolved oxygen.

Turnovers and plankton die-offs are two other situations in which dissolved oxygen levels may fall below critical levels. Turnovers occur during cold rains, heavy winds, and prolonged cold spells in summer. These conditions cause the upper oxygenated layer of water to mix with the cold oxygen- depleted layer of water on the bottom of the pond. The mixing of the two layers reduces the total dissolved oxygen in the whole pond to critical levels due to both dilution and chemical reduction. Turnovers can be particularly common in deep ponds with large watersheds.

Plankton die-offs can occur as a natural consequence of algal population dynamics due to seasonal changes in temperature, pH, light intensity, nutrients, diseases, parasites, toxins, or other factors which are not clearly understood. Plankton die-offs can also occur as a consequence of nighttime low dissolved oxygen. In this case, the density and biomass of the plankton become so great that a critical dissolved oxygen concentration is reached in the pond due to nighttime respiration demands. The plankton dies from lack of oxygen along with the fish.

Dissolved oxygen management is one of the most critical management techniques that must be learned by a fish farmer. Dissolved oxygen management includes both biological and mechanical manipulation. Biological manipulation can include fertilization and submerged aquatic plant control to maintain a healthy phytoplankton bloom. Mechanical manipulation through aeration may help maintain adequate dissolved oxygen concentrations and may save fish during chronic low Dos, turnovers, and plankton die-offs.

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