By: Dr Mustapha ABA. Scientific Expert in Aquaculture. Fish Nutrition. Rabat. Morocco.
Dissolved oxygen control is critical in aquaculture, the volume of oxygen contained in water, is often a critical parameter in the health and well-being of your livestock. In general, most shrimp/fish species will grow and thrive within a DO range of 5-12 mg/L (ppm). However, if levels drop below 4 mg/L they may stop feeding, become stressed and possibly lead to catastrophic fatalities. This series of events can start a chain reaction in a pond aquaculture system that could prove detrimental.
Oxygen depletion usually occurs in the summer months because warmer water holds less oxygen than cooler water. For example, water with a temperature of 32°C can hold up to 7.3 mg/L of oxygen, while 7°C water can hold 12.1 mg/L. As water temperatures rise, oxygen levels decrease. Higher temperatures also increase the metabolic rate of shrimp/fish resulting in the need for more oxygen.
In summer months, ponds can undergo stratification because of differences in water densities with differing temperatures. Cooler water sinks, warmer water rises, and the water at the top of the pond is heated more rapidly through radiation by the sun. Although the pond’s cooler bottom temperature holds more dissolved oxygen when the summer starts, as the summer progresses, microbial decomposition of organic materials depletes oxygen at the pond’s bottom.
In addition, an average pond experiences a DO diurnal cycle, which is a fluctuation in DO levels from day to night. Fluctuations can range from 2 to 3 mg/L during a 24 hour period. DO increases during the daylight hours when photosynthesis is occurring and decreases at night when respiration continues but photosynthesis does not. This natural diurnal cycle in aquatic systems fluctuates greatly and can be exacerbated in aquaculture systems with high stocking densities.
In order to accurately understand these differences and changes, most aquaculture facilities use oxygen sensors to monitor dissolved oxygen and prevent oxygen depletion through monitoring and control. If levels drop too low, turning on an aeration system can quickly increase dissolved oxygen levels.
But in order to enrich the aquifer in dissolved oxygen, aerators are used, despite their use of energy, however we must encourage aquaculturists to overcome this problem to use solar energy.
