Freshwater aquaculture (2+1)

Unit 3 - Nursery, Rearing and grow-out ponds

3.3.4. Water quality management
Good growth and production of fish not only depends on availability of good quality feed, but also and the quality of the water in which they live. Physico-chemical properties of the water should be within the range of tolerance of the species being cultured.

Normally water quality will not be an issue with lower stocking densities and lower yields (extensive and lower level of semi-intensive culture). However, when higher stocking densities are followed, water quality tends to deteriorate, particularly when the stock grows and biomass increase

The most important water quality parameters are the dissolved oxygen. Oxygen dissolves from the atmosphere and also produced by photosynthesis phytoplankton and higher macrophytes which produce liberate oxygen during the day but consume oxygen during night. The levels of DO in culture systems should be above 5 mg/L. When oxygen tends to fall below this level, which usually occurs during night, aeration of the ponds becomes necessary. Various types of aerators are commercially available, which may be expensive. A simple way to aerate is to circulate the pond water such that bottom water comes to the surface and surface water goes to the bottom. This can be done using a pump whose intake (foot valve) is lowered into the pond and water is pumped such that the water splashes back into the same pond. Aeration may also become necessary during cloudy weather since lack of sunlight prevents photo-synthetic production of oxygen by the algae and higher plants.

Accumulation of metabolites and decayed matter will also result in poor water quality in ponds. Ammonia may get accumulated which is toxic to fish. In order to maintain water quality, water exchange may be required to be carried out. The frequency and amount of water to be replaced with freshwater depends on the quality of the water. This requires experience as well as analysis of the water quality.