Prawns: Water Quality

Dissolved Oxygen - Water quality influences the level of growth that can be achieved with freshwater prawns. Dissolved oxygen is particularly important, and a good oxygen-monitoring program is necessary. Electronic oxygen meters are the most reliable and accurate means to determine levels of dissolved oxygen. They are comparatively expensive and require careful maintenance and calibration to ensure good operating condition and the collection of accurate data. The need for an electronic oxygen meter increases as the quantity of ponds that need to be managed increases. If an enterprise consists of only a few ponds, then monitoring of levels of dissolved oxygen can be readily accomplished through use of a commercially available chemical oxygen test kit that is generally equipped to conduct 100 independent tests.

Water being dischargedBecause prawns live on the bottom, levels of dissolved oxygen should be routinely monitored within the bottom 0.3 m (1 ft) depth of water. Oxygen levels at the surface can potentially be lower than those at the bottom. Water samples for dissolved oxygen analysis can be collected from the lowest foot of water column with either commercially available or individually fashioned devices.

Dissolved oxygen concentrations should always be maintained above 3 ppm. At lower concentrations, stressful conditions and eventually mortality will occur. Levels of dissolved oxygen that are substantially higher than 3 ppm are recommended because chronically lower levels of dissolved oxygen throughout the growing season can markedly impact yields. Emergency aeration can be achieved by an aerator, a device that increases the rate of transfer of oxygen from air to water. The type and power of the aeration device(s) will be principally determined by the size and shape of the culture pond. Generally, aeration generated by one horsepower per surface acre of water is recommended.

Nitrogenous Compounds - At concentrations of 1.8 ppm, nitrite has been associated with mortality in hatcheries, but no experiential or experimental definitive information about the toxicity of nitrite to prawns in pond situations is available. During the pond growout phase, high nitrite concentrations in ponds would not be anticipated given the level of prawn biomass associated with the recommended stocking densities and feeding rates. Levels of un-ionized ammonia that exceed 0.1 ppm, can adversely affect fish in ponds. At concentrations of un-ionized ammonia as low as 0.26 ppm at a pH of 6.83, 50% of the prawns in the population died within 144 hours. Therefore, concentrations of un-ionized ammonia that exceed 0.1 ppm must be avoided.

pH - A high pH can cause mortality either directly by means of creating a pH imbalance relative to the prawn tissue or indirectly by causing a larger proportion of ammonia to exist in the toxic un-ionized form. Although freshwater prawns have been successfully raised in ponds where a pH has ranged from 6.0 to 10.0, a pH that remains within the range 6.5 and 9.5 is recommended. High pH values usually occur in water having a total alkalinity 0.5 to 50 ppm often stimulated by the existence of a dense algal bloom. Adding lime to the bottom soil of ponds that are constructed in acid soils can help to minimize severe and possibly lethal fluctuations that might occur during growout.

One management practice that has been implemented to mitigate rising pH in smaller ponds is periodic flushing (removing) of the top 12 inches (30.5 cm) of surface water to reduce the quantity of photosynthetic algae in the pond. However, this procedure is not a cost-effective solution in large ponds and the quality of the effluent (water discharged) may not meet standards established by state or federal agencies. Another management approach to avoid higher than desired pH is the spreading of organic matter, such as corn grain or rice bran, over the surface area of the pond. The organic matter should be introduced gradually, over a period of two weeks, to achieve eventually a level of 30 pounds per acre (32 kg/ha). The decomposition of the organic material releases carbon dioxide that helps to reduce pH. This management procedure must be accompanied by careful monitoring of oxygen levels that would tend to decrease substantially due to the heavy oxygen demand arising from the decomposition of the organic material.

Hardness - Specific information concerning other water quality requirements of freshwater prawns is limited. Although freshwater prawns have been successfully raised in soft water (5-7 ppm total hardness), the shell is noticeably softer and may be more susceptible to bacterial infection. To avoid this condition, water hardness should range between 50 and 200 ppm. Hardness of pond water can be increased through an application of a source of calcium such as agricultural gypsum or calcium chloride. The purity of gypsum varies (70-98%) and generally is more readily available than calcium chloride. Under the assumption of 100% purity, an increase of 1.00 ppm in total hardness can be achieved with an application rate of 1.72 ppm. In very hard water, i.e. 300 ppm or higher, reduced growth and lime encrustations on the exoskeleton have been observed.

MSU Publications & Information

Other Freshwater Prawns Information