Feeding:
Juvenile prawns stocked into grow-out ponds initially are able to obtain sufficient nutrition from natural pond organisms. At the recommended stocking densities, begin feeding when the average weight of the prawn is 5.0g or greater.
Commercially available sinking channel catfish feed (28 to 32 percent crude protein) is an effective and economical feed at the recommended stocking densities.
The feeding rate is based upon the mean weight of the population (Table 1). A feeding schedule can be developed based upon three factors:
- A feed conversion ratio of 2.5;
- One percent mortality in the population per week; and
- Mean individual weight determined from samples obtained every 3 weeks.
At the end of the grow-out season, survival may range from 60 to 85 percent, if you have practiced good water quality maintenance. Yields typically range from 600 to 1,200 pounds per acre. Weights of prawns range from 35 to 45g (13 to 10 per pound). These yields and average sizes, however, will be significantly influenced by initial stocking density.
Water quality management:
Water quality is just as important in raising freshwater prawns as it is in raising catfish or any other species of aquatic animal. Dissolved oxygen (DO) is particularly important, and a good oxygen monitoring program is necessary to achieve maximum yields. You should routinely check and monitor dissolved oxygen in the bottom 1 foot of the pond which the prawns occupy. Electronic oxygen meters are best for this purpose but are rather expensive and require careful maintenance to ensure good operating condition.
The need for an electronic oxygen meter increases as the quantity of ponds to be managed increases. With only one or two small ponds, a chemical oxygen test kit is sufficient. Chemical oxygen tests kits that perform 100 tests are commercially available from several manufacturers. Use a sampler for collecting samples from an appropriate water depth for dissolved oxygen analysis.
These sampling devices are commercially available or can be fashioned. It is important the dissolved oxygen concentration in the bottom 1 foot of water does not fall below 3 parts per million (ppm). Dissolved oxygen concentrations of 3 ppm are stressful, and lower concentrations can be lethal.
Chronically low levels of dissolved oxygen result in less than anticipated yields at the end of the growing season. Emergency aeration can be achieved by an aerator. The design and size of the aerator depend on the size and shape of the culture pond, but a good rule of thumb is to have available 1 HP of aerator per surface acre of pond.
Prevention of thermal stratification is important, since prawns are bottom dwellers, and stratification results in two potential problems. First, since the bottom layer is cool, prawn growth can be severely retarded. Secondly, these bottom layers often become anoxic, and stress or mortality of the prawns can result. If ponds do not exceed the recommended depth, and aeration is properly employed, thermal stratification is unlikely.
Oxygen depletions can be avoided. An increasingly popular method is to employ either fulltime or nightly aeration. Since standing crop biomass seldom exceeds 1,000 lb/acre, this effectively prevents oxygen depletions.
Another reliable method to predict low DO levels is to plot the level an hour after sunset and approximately 2 hours later. Plot these two readings on a piece of graph paper and connect them with a straight line (Figures 1 and 2). Oxygen consumption during the late evening and early morning proceeds at a constant rate, caused by the respiration of the animals and plants in the water.
By extending the line from these two points over time you can quickly determine if the dawn DO concentration will decrease to the 3 ppm concentration that will stress or possibly kill the prawns. This method indicates whether emergency aeration is necessary and when to provide it. Additional information of pond aeration is available in SRAC Publications # 370, Pond Aeration, and 371, Types and Uses of Aeration Equipment.
Specific information on water quality requirements of freshwater prawns is limited. Although freshwater prawns have been successfully raised in soft water (5 to 7 ppm total hardness) in South Carolina, a softening of the shell was noticed. Hard water, 300-plus ppm, has been implicated in reduced growth and lime encrustations on freshwater prawns. Therefore, use of water with a hardness of 300-plus ppm is not recommended.
Authors:
Louis R. D’Abramo and Martin W. Brunson
