Nitrites at concentrations of 1.8 ppm have caused problems in hatcheries but there is no definitive information as to the toxicity of nitrite to prawns in pond situations.
High nitrite concentrations in ponds would not be expected given the anticipated biomass of prawns at harvest. High levels of un-ionized ammonia, above 0.1 ppm, in fish ponds can be detrimental.
Concentrations of un-ionized ammonia as low as 0.26 ppm at a pH of 6.83 have been reported to kill 50 percent of the prawns in a population in 144 hours. Therefore, you must make every effort to prevent concentrations of 0.1 or higher ppm un-ionized ammonia.
For more information on ammonia and nitrites in fish ponds, request SRAC Publications # 462, Nitrite in Fish Ponds, and #463, Ammonia in Fish Ponds.
Acceptable pH ranges:
A high pH can cause mortality through direct pH toxicity, and indirectly because a higher percentage of the total ammonia in the water exists in the toxic, unionized form. Although freshwater prawns have been raised in ponds with a pH range of 6.0 to 10.5 with no apparent short-term adverse effects, it is best to avoid a pH below 6.5 or above 9.5, if possible.
Constantly high pH stresses the prawns and reduces growth rates. High pH values usually occur in waters with total alkalinity of 50 or less ppm and when a dense algae bloom is present.
Before stocking, liming ponds that are built in acid soils can help minimize severe pH fluctuations. Another way to manage to avoid any anticipated problems of high pH is to reduce the quantity of algae in the pond by periodic flushing (removing) the top 12 inches of surface water. Alternatively, organic matter, such as corn grain or rice bran, can be distributed over the surface area of the pond. This procedure must be accompanied by careful monitoring of oxygen levels, which may dramatically decrease due to decay processes.
Diseases:
Diseases so far do not appear to be a significant problem in the production of freshwater prawns, but, as densities are increased to improve production, disease problems are bound to become more prevalent. One disease you may encounter is “blackspot” or “shell disease,” which is caused by bacteria that break down the outer skeleton.
Usually it follows physical damage and can be avoided by careful handling. At other times, algae or insect eggs may be present on the shell. This condition is not a disease, but rather an indication of slow growth, and is eliminated when the prawn molts.
Harvesting:
At the end of the grow-out season, prawns may be seine- or drain-harvested. For seining, depth (or water volume) should be decreased by one-half before seining. Alternatively, ponds could be drained into an interior large rectangular borrow pit (ditch) where prawns are concentrated before seining.
You can effectively drain harvest only if ponds have a smooth bottom and a slope that will insure rapid and complete draining. During the complete drain-down harvest procedure, prawns generally are collected on the outside of the pond levee as they travel through the drain pipe into a collecting device.
To avoid stress and possible mortality, provide sufficient aeration to the water in the collection device.
Selective harvest of large prawns:
during a period of 4 to 6 weeks before final harvest is recommended to increase total production in the pond. Selective harvesting usually is performed with a 1- to 2-inch bar-mesh seine, allowing those that pass through the seine to remain in the pond and to continue to grow, while the larger prawns are removed.
Selective harvest may also be accomplished with properly designed traps. Prawns can be trapped using an array of traditionally designed crawfish traps. Selective harvest can help to extend the duration of the availability of the fresh or live prawn product to the market. However, there is a lack of research to show whether selective harvesting or a complete bulk harvest is the most economical approach.
Regardless of the harvest method employed, some prawns will remain in the bottom of the pond and will have to be manually picked up. Rapid draining or careful seining can minimize this residual crop left in the pond.
Prawns should be quickly chilled after harvest to preserve integrity of the muscle tissue, thus maintaining a firm, high quality texture to the product. The product may be marketed fresh on ice, or may be either processed and frozen or frozen whole for storage and shipment.
Authors:
Louis R. D’Abramo and Martin W. Brunson
