Grow-out of fresh-water prawns
The preferred size of ponds for grow-out appears to be 0.2–0.6ha, with an average depth of 0.9m. The ponds have an adequate supply of fresh or slightly brackish water, not exceeding 4ppt in salinity. Water quality management is generally through water exchange, though paddle wheels or other aeration devices are maintained for use in emergencies. High growth rates are obtained at temperatures between 26 and 30°C. In semi-tropical areas, the ponds are stocked only when the temperatures are not likely to drop below 20°C. Many farmers do not fertilize their ponds except when they are newly built and lack nutrients in the soil. Prawn ponds in Taiwan with earth or sandy bottoms are dried by exposure to sun and treated with lime and teaseed cake. Filtered water is used to fill the ponds, in order to prevent the entry of eggs of unwanted fish. The water is ‘seasoned’ with green water and aerated for several days. Fermented chicken manure, pig manure and inorganic fertilizers are applied to stimulate phytoplankton production (see New, 2002).
Ponds are normally stocked with one-to-four-week-old post-larvae. The stocking rate depends on the preferred market size and the length of the growing season. New and Singholka (1985) recommended a density of five one-month-old post-larvae per m2 (50000 per ha) for a growing season of eight months to harvest prawns of about 70g (head on). Higher stocking rates are adopted in commercial operations in Hawaii, averaging 16 per m2 (160000 per ha), allowing for a mortality that would bring the stock density to 11 per m2 (110000 per ha). The recommended rate under the temperature conditions in South Carolina (USA) is 4.3–6.5 per m2 of nursed juveniles or a mixture of nursed juveniles and post-larvae (43000– 65000 per ha), for a growing period of five to six months, yielding 700–1200kg per ha.
The most common grow-out practice has been described as the ‘continuous culture’ or ‘continuous stocking and harvesting system’, in which the ponds are stocked once or several times a year and are never drained, except for repairs. Harvesting is carried out selectively for marketable prawns about 30 or 45g size, at regular intervals. A considerable disparity in growth rates between individual prawns (especially between males and females) occurs in ponds. So selective fishing is performed as a means of stock management and to grow under-sized individuals from previous stockings to marketable size. If the whole stock is harvested together, there will be a number of small prawns which may not be acceptable in the market. Prawn ponds following this system in Hawaii are reported to produce about 276g/ha per month by selective harvesting, yielding about 3314kg/ha per year (Malecha, 1983).
Some farms in Hawaii have started incorporating a nursery phase or an intermediate grow-out phase, to avoid some of the problems of continuous stocking. This requires specially constructed ponds with harvest sumps.