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.
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