Fish, chitin, and chitosan production 289- manual separation of the sexes, which can be difficult when small
fish fry is used; - production of monosex broods through hybridization;
- sex reversal with the use of sex steroids (such as androgen,
methyltestosterone and ethynyltestosterone) in the diet of fish fry
during the period of gonadal differentiation (or the production of
male or female organs). The effectiveness of these steroids was
reported to be 93 – 100% (Nacario 1987).
Tilapia species is most suitable for waste-fed ponds due to its hardy
characteristics. Other herbivorous fish such as silver carp and bighead carp are
also promising species. Polyculture is recommended where benthic feeding fish
such as mud carp are stocked together with omnivorous fish like common carp.
Stocking density should be of primary concern in fish culture. As shown in
Figure 6.11 and Equation 6.6, high stocking density gives high yield per pond
volume, but with the smaller mean size of fish. To produce trash fish for animal
feed, stocking density of 10 or more fish/m^2 is recommended and the period of
fish culture may be as short as 3 months. Ponds producing marketable-size fish,
for possible use as human food, should be stocked at a lower density such as 1-2
fish/m^2 or even less, but the rearing period may be up to 1 year or longer.
However, several commercial fish farms are reportedly able to stock fish at
densities greater than 50 fish/m^2 provided that pond water quality (listed in
section 6.5.1) and fish food are maintained at the levels favourable for fish
growth.
Water supply
Water supply should be sufficient in quantity and of good quality since ponds
fed with solid or semi-solid organic wastes e.g. animal manures or nightsoil,
need a certain amount of water to compensate for losses through evaporation
and seepage. Ponds fed with sewage or effluent of wastewater treatment plant
sometimes needs dilution water. The quality of water should be in the
permissible range for fish growth.
Pond size
Generally pond depth of 1-1.5 m is suitable for fish with the provision of free
board of 0.3 to 0.6 m. In rain-fed pond, if the organic input is in solid form,
deeper water level should be provided to compensate the water loss during dry
season. A polyculture fish pond is recommended to have 2-3 m depth with an
area of manageable size; normally the 1,000-4,000 m^2 pond area is suitable in
the tropics. The width of ponds should not exceed 30 m to facilitate seining