A slightly more sophisticated system is to remove the young from the pond as they are produced, rear them in fry ponds and then stock them into grow-out ponds. Again the fish tend to breed before they are of market size and over-population can result. The over-population can be controlled by the use of a predator fish. In this system the tilapia are stocked from a fry pond into the grow-out pond and when they have reached a certain size too large for them to be eaten by predators, a predator fish is introduced into the pond. This fish will then eat the majority of tilapia fry when the adults starts to breed and so will prevent over-population. Various predators are used in different part of the world:
cichlasoma manguense, El Salvador; Hemichromis fasciatus, Zaire; Lates niloticus, Nigeria; Micropterus samonides, Madagascar; Bagrus docmac, Uganda. The correct balance between the predator and the tilapia is not easy to maintain but this method is an improvement over the straight production systems.
The most effective method of production control is to grow a unisex population so that there can be no breeding. All male populations are used because males grows faster than females do.
There are three methods of by which unisex populations can be acheived: In the first, the fish are sexed by hand when young and the females are discarded, the difference between the genitalia of the sexes being the criteria used for sorting. This system is slow and requires skilled operatives otherwise a few female fish will be added to the population and breeding will occur.
In the second method all-male populations are achieved by the use of hybrids. It has been found that if certain species of tilapia are cross-bred the resulting offspring are all males; for example male Sarotherodon nilotica crossed with female Sarotherodon mossambica give all-male populations.
In the third system the young fish resulting from normal breeding are treated at an early ege with male sex hormone; this causes a sex reversal in the young females resulting in all-male population. Both methy-testosterone is fed at the rate of 30u/g of feed which is fed at 4o/o of the body weight of the fish for three weeks then 3o/o for seventeen weeks. The treatment is started during the first thirty to fifty days of life.
If the tilapia are reared in cages they cannot breed succesfully as there is no pond bottom on which to spawn, or if they do spawn the eggs fall out of the cage are lost. The disadvantage of this system is that the fish have to be fed by the farmer.
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