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Abstract
INTRODUCTION
Tilapias belong to the family Cichlidae which is widely distributed in Africa, the Middle East, South and Central America, southern India and Sri Lanka. It contains approximately 1300 species, of which approximately 150 can be called tilapia. Tilapia is perciform fish that originated in Africa and the Jordan valley. They have adapted to diverse habitats: permanent and temporary rivers, rivers with rapids, large equatorial lakes, tropical and subtropical rivers, open and closed estuaries, lagoons, swampy lakes, deep lakes and coastal brackish lakes (Trewaves, 1982).
The most commercially important tilapia genera: Oreochromis, Tilapia and Sarotherodon. The genus Oreochromis is the largest, with approximately 79 species, followed by Tilapia with approximately 41 species and the genus Sarotherodon with approximately 10 species.
The rapid growth of tilapia, their resistance to poor environment quality, ability to grow under sub optimal nutritional condition, and high fecundity all make them suited for aquaculture (Robert, 2003; Amal et al., 2005).
All tilapia species exhibit these traits there can be brood variations among species in environmental tolerances such as salinity (Villegas, 1990a).
The culture of tilapia is not new but its mostly conducted in freshwater and brakshwater. Fresh water are very scarce and expensive resources in many areas of the worlds, on other hand seawater is abundant and can easily be utilized for culturing fish (Cruz et al., 1990), and the efficient use of marine and brackish water for aquaculture becomes a vital alternative and the production of tilapia in salinity water is important in costal areas and regions with limited supplies of freshwater.
Development of tilapia hybrids has been pursued to incorporate desirable traits into hybrids and hybrid- based varieties. The development of hybrid- based tilapia varieties is an important step toward increasing production and marketability of tilapia (Lutz, 2001). By combing the beneficial traits found in several tilapia species into hybrids, scientists and producers create fish that grow faster and are less expensive to produce.
These efforts have had varying degrees of success (Rakocy et al., 1993). Some hybrid–based varieties may exhibit traits beneficial to commercial – scale farming such as cold or salinity tolerance (Robert, 2003). And the progeny highly tolerant to saline condition might be produced through hybridization and other genetic manipulation techniques.
The main reasons for genetic manipulation of species used in aquaculture are all directly connected to improved output / input ratios. They are:-
• To enhance growth and /or efficiency of feed conversion.
• To enhance commercially significant flesh characteristics.