الفهرس 
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Abstract
This study aimed to : The aim of this research study effects of polyculture and monoculture between (Tilapia and carp fishes) with different stocking rates in cages. Methods& Results : This experiment was designed in order to study the effect of intensive rearing of mono-sex Nile tilapia and Silver carp under mono-or polyculture systems at different stocking densities in floating net cages. Fish were male mono-sex Nile tilapia with an average initial body weight (19.30 g) and Silver carp with an average initial body weight (73.6 g). The experimental fish were adapted for one week in floating net-cages and then distributed randomly into five experimental cages. The first cage was stocked with Nile tilapia, at a rate of 30 fish / m3, while the 2nd, the 3rd and 4th cages were stocked with different densities of Nile tilapia with Silver carp, at a rate of 30 fish / m3 and the fifth cage was stocked with Silver carp only at a rate of 6 fish / m3. All fish groups were feed the same artificial pelleted diet (except 5th group fed natural water food). The diets were contained of 26.58% crude protein and gross energy 410.35 Kcal/100g DM, and offered twice daily (at 8.30 a.m. and 15.00 p.m.). The feeding rate represented 4% of mass body weight during all experimental periods. The main results showed that the best treatment was the monoculture of mono-sex Nile tilapia (30 fish / m3), then followed by the polyculture (88% Nile tilapia) and (12% Silver carp) in most of the examined traits as well as in the economic efficiency. The obtained results showed also that the intensive monoculture systems of silver carp had the lowest economic efficiency related to the only natural food in water only. Conclusion : The present obtained results lead to the importance using the balanced pelleted complete diet for feeding tilapia fish especially under intensive cultured (monoculture); as well as, depending of green water for fed Silver carp in intensive monoculture. Also, showed the superiority of stocking densities in polyculture system were 88% and 12% for Nile tilapia and Silver carp, respectively As well as from environmentally point of view, it could be concluded that the controlling on different source of water pollution in Manazala Lake are very important to get high quality and safety fish production.