الفهرس 
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Abstract
Objectives: The present study was designed as an attempt to improve Nile tilapia mono-sex production in El-Manzla Lake, through the graded replacement levels (0, 4, 8 and 12%) of density of monosex Nile tilapia, Oreochromis niloticus by silver carp, Hypophthalmichthys molitrix.Methods & Results: The present study was carried out from 22th July to 11th November, 2013, in a private fish cages farm in Manzala Lake, (Raswah Lisa Al-Gamaliah), Dakahlia Governorate, Egypt. An experimental field study was conducted in covered floating net cages in the Lake Manzala to investigate the effect of the graded replacement of stocking density rate (0, 4, 8 and 12%) of the monosex O. niloticus by the silver carp, H. molitrix as a mono (T1) or polyculture systems (T2, T3, and T4, respectively) for 124 days (4 months) on water quality parameters, growth performance and feed efficiency parameters, condition factor, fish organs indices, fish body composition. The total production of O. niloticus and economic efficiency parameters were evaluated in each treatment also. The obtained results in the present study revealed that:1- The positive effects of rearing O. niloticus in polyculture systems with H. molitrix compared to mono-culture system of O. niloticus (T1). O. niloticus reared in polyculture system at stocking rate 4% (T2) or 8% (T3) with H. molitrix significantly (P ≤ 0.05) improved the growth rate, feed efficiency (feed intake, feed conversion ratio, protein and energy utilization), and chemical composition of the fish body (crude protein, ether extract, energy content) parameters of both fish species. 2- O. niloticus reared in polyculture system with 12% silver carp (T4) gave the highest total output, total profit, and economic efficiency (%) among all the experimental mono (T1) or polyculture systems with silver carp (T2 and T3).Conclusion:from the obtained results, it could be concluded that monosex O. niloticus reared with H. molitrix at 4% (T2), and 8% (T3) in polyculture systems are the best aquaculture systems among the experimental mono (T1) or other polyculture system with 12% H. molitrix (T4). But economically, O. niloticus reared in polyculture system with 12% silver carp (T4) is the best aquaculture system among all the experimental systems.