The aim of this study was to investigate the effect of the graded replacement of stocking density rate (0, 4, 8 and 12%) of the monosex Oreochromis niloticus by the silver carp, Hypophthalmichthys molitrix as a mono (T1) or poly-culture 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. An experimental field study was
conducted in covered floating net cages in the Lake Manzala. The obtained results revealed the positive effects of rearing O. niloticus in poly-culture systems with H. molitrix compared to mono-culture system of O. niloticus (T1). O. niloticus reared in poly-culture 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. While, O. niloticus reared in poly-culture system with 12% silver carp (T4) gave the highest total output, total profit, and economic efficiency (%) among all the experimental mono (T1) or poly-culture systems with silver carp (T2 and T3). Thus, it could be
concluded that monosex O. niloticus reared with H. molitrix at 4% (T2), and 8% (T3) in poly-culture systems are the best aquaculture systems among the experimental mono (T1) or other poly-culture system with 12% H. molitrix (T4). But economically, O. niloticus reared in poly-culture system with 12% silver carp (T4) is the best
aquaculture system among all the experimental systems.