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 (T 1 ) or poly-culture systems (T 2 , T 3 , and T 4 , 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 (T 1 ). O. niloticus reared in poly-culture system at stocking rate 4% (T 2 ) or 8% (T 3 ) 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 (T 4 ) gave the highest total output, total profit, and economic efficiency (%) among all the experimental mono (T 1 ) or poly-culture systems with silver carp (T 2 and T 3 ). Thus, it could be concluded that monosex O. niloticus reared with H. molitrix at 4% (T 2 ), and 8% (T 3 ) in poly-culture systems are the best aquaculture systems among the experimental mono (T 1 ) or other poly-culture system with 12% H. molitrix (T 4 ). But economically, O. niloticus reared in poly-culture system with 12% silver carp (T 4 ) is the best aquaculture system among all the experimental systems.

ABSTRACTver the last decades, there has been a growing interest in studying the main problems that could be harmful to the River Nile system in Egypt. Attention was paid to one of the most important problems that face the River Nile, this is called River Nile transport project. So the main objective of the present study was to investigate the adverse effects that can result from spilled gasoline as petroleum product from various motor ships into the River Nile on one of the most important popular fishes in Egypt, Oreochromis niloticus to ascertain whether spilled gasoline into aquatic ecosystems can affect on the aquatic biota or not and to identify any potential public health risks that can be associated with dietary intake of such fish by histological and ultrastructural studies on its edible muscles. Levels of the growth hormone secreted by the pituitary gland which is essential to the development of skeletal muscles and growth rate was also examined. So, the fish were exposed to increasing concentrations of gasoline: 0 (control), 100, 200 and 500 mg/L for 30 days. Unexposed fish exhibited significantly higher levels of serum GH compared to that of tested fish. The GH levels decreased significantly with increasing gasoline concentrations. These data together indicate that Gasoline presents a hazardous substance for both the fish as well as the human consumers . Finally, the results revealed that the River Nile transport project can lead to serious harm to fish community that eventually become unsafe for human consumption. Therefore, this project must be carefully revised forbidden by health agencies.