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¹ Fish Ecology Department, Central Laboratory for Aquaculture Research, Abbassa, Abo-Hammad, Sharqia, Egypt Viewed : 4653 - Downloaded : 3896 The overpopulation of tilapia in confined ponds is an obvious problem, and causes stunted growth due to the shortage of natural food, particularly in semi-intensive culture. However, the control of tilapias population by predator culture has been practiced worldwide. The factors affecting predation efficiency of Nile catfish, Clarias gariepinus (B.) for controlling the overpopulation of Nile tilapia, Oreochromis niloticus (L.) were studied in four indoor experiments. Nile catfish with different sizes was stocked with tilapia fry (2-3 g) at the ratios of 1:10 or 1:15 (catfish : tilapia) without feeding. In another trial, Nile catfish : tilapia (1:15 ratio) with different sizes was frequently fed to satiation with fish diet (25% crude protein). The number of eaten fry was counted after 6, 24, 48 and 72 hours. The predation rate of Nile catfish at different predator sizes increased at the ratio of 1:15 more than 1:10 (catfish : tilapia) ratio. Predation rate of large Nile catfish was greater than small ones. Artificial feeding reduced the predation rate of Nile catfish at different predator sizes, while it increased with increasing predator size and tilapia stocking. In the fourth experiment, the leafless stems of phragmites plants (0.7 m long and 0.5 cm diameter) were used in this study at densities of 0, 5, 10, 20, 30 and 50 stems/m². The aquarium was stocked with 15 fry (2.2 g) and 1 catfish (400 g). The number of eaten fry was counted closely for 6, 24, 48 and 72 hours. The fry used the submerged macrophytes as a refuge to protect themselves from predator attack. It was concluded that predation rate of Nile catfish is dependent on predator size, prey stocking density, supplemental feeding and period of stocking. Moreover, the presence of submerged vegetation at moderate density (20-30 stem/m²) may reduce the number of eaten fry. Keywords : Nile catfish, Clarias gariepinus, predator, Nile tilapia, Oreochromis niloticus, prey, stocking ratio, artificial feeding, macrophytes