The study aims to determine the optimum dietary protein level for growing Nile tilapia fingerlings (Oreochromis niloticus) in fertilized ponds in winter, and investigate the possibility of depending entirely on natural food enhanced by fertilization as the sole source of nutrition. The experiment consisted of four treatments, each replicated in three hapas erected in an earthen pond. Each hapa was stocked with fifty fingerlings averaging about 5g. The pond was fertilized with superphosphate and chicken manure. Fish in one treatment were left without supplemental feeding as a control treatment, whereas fish in each of the other three treatments received one of the three experimental diets containing 28%, 32% or 36% protein. Water quality was monitored and plankton samples were collected for qualitative and quantitative analysis. Statistical analysis using a computerized SPSS program one way ANOVA (Version13) revealed a significant difference between the control treatment and the fed treatments with respect to final fish weight, weight gain, %weight gain, daily gain and specific growth rate. The treatment with the highest protein level (36 % protein) was superior to the other three treatments having the best weight gain in grams (37 g), the best % weight gain (712 %), the best daily gain (0.26g/day), the best specific growth rate (1.51%), the best food conversion ratio ( 2.8) and the best survival rate (84 %). Treatments with 28% and 32% protein level gave almost similar results with regard to all evaluated performance parameters. Fish fed diet 1 (28% protein) showed the lowest feed cost/kg fish produced and having no significant difference (P ≥ 0.05) in terms of growth performance as compared to the fish that were fed the diet which produced the highest growth rate (36% protein diet). Hence, diet 1 (28% protein) is the best diet for feeding fingerling Nile tilapia in winter. To reduce the cost of feeding, this diet should be fed to fish during warm periods in winter, leaving the fish to depend entirely on cheap natural food production stimulated by fertilization during cold periods.

Keywords: Nile tilapia fingerlings, protein, optimum diets, supplemental feeding.