Effects of stocking density on growth, yield and profitability of farming Nile tilapia, Oreochromis niloticus L., fed Azolla diet, in earthen ponds

Two consecutive experiments were conducted to study the effects of stocking density on growth, food utilization, production and farming profitability of Nile tilapia (Oreochromis niloticus) fingerlings (initial mean weight: 16.2 +/- 0.2 g) fed Azolla, as a main component in diet. In experiment 1, fish were hand-fed twice daily with three isonitrogenous (28.5% crude protein) and isocaloric (14.5 kJ g(-1)) diets A30, A35 and A40 containing 30%, 35% and 40% Azolla, respectively, for 90 days. Diets were formulated by mixing Azolla with locally available by-products. No significant differences were found in growth parameters and production (P > 0.05). Total investment cost was significantly higher with A30 (P < 0.05), but same profitability values were obtained with all diets (P > 0.05). In experiment 2, three stocking densities, 1, 3 and 5 m(-2), were assigned to three treatments T-1, T-2 and T-3 respectively. Fish were hand-fed twice daily with diet A40. The final mean weight (89.53-115.12 g), the mean weight gain (0.81-1.10 g day(-1)), the specific growth rate (1.90-2.20% day(-1)) and the apparent food conversion ratio (1.29-1.58) were affected by stocking density, with significant difference (P < 0.05) at 5 m(-2), compared with the other densities. Stocking density did not affect survival rate (P > 0.05). Yield and annual production increased with increasing stocking density, ranging from 7.10 +/- 0.90 to 25.01 +/- 1.84 kg are(-1) and 28.79 +/- 3.66 to 101.42 +/- 7.48 kg are(-1) year(-1), respectively, with significant differences between all densities (P < 0.05). Higher stocking density resulted in higher gross return and lower cost of fish production, with significant variations (P < 0.05). The net return increased with increasing stocking density (P < 0.05). However, both densities of 3 and 5 m(-2) produced the same profitability values. On the basis of growth values and economic return, it was concluded that Nile tilapia could be raised at a density of 3 fish m(-2) with A40 to improve production and generate profit for nutritional security and poverty alleviation in rural areas.