Effects of stocking density and water flow rate on performance, water quality and economic benefits of African catfish larvae (Clarias gariepinus Burchell, 1822) in the aquaponic system integrated with Azolla fern

The effect of stocking on fish performance is known is most systems, and for many fish. Meanwhile water flow rate remains an important factor in recirculating aquaculture systems. However less information is still available on the influence of aquaculture management practices on the performance of aquaponics. This study determined the effect of stocking density and water flow rate on growth performance of African catfish larvae (Clarias gariepinus) and Azolla spp. in catfish-Azolla aquaponics system. The 4 x 4 factorial experiment consisted of four larval stocking densities (1500 larvae/m(,)(3) 3000 larvae/m(3), 4500 larvae/m(3) and 6000 larvae/m(3)) and four water flow rates (0.25 L/min, 0.5 L/min, 0.75 L/min and 1 L/min) executed in triplicate. The stocking density and water flow rate had an interactive influence (P < 0.05) on the concentration of DO, TAN, and NO3-. The stocking density and water flow rate had an interactive influence (P < 0.05) on the final mean weight, SGR, and survival of C. gariepinus larvae, but not for FCR and yield (P > 0.05). The highest fish weight was achieved under combination of stocking density 3245 larvae/m(3) and water flow rate of 0.87 L/min, but growth will still be optimized upto a stocking density of 4850 larvae/m(3) but at higher combinations of water flow rates (> 0.875 L/min). The stocking density and water flow rate had an interactive influence (P < 0.05) on chlorophyll a concentration, but not fresh weight, dry weight and yield (P > 0.05). Enterprise budgets for aquaponic with different stocking densities and water flow rate for fish and Azolla production allowed all the treatments to post positive returns to risk and were viable investments. Cash flow projections were made over a 10 year period for each of the units with the net present value (NPV) and internal rate of return (IRR) revealed that sales of larval fish and Azolla sp. at stocking density of 1500 larvae/m(3) were not profitable. However, all the other stocking densities were profitable except at 3000 larvae/m(3) and water flow rate of 0.25 L/min. Although at all stocking density the revenue were able to cover the variable costs, this study recommends stocking at 3000 to 4500 larvae/m(3) and water flow rate of 0.75 L/min and 1.00 L/min.