Effects of water exchange on Oreochromis niloticus (L.) growth and water quality in outdoor concrete tanks

Abstract

Fingerlings of Oreochromis niloticus (L.) (av. wt. 19.0 g) were stocked in 3.75-m 3 outdoor concrete tanks at 240/tank, and reared for 98 days to evaluate the effects of rate of water exchange on growth, feed conversion and water quality. The average harvest weight of fish ranged from 122.4 to 87.6 g. The best growth was obtained with a continuous flow rate of 11/min per kg biomass (specific growth rate (SGR) 1.80, average daily gain (ADG) 1.06 g/day), closely followed by fishes receiving 0.5 l/min per kg (SGR 1.74, ADG 0.94 g/day). The poorest growth (SGR 1.46, ADG 0.7 g/day) occurred when the water was batch changed once every 24 h. With two batch changes daily, the average SGR and ADG were 1.63 and 0.86 g/day respectively. The feed conversion ratios ranged from 1.7 to 2.3, being low in tanks with continuous flow and high in tanks with batch exchange. As there was no significant difference in growth performance in two sets of flow-through tanks, a flow rate of 0.5 l/min per kg appears to be desirable for intensive culture of tilapia.