EFFECTS OF POND WATER REPLACEMENT AND STOCKING DENSITY ON WATER QUALITY, PLANKTON ABUNDANCE AND GROWTH PERFORMANCE OF TILAPIA Oreochromis niloticus
To determine optimum water replacement rate for fish pond production, we investigated the effects of pond water replacement and stocking density on water quality, plankton abundance, fish growth rate and yield. Nile Tilapia (Oreochromis niloticus) fingerlings with average body weight of 0.4 g were stocked into four 300 m2 earthen ponds. Each pond was partitioned into six equal parts using fishnets. Three parts of each pond were stocked with 2 fingerlings/m2 and the other three parts with 4 fingerling/m2. Each pond was fertilized using 10 kg of biogas slurry three times a week and fish were fed at a feeding level of 10% of their body weight daily for 3 months. For the first two ponds 50% of water was replaced weekly and for the other two ponds water was not replaced. Water quality, plankton abundance and tilapia growth performance were monitored throughout the 90 day experimental period. The results show significantly higher (p ≤ 0.05) conductivity (168.9 10.7 µS/cm) and nitrate (NO3-N) (5.0±0.5 mg/l) concentrations in ponds with no water replacement while significantly higher (p ≤ 0.05) dissolved oxygen concentration (9.9 mg/l) was observed in ponds in which 50% of water was replaced weekly. Higher plankton (both phytoplankton and zooplankton) abundance was observed in ponds with lower stocking density and 50% water replacement than in ponds with higher stocking density and no water replacement. Stocking density influenced fish yield but water replacement did not affect yield. The average net yield for higher stocking density was superior (4,891±565 kg/ha/yr) to that of lower stocking density (2,105±485 kg/ha/yr). Although stocking density of 2 fingerlings /m2 gave higher growth performance than that of 4 fingerlings/m2, statistically no significant (p ˃ 0.05) differences were observed with regard to growth rate, specific growth rate and survival between the two stocking densities and water replacement regime. Our data suggest that water replacement can improve the productivity of Nile Tilapia (Oreochromis niloticus) in earthen ponds, however, for good results more studies are needed with a wider range of both stocking density and water replacement regimes.