WATER QUALITY IN NILE TILAPIA OREOCHROMIS NILOTICUS L. AND SHARPTOOTHED CATFISH CLARIAS GARIEPINUS (BURCHELL 1822) POLYCULTURE IN KENYA

摘要

In this 20-week study at Sagana Fish Farm, central Kenya, twelve 0.8 m deep, 800 m~2 ponds, in a randomised block design, were stocked with juvenile (32 g) Oreochromis niloticus at 2 m~(-2) and clarias gariepinus fingerlings (4.6 g) at 0.2 m~(-2). Four treatments (T1 - T4) were applied in triplicates as follows: T1. Urea and DAP to provide 16 kg N ha~(-1)weel~(-1) and 4 kg P ha~(-1) week~(-1); T2. Urea and DAP applied to give 8 kg N ha~(-1) week~(-1) and 2 kg P ha~(-1) week~(-1) plus rice bran (RB) fed at 60 kg ha1 day~(-1); T3. RB fed at 120 kg ha~(-1) day~(-1); T4. RB as in T3 and fertiliser as in T2. Dissolved oxygen (DO), temperature and pH were measured weekly in the morning and afternoon while total alkalinity (TA) and total ammonia nitrogen (TAN) were measured fortnightly at similar times. Mean values were 1-7mgl~(-1) for DO, 25-28 deg C for temperature, 7.2-9.4 for pH, 48-88 mg CaCO-3 l~(-1) M forTA and 0.23-0.42 mg N I~(-1) for TAN. Significant differences were found in the pH (0.3-1.6 units), TA (29.3-40.I mgl~(-1))and afternoon DO (0.4-2.4 mg I~(-1)) and temperature (0.4-0.8 deg C) between the 4 treatments. However, the differences in TAN (0.09-0.20 mg~(-1)) and morning DO (2.3-3.9 mg~(-1)) between the 4 treatments were not significant. As expected, pH, DO and temperature were higher in the afternoon than in the morning. Whereas the daily differences in temperature were slight, the variations in DO and pH could have significant effects, those of pH directly and through effects on TA and TAN. However, the present results show that inorganic fertiliser and rice bran applied at the stated rates, did not adversely affect the fish and all water quality parameters, except the dangerously low morning DO, were within normal ranges experienced in fish culture.