Chronic ammonia toxicity to duckweed-fed tilapia (Oreochromis niloticus)

84 The effects of prolonged exposure to sub-lethal un-ionised ammonia concentrations on the growth performance of juvenile Nile tilapia (Oreochromis niloticus) fed on fresh duckweed (Lemna gibba) grown on pre-treated domestic sewage has been investigated. The experiment was conducted in duplicates over 75 days using juveniles with a mean body weight of 20 g. Five nominal, total ammonia nitrogen concentrations (control, 2.5, 5, 7.5 and 10 mg https://www.w3.org/1998/Math/MathML" display="inline"> N l - 1 ) were established as treatment groups. Statistical analysis of the specific growth rate (SGR) showed no significant ( https://www.w3.org/1998/Math/MathML" display="inline"> p > 0 . 05 ) differences between the SGR (0.71) of tilapia in the control (0.004 mg https://www.w3.org/1998/Math/MathML" display="inline"> UIA - N I - 1 ) and the SGR (0.67) of those exposed to 0.068 mg https://www.w3.org/1998/Math/MathML" display="inline"> UIA - N I - 1 . The SGR of tilapia exposed to un-ionised ammonia nitrogen over 0.068 mg https://www.w3.org/1998/Math/MathML" display="inline"> UIA - N I - 1 (0.144, 0.262 and 0.434 mg https://www.w3.org/1998/Math/MathML" display="inline"> UIA - N I - 1 ) was significantly reduced (p< 0.01). The non-observable effect concentration was 0.068 mg https://www.w3.org/1998/Math/MathML" display="inline"> UIA - N I - 1 , while the lowest observable effect concentration was 0.144 mg https://www.w3.org/1998/Math/MathML" display="inline"> UIA - N I - 1 . Increasing the un-ionised ammonia concentration increased the feed conversion ratio (FCR). At 0.144 mg https://www.w3.org/1998/Math/MathML" display="inline"> UIA - N I - 1 , the FCR increased by a factor of 1.6 of the value observed in the control, while at 0.262 mg https://www.w3.org/1998/Math/MathML" display="inline"> UIA - N I - 1 the FCR increased by a factor of 2.7. At 0.434 mg https://www.w3.org/1998/Math/MathML" display="inline"> UIA - N I - 1 , the FCR increased by a factor of 4.3. Protein efficiency ratio (PER) was also negatively correlated with the un-ionised ammonia concentration above 0.068 mg https://www.w3.org/1998/Math/MathML" display="inline"> UIA - N I - 1 . This study concluded that, for raising O. niloticus in fishponds fed on fresh duckweed, the toxic level of UIA-N and its negative effect on the growth performance, lies between 0.07 and 0.14 mg https://www.w3.org/1998/Math/MathML" display="inline"> UIA - N I - 1 . It is recommended that the un-ionised ammonia concentration be maintained below 0.1 mg https://www.w3.org/1998/Math/MathML" display="inline"> UIA - N I - 1 .