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

The effects of prolonged exposure to sub-lethal un-ionised ammonia nitrogen (UIA-N) concentrations on the growth performance of juvenile Nile tilapia (Oreochromis niloticus) fed on fresh duckweed (Lemna gibba) grown on pretreated domestic sewage have been investigated. The experiment was conducted 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 N l(-1)) were established as treatment groups. Statistical analysis of the specific growth rate (SGR) showed no significant (p>0.05) differences between the SGR (0.71) of tilapia in the control (0.004 mg UIA-N l(-1)) and the SGR (0.67) of those exposed to 0.068 mg; UIA-N l(-1). The SGR of tilapia exposed to un-ionised ammonia nitrogen over 0.068 mg UIA-N l(-1) (0.144, 0.262 and 0.434 mg UIA-N l(-1)) was significantly reduced (p < 0.01). The no-observable effect concentration was 0.068 mg UIA-N l(-1), while the lowest observable effect concentration was 0.144 mg UIA-N l(-1). Increasing the unionised ammonia concentration increased the feed conversion ratio (FCR). At 0.144 mg UIA-N l(-1), the FCR increased by a factor 1.6 of the value observed in the control, while at 0.262 mg UIA-N l(-1) the FCR increased by a factor of 2.7. At 0.434 mg UIA-N l(-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 UIA-N l(-1). This study concluded that, for raising Nile tilapia in fishponds fed on fresh duckweed or other feed, the toxic level of UIA-N and its negative effect on the growth performance lies between 0.07 and 0.14 mg UIA-N l(-1). It is recommended that the UIA-N concentration be maintained below 0.1 mg l(-1). (C) 2004 Elsevier B.V. All rights reserved.