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

Abstract

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 un-ionised 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.