Toxicity of ammonia and nitrite to silver perch (Bidyanus bidyanus)

Acutely toxic concentrations (LC\\(_{50}\\)) of ammonia and nitrite were estimated for juvenile silver perch (Bidyanus bidyanus) during 96h experiments. Additional experiments were conducted for at least 25 days to determine the growth-limiting concentrations of ammonia and nitrite for juvenile silver perch. Furthermore, the effects of exposure to metabolic and reagent ammonia were compared. Gill histopathology was examined in each experiment to determine if changes in gill structure correlated with exposure to toxicant, and whether gill histopathology was a useful indicator of sub-acute intoxication for this species. The acutely toxic concentration (LC\\(_{50}\\)) of un-ionised ammonia (UAN) was defined as the concentration at which silver perch lost the ability to orientate. At this point, fish were removed from the experiment, as preliminary research had shown that prolonged exposure beyond this point always led to death. The LC\\(_{50}\\) was estimated to be 1.2 mg L\\(^{-1}\\) UAN. The acutely toxic concentration (LC\\(_{50}\\)) of nitrite was estimated to be 160 mg L\\(^{-1}\\)NO\\(_2\\) -N. Following a 39 day exposure, growth (wet weight gain) of juvenile silver perch was reduced at concentrations of ammonia above 0.36 mg L\\(^{-1}\\) UAN. There was no significant difference in wet weight gain of control fish or those exposed to 0.02 mg L\\(^{-1}\\) metabolic ammonia. Growth of silver perch exposed to nitrite at concentrations above 1.43 mg L\\(^{-1}\\)NO\\(_2\\) -N for 25 days was reduced. Following acute exposure to ammonia, there was a significant increase in the percentage of filaments affected by epithelial lifting, but no other significant change in gill histopathology was observed. Acute exposure to nitrite resulted in no significant changes in gill histopathology, although macroscopic observation indicated browning and discolouration of gill tissue at concentrations above 130 mg L\\(^{-1}\\)NO\\(_2\\) -N. After 39 days' exposure to ammonia, there was an increase in the occurrence of epithelial lifting, while no other index of gill histopathology was significantly different. Exposure to metabolic ammonia also increased the percentage of filaments affected by epithelial lifting. Following 25 days' exposure to nitrite, there was a significant difference in the percentage of gill filaments affected by epithelial lifting and hypertrophy. These results indicate critical concentrations of ammonia and nitrite which will assist in the management of nitrogenous wastes in pond or tank culture of silver perch. Understanding of these concentrations may increase production and reduce the incidence of critical water quality conditions.