The impact of elevated water ammonia concentration on physiology, growth and feed intake of African catfish (Clarius gariepinus)

The threshold concentration for NH3 in rearing water of African catfish (Clarias gariepinus) was assessed. African catfish with an initial mean (SD) weight of 141.0 (24) g were exposed to five different Tamm [sum of NH3 and NH4+] concentrations: 0.37 (Control), 1.06, 2.12, 5.16 and 19.7 mM, which concurs with NH3 concentrations of 4 (Control), 14, 38, 176 and 1084 µM. Plasma concentrations of NH4+, cortisol, glucose and lactate, plasma osmolality, gill morphology, branchial Na+/K+-ATPase activity, feed intake and specific growth rate were monitored. No effect of water NH3 on plasma NH4+ concentrations was detected. Feed intake and specific growth rate were severely affected at exposure to water NH3 concentrations above 90 µM (calculated EC10 values: 89 and 122 µM). No major disturbances in physiological blood parameters were observed at these NH3 concentrations, but gill morphology (a remarkably sensitive stress indicator) deteriorated significantly. Based on the lower limit of the 95% confidence interval for EC10, we advise for African catfish not to exceed a water NH3 concentration of 24 µM (0.34 mg NH3-N/L). This finding is relevant for design and management of African catfish production systems.