Objective  Chronic ammonia nitrogen stress on juvenile crucians were studied with observations on liver and kidney histology and non-specific immunity of the fish for an improved aquaculture water quality control.

  Method  Juvenile Carassius auratus with a body mass of (3.10±0.15) g were raised in an environmentally controlled pond. By an acute toxicity test on the fish, the semi-lethal (LC₅₀) and safe concentrations (SC) of ammonia nitrogen over 96 h were determined. Accordingly, a control at 0 mg·L⁻¹ and three treatments groups (i.e., low concentration L at 6 mg·L⁻¹, medium concentration M at 15 mg·L⁻¹, and high concentration H at 24 mg·L⁻¹) of ammonia nitrogen in water were applied to the pond with the juvenile fish for 60 d. Histopathological changes induced by the treatments in the liver and kidney as well as the alternations occurred to the activities of alanine aminotransferase (ALT), aspartate aminotransferase (AST), catalase (CAT), and superoxide dismutase (SOD) of the fish were monitored continuously on the 15^th, 30^th, 45^th, and 60^th days.

  Result  The acute ammonia nitrogen toxicity test on the fish showed the LC₅₀ of 289.29 mg·L⁻¹ and SC of 28.9 mg·L⁻¹. The microscopic examination revealed the increasing ammonia nitrogen concentration and stress duration caused symptoms in the fish included blurred and disorganized hepatocytes with some vacuolated and cell nuclei dissolved in hepatic tissue, tumefacient renal tubular epithelial cells, narrowed renal tubular lumen, and atrophied glomerulus in nephridial tissue. The ALT and AST activities in the liver and the kidney rose with increasing ammonia nitrogen in water. The ALT in the livers of fish in L and M groups did not significantly differ from control on the 15^th day (P＞0.05). On the other hand, the ALT and AST in livers and kidneys of all 3 treatment groups were significantly different from control at all testing times (P＜0.05). The specific activity of CAT and SOD in liver of the fish increased initially and decreased after a period of time of the treatments; that of CAT in treatment groups significantly differed from control on the 45^th day; that of SOD significantly differed on the 15^th day, and that of CAT in renal tissue significantly higher than that in control group on the same day; while that of SOD in all treatment groups significantly higher than that in control at all time periods.

  Conclusion  The stress of prolonged high concentrations of ammonia nitrogen in aquacultural water caused severe damage to the liver and kidney tissues in juvenile crucians. The histopathological injuries could result in malfunctioned metabolism, compromised detoxification capacity, and reduced non-specific immunity of the fish. Consequently, for a healthy and productive aquaculture, it was deemed imperative the ammonia nitrogen content in the water be continuously monitored and strictly controlled.