Effects of long-term low-concentration nitrite exposure and detoxification on growth performance, antioxidant capacities, and immune responses in Chinese perch (Siniperca chuatsi)

Few studies have investigated the effects of long-term low-concentration nitrite exposure and detoxification on growth and physiology of fish. Considering this, the present study focuses on the effect of nitrite exposure and detoxification on Chinese perch. Chinese perch were exposed to water with 0 mg/L (C group) and 0.3 mg/L nitrite (N group) for 30 and 60 days, respectively (C30, C60, N30, N60). No significant differences in the average daily growth (ADG), food intake (FI), feed conversion ratio (FCR), and histology were observed between C30 group and N30 group, while N30 group exhibited significantly lower protein retention and crude protein content and significantly higher levels of blood urea nitrogen (BUN) and higher activities of superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH-Px) than C30 group. The ADG, FL plasma glucose, total triglyceride, total cholesterol, the activities of SOD, CAT and GSH-Px were significantly decreased in N60 group, whereas the levels of methemoglobin, BUN, and immune responses-related indicators (globulin, aspartate aminotransferase, and alanine aminotransferase) were significantly increased. The damages to the gill and liver of fish and the increased expressions of pro-inflammatory cytokines genes (IL-1 beta, p65-NF-kappa B, TNF alpha and PAI) were observed in N60 group. In addition, fish exposed to nitrite for 30 and 60 days were detoxified in nitrite-free water for 30 days (DN30 and DN60). The detoxification significantly increased the protein retention and protein content. However, detoxification did not improve the appetite or fully restore histopathological injury in DN60 group. Those results indicated that 30-day exposure to 0.3 mg/L of nitrite promoted protein catabolism for providing energy to activate the defense, and that the detoxification could completely reverse the effects. The 60-day exposure to nitrite could disrupt defense system, cause histopathological damage, and affect energy metabolism. Moreover, nitrite exposure significantly decreased appetite. Above factors might be responsible for growth inhibition. The detoxification post 60-day nitrite exposure only restored partial physiological functions. Our findings provide a reference for the concentration control of nitrite in feed farming of Chinese perch.