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This study aimed to evaluate the effects of nitrate (NO3–N) in juvenile mullet in freshwater. Experiment 1 evaluated the LC50‐96h of fish at five different NO3–N concentrations (0.01, 85.32, 462.12, 851.25 and 955.25 mg L−1). Experiment 2 evaluated haematological and oxidative stress variables in fish exposed to four NO3–N concentrations (0.01, 8.16, 33.83 and 52.66 mg L−1) for 5 days. The safe concentration for 96 h was 60.61 mg NO3–N L−1. In experiment 2, blood glucose and haematological values were lower in fish subjected to 52.66 mg NO3–N L−1 compared with the other treatments on day 1. On day 5, fish subjected to the two highest NO3–N concentrations had blood glucose and erythrocytes levels as well as gills and liver ACAP values significantly higher than the control group. Fish exposed to 52.66 mg NO3–N L−1 showed reduced GST activity in gills, liver and muscle on day 5 compared with day 1. In general, exposure to nitrate did not induce an increase in the TBARS levels in the tissues evaluated. In conclusion, acute exposure to 52.66 mg NO3–N L−1 impacted the physiological homeostasis of mullets, but it did not cause lipid peroxidation.

The objective of this study was to determine the effect of different waterborne nitrite (NO 2 -) and calcium (Ca 2+ ) levels on growth, biochemical and hematological parameters of silver catfish juvenile (Rhamdia quelen). Fish were submitted to low (0.05 mg L -1 ) or high (1.3 mg L -1 ) NO 2 and low (7 mg L -1 ) or high (14 mg L -1 ) Ca 2+ levels (four replicates) for 60 days. At the end of the experimental period, fish exposed to high NO 2 showed lower weight gain, biomass and specific growth rate than those maintained at low NO 2 , irrespective of Ca 2+ levels.