Tolerance of larval and juvenile tambaqui (Colossoma macropomum) to nitrogenous compounds

Nitrogenous compounds are among the main toxic substances produced in aquaculture systems, and their tolerance limits vary among species and developmental stages. In this study, six acute bioassays were conducted to evaluate the 96 h median lethal concentration (LC_50-96 h) of total ammonia nitrogen (TAN = NH₃-N + NH₄⁺-N), non-ionized ammonia (NH₃-N), nitrite (NO₂^--N), and nitrate (NO₃^--N) in larval (0.05 ± 0.01 g) and juvenile (0.50 ± 0.3 g) tambaqui (Colossoma macropomum). Larvae were exposed in 0.5 L aquaria containing 20 larvae, whereas juveniles were exposed in 30 L tanks containing 10 fish, with three replicates per treatment and five concentrations, plus a control for each compound. Water quality, mortality, and behavioral alterations were monitored for 96 h. The estimated LC_50-96 h values for larvae were 6.02 mg L^-1 TAN, 0.17 mg L^-1 NH₃-N, 5.36 mg L^-1 NO₂^--N, and 122.78 mg L^-1 NO₃^--N. For juveniles, the estimated LC_50-96 h values were 32.52 mg L^-1 TAN, 0.60 mg L^-1 NH₃-N, 10.80 mg L^-1 NO₂^--N, and 405.24 mg L^-1 NO₃^--N. The main behavioral alterations observed in juveniles included lethargy, rapid and erratic swimming, surface breathing, loss of equilibrium, and muscle spasms. The results indicate that C. macropomum show high tolerance to nitrogenous compounds under the tested experimental conditions, although toxicity thresholds varied between developmental stages.