Tolerance of juvenile pompano Trachinotus marginatus to acute ammonia and nitrite exposure at different salinity levels

The present study was conducted to estimate the acute toxicity of unionized ammonia–nitrogen (NH3–N)and nitrite–nitrogen (NO2–N) to juvenile pompano Trachinotus marginatus (0.86±0.21 g) at different salinity levels: 5, 10 (equivalent to its isosmotic point), and 30‰. Fish were acclimated to the different salinities for 10 days and fed ad libitum daily. Groups offivefishes were exposed tofive concentrations of NH3–N and NO2–N for 96 h plus control groups for each salinity, where no toxicant was added. Test concentrations ranged from 0.28 to 3.53 mg NH3–N/L and 24.8 to 191.1 mg NO2–N/L with three replicates per treatment. Tests were run using a standard semi-static system with 100% daily renewal of water and toxicants. The results were based on mortality data registered in different concentrations tested, using the software Trimmed Spearman Karber method. The median lethal concentrations (LC50) after 96 h of exposure to NH3–N were 0.66 (0.53–0.81),1.87 (1.65–2.12) and 1.06 (0.94–1.20) mg NH3–N/L for 5,10, and 30‰. The 96 h LC50 to NO2–N were 39.94 (36.39–43.84),116.68 (112.52–121.00) and 37.55 (20.91–67.44) mg NO2–N/L for 5,10, and 30‰. Acute toxicity of NH3–N and NO2–N to pompano was affected by salinity. Results of the present study show that pompano reared at an isosmotic environment are less sensitive to NH3–N and NO2–N. Tolerance to NH3–N is compromised at reduced salinities, while toxicity of NO2–N is similar at 5 and 30‰