Acute toxicity of ammonia and nitrite for juvenile Pompano Trachinotus marginatus (Cuvier, 1832) at different salinities
The present study was conducted with the objective of estimating the acute toxicity of ammonia (N-NH3) and nitrite (N-NO2 -) for juvenile pampo Trachinotus marginatus (0.86 ± 0.21g) at different salinities: 5, 10 and 30‰. The juveniles were captured at Praia do Cassino (Rio Grande, RS), taken to the laboratory and acclimated to the desired salinities for 10 days. During this period they were overfed daily. The temperature and photoperiod were fixed at 24ºC and 12C: 12E, the dissolved oxygen concentration and pH were measured daily. Groups of five fish were exposed to five concentrations of ammonia (actual concentrations between 0.28 to 3.53 mg/L N-NH3) and nitrite (actual concentrations between 24.8-191.1 mg/L N-NO2-) for 96 h, plus a control group for each salinity (three repetitions each).The tests were conducted in a semi-static system with total daily renewal of the experimental media. The median lethal concentrations and their respective confidence intervals (95%) were estimated based on mortality data recorded at the different concentrations tested, using the Trimmed Spearman Karber Method software. The LC50 - 96 h of ammonia were: 0.66 (0.53-0.81; 5‰), 1.87 (1.65-2.12; 10‰) and 1.06 (0.94-1 , 20; 30‰) mg/L N-NH3. The LC50 - 96h for nitrite were 39.94 (36.39 - 43.84; 5‰), 116.68 (112.52-121.00; 10‰) and 37.55 (20.91-67.44 ; 30‰) mg/L N-NO2 -. The acute toxicity of ammonia and nitrite to the pampo was affected by salinity. The results of the present study showed that fields raised in an isosmotic environment are less sensitive to N-NH3 and N-NO2 -.Tolerance for ammonia is compromised at reduced salinity, while nitrite toxicity is similar at salinity 5 and 30‰.