Acute toxicity of ammonia on juvenile cobia (Rachycentron canadum, Linnaeus, 1766) according to the salinity

Juvenile cobia (Rachycentron canadum) (total length 15.16 +/- 0.92 cm and weight 19.26 +/- 4.5 g) were exposed to different concentrations of ammonia-N (unionized plus ionized ammonia as nitrogen), using the static renewal method at different salinity levels of 5, 20, and 35% at pH 8.1 and 25 degrees C. The 24, 48, 72, 96 h LC50 values of ammonia-N for R. canadum juveniles were 60.28, 48.57, 37.42, 22.73 mg l(-1) at 35%; 51.25, 43.63, 28.17, 19.05 mg l(-1) at 20%; and 39.48, 25.31, 19.50, 8.13 mg l(-1) at 5%, respectively. The 24, 48, 72, 96 h LC50 values of NH3-N (unionized ammonia as nitrogen) were 1.81, 1.46, 1.12, and 0.68 mg l (1) at 35%; 1.75, 1.49, 0.96, and 0.65 mg l(-1) at 20%; and 1.52, 0.97, 0.71, and 0.31 mg l(-1) at 5%, respectively. As the salinity decreased from 35 to 5%, susceptibility of ammonia-N increased by 34.5, 47.88, 50.56, and 64.23% after 24, 48, 72, and 96 h exposure, respectively. Furthermore, we found that exposure of fish to ammonia-N caused an increase in oxygen consumption of 129.1, 157.5, and 192% and a decrease in the ammonia excretion level of 53.4, 38.2, and 23.3% with respect to the control.