Acute toxicity and sublethal effects of ammonia and nitrite for juvenile cobia Rachycentron canadum

Nitrogenous compounds can be toxic to aquatic animals especially when they are reared at high stocking densities. Cobia (Rachycentron canadum) is a fast growing fish currently reared in cages, but with expanding production in intensive recirculating aquaculture systems (RAS). Therefore, the objective of this study was to evaluate the acute toxicity of ammonia and nitrite to juvenile cobia. Juveniles (1.74 ± 0.11 g for ammonia and 0.88 ± 0.06 g for nitrite toxicity evaluation) were acclimated to test conditions (temperature 26 °C and salinity 22‰) and acutely exposed to ammonia (0.25–1.30 ppm NH3-N) and nitrite (30–210 ppm NO2-N) at 0.2 fish L− 1. Tests were run in 50 L semi-static tanks, experimental water was fully renewed daily, and all test concentrations plus the controls were run in triplicate. Mortality, feeding and swimming behavior were observed during 96 h, toxic concentrations for 50% the population and the respective 95% confidence intervals for these three end points were estimated using the Trimmed Spearman Karber Method. Cobia ceased to eat at 0.62 (0.56–0.70) ppm NH3-N and 76.1 (73.2–79.0) ppm NO2-N. Swimming behavior was affected at higher concentrations: 0.80 (0.74–0.85) ppm NH3-N and 88.8 (82.6–95.5) ppm NO2-N. Even higher concentrations were necessary to kill juvenile cobia, LC50–96 h for ammonia was estimated at 1.13 (1.06–1.19) ppm NH3-N, and within the range of concentrations tested for nitrite it was not possible to estimate the LC50–96 h, as only 30% of the individuals died at the highest concentration after 96 h (210 ppm NO2-N). The results of the present experiments demonstrate that ammonia could be problematic at relatively low levels for the intensive rearing of juvenile cobia; however, it is unlikely that the high levels of nitrite needed to harm juvenile cobia would be reached in a well designed and properly operating RAS.