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

Abstract

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(temperature26°Candsalinity22‰)andacutelyexposedtoammonia(0.25–1.30ppmNH 3-N)andnitrite(30–210ppm 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 NO 2-N. Even higherconcentrationswerenecessarytokilljuvenilecobia,LC50–96hforammoniawasestimatedat1.13(1.06–1.19)ppmNH 3-N, andwithintherangeofconcentrationstestedfornitriteitwasnotpossibletoestimatetheLC50–96h,asonly30%oftheindividuals died at the highest concentration after 96 h (210 ppm NO2-N). The results of the present experiments demonstrate that ammonia couldbeproblematicatrelativelylowlevelsfortheintensiverearingofjuvenilecobia;however,itisunlikelythatthehighlevelsof nitrite needed to harm juvenile cobia would be reached in a well designed and properly operating RAS.