Effect of dissolved oxygen and the molt stage on the acute toxicity of ammonia to juvenile green tiger prawn Penaeus semisulcatus

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Abstract
The mean 96‐h LC50 value of ammonia for juvenile Penaeus semisulcatus (0.35–2.4 g) was found to be 23.7 mg TAN/L (19.3–28.7, 95% C.L.). There was no significant effect of size on the sensitivity of juvenile shrimp to ammonia. Increased toxicity of ammonia to juvenile P. semisulcatus was observed at dissolved oxygen (DO) levels below 55% saturation (3.7 ppm). At 27% DO saturation, the ammonia toxicity (96‐h LC50) was doubled. In addition, the time of exposure to ammonia required for a given lethal effect decreased with reduced dissolved oxygen concentrations. Before and immediately after molt, the shrimps were more sensitive to ammonia.Due to the effect of the molt stage on the apparent ammonia toxicity, we recommend a minimum of 96‐h bioassay for determining toxicity levels in shrimps. In a shorter period, the physiological changes occurring at the time of molting can confound the results of the experiment.