Joint Action of Ammonia and Nitrite on Tiger Prawn Penaeus monodon Postlarvae

Tiger prawn Penaeus monodon postlarvae (PL6) were exposed to a mixture of ammonia and nitrite by the static renewal method. When the toxicity ratio of the ammonia‐nitrite mixture was 1:1, the 48 h, 72 h and 96 h LC50 values were 2.20, 1.43 and 0.84 toxicity units which were equivalent to 12.66 mg/L ammonia‐N (0.60 mg/L NH3‐N) plus 14.91 mg/L nitrite‐N, 8.23 mg/L ammonia‐N (0.39 mg/L NH3‐N) plus 9.69 mg/L nitrite‐N, and 4.83 mg/L ammonia‐N (0.23 mg/ L NH3‐N) plus 5.69 mg/L nitrite‐N. When the toxicity ratio of the ammonia‐nitrite mixture was 51, the 48 h, 72 h and 96 h LC50 values were 2.50, 1.56 and 0.85 toxicity units which were equivalent to 23.98 mg/L ammonia‐N (1.13 mg/L NH3‐N) plus 5.65 mg/L nitrite‐N; 14.96 mg/L ammonia‐N (0.71 mg/L NH3‐N) plus 3.52 mg/L nitrite‐N, and 8.15 mg/L ammonia‐N (0.39 mg/ L NH, N) plus 1.92 mg/L nitrite‐N. A mixture of ammonia and nitrite exerted greater toxicity than high concentrations of either ammonia or nitrite alone. The combined effect of ammonia and nitrite on P. monodon postlarvae was antagonistic for 48 and 72 hours exposure, but synergistic after 96 hours exposure.