ACUTE AND CHRONIC TOXICITY OF AMMONIA TO JUVENILE METAPENAEUS-MACLEAYI AND PENAEUS-MONODON AND THE INFLUENCE OF LOW DISSOLVED-OXYGEN LEVELS

Acute toxicity of ammonia was estimated as 96-h LC50 values. For juvenile school prawns, Metapenaeus macleayi, and leader prawns, Penaeus monodon, these were 1.39 and 1.69 mg un-ionised ammonia, NH3-N/l (26.3 and 37.4 mg total ammonia-N/l), respectively. Reduced dissolved-oxygen (DO) levels significantly (P<0.05) increased the acute toxicity of ammonia to P. monodon. Ninety percent of prawns held for 96 h at a DO level of 2.3 mg/l and an ammonia concentration of 1.60 mg NH3-N/l (33.5 mg total ammonia-N/l) died, whereas only 33.3% died at a DO level of 5.7 mg/l and a similar ammonia concentration of 1.63 mg NH3-N/l (33.9 mg total ammonia-N/l). The "maximum acceptable" level of ammonia was defined as that which reduced growth by 5% over 3 weeks. For M. macleayi and P. monodon these levels were 0.35 and 0.21 mg NH3-N/l (7.7 and 4.1 mg total ammonia-N/l), respectively. © 1990.