Critical levels of low pH for juvenile Penaeus monodon (4.2-5.5 g average weight), were determined using static bioassays with seawater acidified by using hydrochloric acid (HCI). The lethal pH (96 h LC50) was 3.7 (95% confidence limits; 3.4 and 4.1 ) at a salinity of 32 parts per thousand. The minimum acceptable pH, defined as that pH which reduced growth by 5% over 23 days, was estimated as 5.9 al a salinity of 30 parts per thousand. In comparison to a pH of 7.8, long-term (23 days) exposure to low pH (4.9) at 30 parts per thousand salinity also significantly decreased dry matter content of the prawns (P < 0.001) and increased moulting frequency (P < 0.05). For prawns held at different pH (5.5 or 7.8) and salinity ( 15 or 30 parts per thousand combinations, growth was reduced by low pH (P < 0.001) but was unaffected by salinity (P > 0.05) while the pH x salinity interaction was significant (P < 0.05). In a separate factorial experiment with prawns held at different pH (5.6 or 7.8) and salinity ( 15 or 30 parts per thousand) combinations, haemolymph osmoregulation was worse at high salinity (P < 0.001 ) and low pH (P < 0.01), compared with low salinity and high pH and there was no significant interaction (P > 0.05 ). The estimation of lethal and minimum acceptable low pH values should assist prawn farmers with the management of acid ponds.
