Effects of pH and salinity on survival, growth and osmoregulation in Penaeus monodon Fabricius

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 ( HCL ). The lethal pH (96 h LC 50) was 3.7 (95% confidence limits; 3.4 and 4.1 ) at a salinity of 32‰. The minimum acceptable pH, defined as that pH which reduced growth by 5% over 23 days, was estimated as 5.9 at a salinity of 30&permil;. In comparison to a pH of 7.8, long-term (23 days) exposure to low pH (4.9) at 30‰ 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‰) combinations, growth was reduced by low pH ( P < 0.001) but was unaffected by salinity ( P > 0.05) while the pH × 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‰) 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.