Critical dissolved oxygen level to Penaeus setiferus and Penaeus schmitti postlarvae (PL10-18) exposed to salinity changes.
A total of 200 post-larvae (PL10) of P. schmitti (2.3±0.1 mg) or P. setiferus (2.4±0.05 mg) were placed in 24-litre containers at a density of 25 PL/litre. Post-larvae of P. schmitti were maintained at 25±1°C, pH 8.1 and oxygen concentration >5 mg/litre; those of P. setiferus were maintained at 28±1°, pH 8.0 and oxygen concentration >5.0 mg/litre. Except for the first water change (38 to 30 ppt salinity for P. schmitti and 37 to 30 ppt salinity for P. setiferus), PL from both species were exposed to salinity changes at intervals of 5 ppt daily. In both species, the critical oxygen level (COL, estimated as the point of inflection of the curve obtained from the relation between oxygen consumption and oxygen concentration) was affected by salinity. In P. schmitti, COL was 5 mg/litre in salinities of 38, 30, 20 and 15 ppt, and 4.5 mg/litre for those kept at 25 ppt. In P. setiferus, COL was 5.0 mg/litre in salinities of 37, 30, 25, 20 and 10 ppt and 4.5 mg/litre in 15, 5 and 1 ppt. In P. schmitti, energy deficit (END, deficit of metabolic energy) caused by the metabolic oxygen critical concentration was 13.9 (38 ppt) and 26.3% (30 ppt), with intermediate values of 17.2, 22.7 and 24.7% in 25, 20 and 15 ppt, respectively. In P. setiferus, END was between 9.1 (1 ppt) and 25.1% (30 ppt), with intermediate values for the remaining salinities. It is concluded that optimum salinity for P. setiferus postlarvae is between 5 and 15 ppt, and 25 ppt for P. schmitti postlarvae. At these salinities the tolerance for the decrease in oxygen concentration was greater than in the other salinities. The high COL observed for the postlarvae of both species indicates that culture conditions must be carefully maintained at this developmental stage to obtain the maximum metabolic performance of the shrimp.