Effect of pH variation on survival of Litopenaeus vannamei reared in ponds and its physiological mechanisms

Generally, pH in the ponds of Litopenaeus vannamei farming exhibits long term trend as well as diel variation. Understanding the effect of pH variation on survival of L. vannamei is critical for improving water quality management in commercial white shrimp farming. In the present study, two independent experiments each with single factor design were conducted to evaluate the effects of pH level and amplitude of pH diel variation on survival of L. vannamei. Meanwhile, the physiological mechanisms were analyzed. In experimental I, four pH levels (6.6, 7.6, 8.6 and 9.6) were evaluated, and the average pH level (8.2) observed in commercial ponds was used as control. In experimental II, three amplitude of pH diel variation (7.5−8.2, 7.5−9.5, 7.5−10.0) were evaluated, and the average amplitude of pH diel variation (7.5−8.7) observed in commercial ponds was used as control. The results showed that pH level can significantly affect survival of L. vannamei, hemolymph cell count, and activities of superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), glutathionereductase (GR), catalase (CAT), phenoloxidase (PO) and inducible nitric oxide synthase (iNOS) in vivo. Mortality of L. vannamei significantly increased, while activities of SOD, GSH-Px, GR, CAT, PO and iNOS declined, when environmental pH climbed from 7.6 to 9.6. On the other hand, amplitude of diel pH variation can significantly affect survival of L. vannamei and activities of antioxidant enzymes (SOD, GSH-Px, GR and CAT), but did not result in significant change in glutathione content. Mortality of L. vannamei significantly increased, while activities of SOD, GSH-Px, GR and CAT decreased, when amplitude of diel pH variation was extended from 7.5−8.2 to 7.5−10.0. This study reveals that pH level and amplitude of diel pH variation are important environmental factors responsible for survival of L. vannamei, and stress induced by unsuitable pH should be the physiology mechanism since high mortality of L. vannamei was accompanied by the change in activities of SOD, GSH-Px, CAT and PO. According to the present study, it is recommended that proper pH control should be listed as an important content for environmental management in L. vannamei pond farming.