Hypoxia tolerance and alternation of hematology and gill morphology in black rockfish (Sebastes schlegelii)

Dissolved oxygen is an important environmental factor affecting the survival of fish. In this study, the value of dissolved oxygen at critical oxygen partial pressure (Pcrit) and loss of equilibrium (LOE), respiratory rate, plasma cortisol, glucose, white blood cell (WBC), red blood cell (RBC), hemoglobin (Hb), hematocrit (HCT) and the alternation of gill morphology and related parameters lamellar length (SLL) and width (SLW), interlamellar distance (ID) and perimeter as well as the proportion of secondary lamellae available for gas exchange (PAGE) were observed during hypoxia to illustrate hypoxia tolerance, physio-biological response and morphological changes in the gills of Sebastes schlegelii during hypoxia stress. Results showed that the value of dissolved oxygen at Pcrit and LOE of S. schlegelii at (88.01±5.34) g were (3.96±0.11) mg/L and (2.60±0.21) mg/L respectively under the condition of water temperature (15.6±0.2) °C, pH value 7.85, and salinity 30.0. The PAGE and respiratory frequency increased first and then decreased, the highest value respectively obtained at Pcrit and LOE (P<0.05) throughout hypoxia stress. Meanwhile, plasma cortisol and glucose levels significantly increased, the highest values observed at LOE andPcrit (P<0.05). In blood physiology, Hb, HCT, WBC similar results like plasma cortisol, with the highest value at LOE (P<0.05), whereas RBC remained unchanged during hypoxia stress. Besides, the SLL, ID, Perimeter of gills increased significantly during hypoxia stress, and reached the maximum value at LOE point (P<0.05) either; the SLW decreased and reached the minimum value at the LOE (P<0.05). At the same time, the clubbing, hypertrophy, hyperplasia and epithelial cell edema were observed in the secondary lamella. All the aforementioned parameters recovered to normal levels after re-oxygenation 24 h inS. schlegelii. In conclusion, S. schlegelii are sensitive to low dissolved oxygen, have numbers of changes in hematology and morphology for hypoxia stress and reduce hypoxia stress after reoxygenation treatment for 24 h. These findings expand current knowledge on hypoxia tolerance and help in the management of S. schlegeli in captivity.