Abstract:Dissolved oxygen is an important environmental factor affecting the growth and survival of aquatic organisms, including fish. Hypoxia has gradually become common in aquatic ecosystems and poses a significant challenge for fish farming. A decrease of dissolved oxygen levels in the body or a lack of oxygen will lead to a severe stress response for fish, hindering the development of the aquaculture economy. Dissolved oxygen is an important environmental factor that affects the aquaculture production of spotted knifejaw (Oplegnathus punctatus). The solubility of oxygen decreases with the increase of temperature, and the solubility of gaseous substances decreases with the increase of temperature and pressure. These show significant seasonal variation: The higher the temperature, the smaller the gap between the water molecules, and the lower the dissolved oxygen. Fish in a low-oxygen environment breathe normally and this can lead to physiological metabolic disorders, hence affecting fish behavior and physiological and biochemical indicators. However, in the process of evolution, fish have also developed different response modes and adaptive regulation mechanisms to maintain normal physiological functions. To clarify the changes in physiological and biochemical indices in blood during hypoxic tolerance and hypoxic stress, physiological and ecological methods were used in this study. The solubility of dissolved oxygen under two different specifications of critical oxygen tension (Pcrit) and loss equilibrium (LOE) were investigated, and the changes in respiration behavior under natural oxygen consumption were observed. Changes in cortisol and glucose, concentration of hemoglobin (Hb), number of red blood cells (RBC) and white blood cells (WBC), and hematocrit (HCT) during hypoxic stress and recovery were analyzed. Results showed that in a water temperature of (23.0±0.5)℃, the ammonia nitrogen concentration was less than 0.5 mg/L, nitrite concentration 0~0.05 mg/L, salinity 30 and pH 7.80; the dissolved oxygen concentrations at Pcrit were (4.05±0.09) mg/L and (3.15±0.12) mg/L for 200 g and 50 g O. punctatus, respectively. The concentrations of dissolved oxygen at LOE value were (1.16±0.08) mg/L and (0.93±0.11) mg/L. The time until the 50 g O. punctatus reached LOE (t=480 min) was longer than that of the 200 g O. punctatus (t=110 min). In addition, there were significant changes in respiration rate between hypoxia and re-oxygenation in both the 200 g and 50 g O. punctatus. In the process of natural oxygen consumption, the respiration rate of the two first increased and then decreased, and increased significantly at the Pcrit value. When the dissolved oxygen concentration decreased to 1.27 mg/L (420 min) and 1.10 mg/L (90 min), respiration rates of the 50 g and the 200 g fish reached their maximum (P<0.05). After recovery in normal dissolved oxygen for 24 h, the respiration rate did not differ significantly between the two sizes of O. punctatus nor with the control group. Plasma glucose and cortisol showed similar results to the respiratory rates; hypoxic stress led to significant increases and the highest values were obtained at LOE (P<0.05). The plasma cortisol concentration was the same in the two sizes of fish. While the values for the 50 g O. punctatus were significantly lower than those of the 200 g fish during hypoxic stress and re-oxygenation (P<0.05), the glucose and cortisol did not differ significantly from the control after recovery in normal dissolved oxygen for 24 h. Hypoxic stress had a significant effect on the blood biochemical indexes of O. punctatus. At Pcrit and LOE values, the number of WBCs, RBCs, Hbs and HCT, at 50 g were significantly higher than those in the control group (P<0.05), and the maximum values were at the Pcrit value. The concentrations of the four indexes were significantly increased in the 200 g O. punctatus compared with the control group (P<0.05). The maximum values of WBC, HCT, and Hb were reached at LOE, and the maximum values of RBC were reached at Pcrit. After recovery in normal dissolved oxygen for 24 h, the two sizes of O. punctatus showed normal swimming behavior and respiratory activities, and the above physiological and biochemical indexes did not differ significantly from those of the control group (P<0.05). Lastly, the body weight of O. punctatus differed during hypoxia-induced hematological and biochemical responses. In O. punctatus, the 50 g fish were more tolerant to hypoxic environments than the 200 g, while the 200 g fish were more sensitive to hypoxic stress. Both sizes of O. punctatus could enhance the absorption and utilization of dissolved oxygen by increasing their respiratory rate, increasing the number of WBCs and RBCs, and increasing the concentrations of cortisol, glucose, and Hb in response to hypoxia stress. In this study, the tolerance to hypoxia and the physiological responses of different sizes of O. punctatus were elucidated; their adaptability to low dissolved oxygen was explored, and their tolerance threshold to hypoxia was determined. The results provide data for land-sea relay breeding of O. punctatus, a theoretical basis for efficient land-sea relay, and an early warning range of hypoxia for breeding O. punctatus, to help reduce economic losses caused by decreased oxygen.
