Abstract:Offshore aquaculture net pens have the characteristics of open spaces, vast waters, and natural environments. This is one of the important ways to expand aquaculture space, create a new production base for high-quality protein, and develop deep and offshore sea aquaculture. A sudden change in ambient water temperature may significantly affect the normal physiological status of fish as they are ectotherms. In the present study, respiratory frequency, serum cortisol and glucose concentrations, the hematological index, and hepatic hsp70 and hsp90 mRNA expression were determined during various thermal durations and recovery periods to illustrate the thermal tolerance and the underlying physiological response of two sizes of spotted knifejaw. The respiratory frequency of the two sizes of spotted knifejaw first increased and then decreased subject to thermal stress at 32℃, with the highest value (P<0.05) observed at 0.3 (20 min) and 1 h in 50 and 200 g fish, respectively. The serum cortisol and glucose concentrations of the two sizes of spotted knifejaw showed similar trends to the respiratory frequency. In 50 g spotted knifejaw, the serum cortisol concentration was significantly higher at 1 h (P<0.05) and the serum glucose concentration was significantly higher at 6 h (P<0.05) than that in the other treatment groups. However, in 200 g spotted knifejaw, serum cortisol and glucose concentrations remained high under thermal stress. Hematological data of the two sizes of spotted knifejaw were rather different. Specifically, the number of white blood cells (WBCs), red blood cells (RBCs), and hemoglobin (HGB) in the blood of 50 g spotted knifejaw first increased and then decreased and were significantly higher than that in the control group at 3 h (P<0.05). In 200 g spotted knifejaw, no significant changes in WBC, RBC, and hematocrit were noted (P>0.05), although HGB showed an initial increase, followed by a decreasing trend and was significantly higher than that in the control group at 3 h (P<0.05). The hepatic hsp70 and hsp90 mRNA expression of the two sizes of spotted knifejaw increased first and then decreased. As such, hsp70 mRNA expression was the highest at 3 h, while hsp90 mRNA expression was the highest at 1 h (P<0.05). In conclusion, acute thermal stress significantly increased the respiratory rate and serum cortisol and glucose concentrations of two different sizes of spotted knifejaw and upregulated hepatic hsp70 and hsp90 mRNA expression; all aforementioned parameters gradually recovered to normal levels after treatment at 25℃ for 12 h. Meanwhile, small spotted knifejaw manifested greater thermal tolerance. These results provide useful data for spotted knifejaw culture in offshore aquaculture net pens and can strengthen our understanding of the regulatory mechanism of spotted knifejaw temperature tolerance.
