A comparison of the physiological responses to heat stress of juvenile and adult starry flounder (Platichthys stellatus)

Water temperature is the most important factor in fish farming as changes in water temperature cause physiological stress in fish. There have been few studies on physiological responses to high temperatures, which vary with aging. This study investigated the responses of juvenile and adult Platichthys stellatus to heat stress. Plasma cortisol, glucose, lactate, and lysozyme levels, antioxidant enzyme activities, and expression of heat shock proteins 70 and 90 (HSP70 and HSP90) of P. stellatus were determined at water temperatures of 16, 20, 24, 28, and 30°C. As a result, it was confirmed that several plasma parameters of adult were significantly higher than that of juvenile under heat stress. Plasma cortisol and glucose levels of adult were increased than juvenile at 24 and 28°C. Plasma lactate level of adult were higher than that of juvenile at 28°C. Comparisons of survival and physiological changes showed that juveniles have better thermal tolerance, resulting in a higher cumulative survival rate. Moreover, the relationship between thermal tolerance and HSP gene expression revealed that expression of HSP70 and HSP90 was significantly upregulated at 28°C in both juvenile and adult fish, and HSP70 expression was significantly higher in juvenile fish than in adult fish. It is judged that the adult's HSP70 activity was lower than juvenile, so the demand for plasma parameters for heat response was relatively high, whereas juvenile's HSP70 activity increased at 24 and 28°C, indicating a relatively stable value of plasma parameters. These results indicate that the thermal tolerance of juvenile fish is greater than that of adult fish, based on the differences in plasma parameters and HSP expression. These findings improve our understanding of age-related changes in P. stellatus during thermal stress and may help guide the management of fish farms