Effects of acute air exposure on the hematological characteristics and physiological stress response of olive flounder (Paralichthys olivaceus) and Japanese croaker (Nibea japonica)

Abstract

We examined the effect of acute air exposure on the hematological characteristics and physiological stress responses of the olive flounder (Paralichthys olivaceus) and Japanese croaker (Nibea japonica). Fish were exposed to air for 30 s (30S), 90 s (90S), and 180 s (180S). The control group was not exposed to air after capture. Experiments were performed in triplicate. The water exchange rate was 32 times the water volume of the tank and dissolved oxygen was maintained at over 5 mg/L. After air exposure (AE), fish were kept in the experimental tanks for 24 h. Blood samples were taken from the fish at 0 h (before air exposure) and at 1, 6, and 24 h AE. We found that the olive flounders and the Japanese croakers differed in their primary (hormonal) and secondary (hydromineral and metabolic) responses to the air exposure. Cortisol levels were lower in the olive flounders than in the Japanese croakers. In the 180S group, the olive flounder cortisol levels were increased by 62-fold whereas the Japanese croaker cortisol levels were increased by 75-fold. The cortisol levels increases were associated with elevated glucose levels in both olive flounders and Japanese croakers. Olive flounders and Japanese croakers (except in 180S) had increased glucose levels that recovered to a stable level or showed a decreasing trend until 24 h AE. Several studies have reported on the resilience of flounders to short-term stress. This was confirmed in the present study by the fast recovery of elevated cortisol, glucose, and lactic acid levels that were induced by the stressor. Compared to the stress response in the olive flounder, the Japanese croaker exhibited a stronger response associated with the cortisol, glucose, and lactic acid levels