Affect of chronic and acute stress on growth and survival of hatchery-reared Japanese flounder juveniles

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1. Physiological and morphological response for chronic stressPost-settlement juveniles were reared for 3 months at three different rearing density. Plasma cortisol level of young fish was not different at the end of the experiment among three treatments but the nuclear area of interrenal cell of fish reared under overcrowded conditions became significantly larger. It suggested that chronic stress promoted metabolism of cortisol. To examine a change in capability of ACTH secretion for chronic stress, juveniles were reared for 12 weeks under overcrowded conditions without sand on the bottom. In this experiment, the nuclear area of ACTH secretion cell significantly increased in size. Thus, morphological change of juveniles was recognized for chronic stress at cell level.2. Response of juveniles for lethal acute-stressorsTo determine a tolerance to acute stress, flounder juveniles were exposed to the air. A mortality of juveniles which were exposed to the air for 15 and 20 minutes was about 65 % and 30 %, respectively. All individuals were dead by 30 minutes exposure to the air. All juveniles which were exposed to the air for 10 minutes once a day for five days before 15 minutes exposure were survived. However, a survival rate of fish which were not stressed before exposure was only 13. 3 %. As a cortisol elevation after exposure was not observed in survival juveniles, it was suggested that tolerance to acute stress was related to the cortisol response.3. Effect of handling at transportation on mortalitySmall size juveniles (35 mm TL) showed clear stress response (cortisol elevation ) for handling at transportation from grow-out facility to release area and mortality rate was high. It was suggested that transportation is one factor of mortality after release for small size fish.