1Department of Fisheries, School of Agriculture, Kinki University, Nara, Nara 631-8505, 2Fisheries laboratory, Kinki University, Nachikatsuura, Wakayama 649-5145, Japan
Changes in tolerances to certain environmental stressors, e.g., low and high temperatures and salinity as well as high concentrations of ammonia, were studied in the red sea bream Pagrus major during its early life stage up to 42 days post-hatch. Relative tolerance was assessed by determining 24-h median lethal level. In laboratory experiments, metamorphosis was observed mainly from days 14-21, and the morphological shift from larva to juvenile was indicated at 9.5mm total length at 28 days post-hatch. Among the stages tested, larva immediately after hatching showed maximum tolerance to all stressors. Tolerances to both low and high temperatures decreased up to 14 days post-hatch and increased thereafter. Hypo- and hyper-salinity tolerances also decreased up to the 14th day post-hatch and then showed a gentle increase. In regard to ammonia tolerance, the 50% lethal concentration diminished drastically between days 2 and 7, showed the lowest level on day 21, and subsequently rose. These results indicate that, in red sea bream under these experimental conditions, all tolerances to temperature, salinity, and ammonia stress temporarily fell during metamorphosis from days 14-21. It is considered that the depression of the scope for activity based on increasing oxygen consumption induced the decrease in various stress tolerances during metamorphosis. It is also suggested that the 50% lethal levels of various environmental stressors can be used as a stress test to evaluate the hardiness of a hatchery-produced red sea bream.