Ontogeny of tolerance to hypoxia and oxygen consumption of larval and juvenile red sea bream, Pagrus major

Changes in tolerance to hypoxic stress and oxygen consumption were studied in the red sea bream, Pagrus major, from its early life stage until 42 days post-hatch. In the experiments, metamorphosis was observed mainly from days 15 to 30, and the morphological shift from larva to juvenile was completed at around 9.5 mm total length (TL). During the larval stage, lethally low dissolved oxygen (DO) levels and mass-specific metabolic rates increased with growth from 2.6 to 5 mm TL (P<0.01). Subsequently, the levels remained high and decreased until about 9.5 mm TL around the flexion stage and post flexion stage. Finally, beginning in the juvenile stage, lethal DO levels and mass-specific metabolic rates decreased as TL increased up to about 30 mm (P<0.01). The relationship between lethal DO levels and mass-specific metabolic rates was significantly linear (r=0.59, p<0.001, n=207) in fish larvae and juveniles. These results indicated that, around the stage of flexion and post flexion larvae in red sea bream, metabolic rates were highest during metamorphosis, and consequently hypoxia tolerance was lowest. It was presumed that the increasing metabolic rate during metamorphosis induced a decrease in the metabolic scope of activity and thereby induced the decrease of the tolerance to some environmental stressors in the background.