Age Determination of Ayu with Otolith

The growth increments in otolith were validated to establish the age determination method of the ayu Plecogdossus altivelis an amphidromous salmonoidei fish with 1 year life span. Newly hatched larvae have ca. 5 embryonic increments in a circular sagitta of ca. 15μmradius. No mark peculiar to the time of hatch was detected in sagitta. Sequential samplings of known age fish from a rearing pond and those released in a river demonstrated that the days after hatching and the number of sagittal increments outside the 15μm radius presented close linear regressions with the slopes of 1.007 (1-150 days), 0.950 (120-180 days) and 0.871 (215-355 days), suggesting a daily deposition of the increment in sagittae. This was supported by the tetracycline marking of otolith in both larvae and juveniles. Ages of fish over 200 days old tended to be slightly underestimated, because the earlier in-crements arround the focus became invisible due to thickening and brownish coloration of otolith.accompanied by fish growth, and further because the occurrence of conspicuous opaque bands (checks) corresponding to the tentative cessation of increment formation increased in older fish. The error, however, did not exceed 5 days (2%) till 300 days old, and the age determination method was applicable to the ecological study of this species for almost the entire life cycle.