耳石輪紋によるワカサギの日齢査定

概要

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Daily otolith increments of the Japanese smelt Hypomesus transpacificus nipponensis, were validated to establish the age determination method for the population study and the early life history of this species. Artificially fertilized Japanese smelt was sequentially sampled from the age of 0 day to 145 days and the number of growth increments on sagittae was counted. The sagittal radius at hatching was about 10μm regardless of water temperature: i.e.9.81±0.73μm at 19.5°C, and 10.03±0.82μm at 10.0°C.The relation between the days after hatching (X) and the number of growth increments on sagittae over 10μm in radius (Y) was expressed by the linear regression of Y=0.966X+0.389(r=0.992, n=96, P<0.01).The slope is not significantly different from 1 (p>0.1), and thus the daily deposition of sagittal increments was confirmed. The maximum deviation from the mean day was 5 days (4.5%) at 145 days. The ageing rate (No. fish successfully aged/No. fish examined) gradually decreased to 50% at 145 days after hatching. Consequently, this method of age determination can be applicable for fish younger than 145 days of the age.