Abstract
Ontogenic change patterns in otolith Sr:Ca ratios were examined in amphidromous, landlocked (Lake Biwa population) and hatchery-reared ayu, so as to determine the usefulness of otolith microchemistry for discrimination between amphidromous and stocked non-amphidromous individuals. Otolith Sr:Ca ratios of amphidromous ayu fluctuated strongly along the life history transect in accordance with the migration pattern from sea to freshwater, via brackish water in the river mouth. On the contrary, the Sr:Ca ratios of Lake Biwa and hatchery-reared ayu remained at consistently low levels throughout the otolith. The higher ratios in amphidromous ayu, in the otolith region from the center to 220-400μm equivalent to the first 90-110 days from hatching, corresponded to the initial sea-going period, suggesting that otolith Sr:Ca ratios are affected by ambient water salinity or the seawater-freshwater gradient in Sr concentration. No relationship between changes in the Sr:Ca ratio and water temperature was found. The findings clearly indicated that otolith Sr:Ca ratios reflected individual life histories, enabling amphidromous ayu to be distinguished from stocked non-amphidromous ayu.
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