Determination of the freshwater mark in otoliths of Japanese eel elvers using microstructure and Sr/Ca ratios |
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Authors: | Yutaka Kawakami Noritaka Mochioka Koichiro Morishita Hidetoshi Toh Akinobu Nakazono |
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Affiliation: | (1) Department of Fisheries, Faculty of Agriculture, Kyushu University, Hakozaki, Higashi-ku, Fukuoka, 812, Japan;(2) Present address: Department of Biology, Faculty of Fisheries, Hokkaido University, Minato, Hakodate, 041-0821, Japan;(3) Fukuoka Dental College, Ta, Sawara-ku, Fukuoka, 814-01, Japan |
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Abstract: | The microstructure, in particular checks within the otolith edge, of Anguilla japonica glass-eels and elvers and changes in otolith Sr/Ca ratios were examined to ascertain the environmental history of the eels, especially with regard to the time when glass-eels entered the river, and as a benchmark for count daily increments. The percentage of glass eels and elvers with checks and the mean number of checks within the otoliths of glass-eels caught at four localities, Tosa Bay off Tosa City, the mouth of the Gokase River, the mouth of the Saigo River and the dam of the Tsuri River were 0% (0), 15.0% (0.2), 51.6% (1.0) and 100.0% (4.2), respectively. The Sr/Ca ratios and Sr content peaked in the region where checks were formed and the values decreased rapidly towards the edge of the checks; on the other hand, these decreased gradually in the otolith when checks were not formed. These checks were estimated to be formed by stress when the glass-eels were affected by ambient fresh water within the river. The innermost check was called the freshwater mark in the present study and this mark may be useful as a benchmark in studying the growth history of the eel before and after entering freshwater. |
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Keywords: | Anguilla japonica check Sr incorporation physiological change |
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