Migratory history of fishes as revealed by otolith Sr : Ca ratio and oxygen isotopic composition

[用SIMS估算鳗鱼产卵温度]我们建立了用二次离子质谱仪(SIMS)原位分析鱼类耳石中氧同位素比值的微观分布的方法，并将其应用于鳗鱼产卵地水温的估算。根据氧同位素比值估算的温度为16~28℃，与产卵区水柱深度250~50m相对应。这些结果表明，日本鳗在北赤道洋流的水团内产卵，日本鳗幼体在此水团中运输。[阿鱼的迁徙机制]用耳石显微结构和微化学方法研究了阿鱼的迁徙历史。阿育幼体孵化后即漂流到海面，在SL(40日龄)30 mm大小的冲浪区迁徙。它们在达到45 mm SL(135日龄)后开始向河口微咸水区迁徙，在50 mm SL(160日龄)前后开始上游迁徙。较早迁徙到河中的幼鱼往往比迁徙较晚的幼鱼在SL中体型较大，产卵时间较早。用耳石锶钙比确定了河边产卵场的成熟阿鱼为两栖和非两栖放养阿鱼(碧洼湖陆生阿鱼)。在三条河流中(米亚河，Mie Pref.；岐阜县那加拉河。；首河，富山首选。)放养的碧瓦湖鱼鱼只在产卵早期才对繁殖有贡献，放养的鱼群占总产卵量的10-30%，这表明碧瓦湖鱼鱼与当地的两栖动物在河流中发生了杂交。

【Estimation of spawning temperature of the Japanese eel by otolith oxygen isotopic ratio using SIMS】 We established the methodology for in situ analyses of micro-distribution of oxygen isotopic ratios (^<18>O/^<16>O) in fish otolith using SIMS (secondary ion mass spectrometry), and applied it for estimating the water temperature of spawning site of the Japanese eel. The temperature estimated from oxygen isotopic ratio was 16-28 ℃, which corresponded to the depth of 250-50 m in the water column at the spawning area. These suggest that the Japanese eel spawns within the water mass of North Equatorial Current, in which the Japanese eel larvae are transported.【Migration mechanism of Ayu】 Migration history of ayu was examined by otolith microstructure and microchemistry. The ayu larvae drifted down to the sea just after hatching, migrating to surf zone at the size of 30 mm in SL (40 days old). They began to migrate to brackish water area in river mouth after they reached 45 mm SL (135 days old), starting upstream migration around 50 mm SL (160 days old). The juveniles which migrated to the river earlier tended to be larger in SL and born at earlier spawning time than those migrated in later time. The mature ayu collected from spawning ground in river was confirmed to be distinguished between amphidromous and non-amphidromous stocked ayu (land-locked ayu from Biwa Lake) using otolith Sr : Ca ratio. In three rivers (Miya river, Mie pref. ; Nagara river, Gifu pref. ; Shou river, Toyama pref.) stocked Lake Biwa ayu was found to contribute to the reproduction only in early spawning season with 10-30 % of total spawning ayu being stocked ayu, suggesting that interbreeding between Lake Biwa ayu and native amphidromous one occurred in river.