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

【利用SIMS技术估算日本鳗鲡产卵温度】建立了利用SIMS（二次离子质谱法）原位分析鱼类耳石中氧同位素比值（^<18>O/^<16>O）微分布的方法，并将其应用于估算日本鳗鲡产卵地点水温。氧同位素比值估算温度为16 ~ 28℃，对应于产卵区水柱深度为250 ~ 50 m。这表明，日本鳗鲡在北赤道洋流的水团中产卵，日本鳗鲡的幼鳗在其中被运输。【阿玉运移机理】通过耳石微观结构和微化学分析考察了阿玉的运移历史。阿育鱼幼虫刚孵化就向海中漂流，在SL（40日龄）大小为30 mm时迁移到冲浪区。至45 mm SL（135日龄）时开始向河口微咸水域迁移，至50 mm SL（160日龄）左右开始向上游迁移。较早迁徙到河流的幼鱼在SL中体型较大，产卵时间较晚的幼鱼出生。利用耳石Sr: Ca比值，证实了从河流产卵场采集的成熟阿鱼可区分为两相和非两相放养的阿鱼（琵琶湖陆生阿鱼）。在3条河流（宫河、三重河、长原河、奇阜河、首河、富山河）中发现，放养的碧和湖只在产卵期早期繁殖，占总产卵量的10- 30%，表明该河流与本地雌雄同体的碧和湖发生了杂交。

【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.