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

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【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.