| Research Abstract |
To determine the early life history of the Japanese eel, otolith microstructure and microchemistry were investigated. Results obtained are follows :
1.Birthdate of the Japanese eel leptocephali sampled near the spawning area (15ﾟN,137ﾟE) in the Philippine Sea 1991 showed a bimodal distribution with 2 weeks interval suggesting that this species spawn periodically.
2.Hatch check of artificially obtained larvae was smaller (9mum) in diameter than that of wild specimen (12mum), which suggested that the wild had a longer incubation time than the reared fish (40hrs). The wild also had some embryonic rings (1-6), while the reared had no ring.
3.Observation of otolith microelements by ICP method detected Ca, Sr, Zn, Fe, Mg, Mn, Al in sagitae of the eel. Ca content was highest (40%), and Sr was the next (0.1-0.3%) contents of Sr varyed among species corresponding to their migratory pattern ; freshwater fishes was about 0.1%, marine fishes 0.2%, and their intermediate values in diadromous fishes.
4.Fluorescent X ray Imaging by syncrotoron showed that the eel otolith generally had a hight concentration of Sr at otolith core. On the other hand, the eels sampled at Danjyo Gunto Is. in the East China Sea showed a high Sr concentration at not only central part but also whole area of the otolith. Based on the difference in Sr content this suggested that these specimens of ambient water between seawater and freshwater, did not show an ordinary migratory patterns, but remained continuously in the sea water after hatching.
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