Studies on the reproduction mechanism in the Japanese eel inferred by DNA analysis and otolith microchemistry.

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1. Analysis of population structure : Population structure analysis through examination of nucleotide sequences (nearly 6O0bp) of the control region in the mitochondrial DNA from a total of 200 glass eels collected in the same year at Taiwan, China, Tanegashima, and Miyagi Prefecture provided no evidence for genetic differentiation among populations. The same was the case for leptocephali samples, which might much reflect genetic characteristics of spawning aggregates. In order to examine genetic structure of glass eels much in detail in the nuclear DNA level, amplified fragment length polymorphism (AFLP) analysis was made for 30 glass eels collected from Miyagi, Kanagawa, and Tanegashima, and also showed no clear geographic differentiation. Altogether, the present DNA analyses of glass eels and leptocephali did not provide any evidence for genetic subdivision of Japanese eel population.2. Analysis of otolith microstructure : Otolith analysis of 150 glass eel collected from five localities in Japan at the beginning, mid and end of fishing season revealed a recruitment mechanism of glass eel : the individuals born earlier in the spawning season tended to recruit earlier to the East Asian estuaries in the lower latitude with the younger age and the larger growth rate than those born later. From EPMA analysis of otolith Sr, about 75% of silver eel collected in the coastal waters of Japan and East China Sea was estimated as sea eel that had never migrated into freshwater, suggesting that reproduction of the Japanese eel might be supported mostly by such sea eel.3. Conclusion : A possible large panmictic population of the Japanese eel suggested a international management of glass eel fishery among four countries in Ease Asia. Furthermore, the importance of estuaries as an eel habitat suggested the protection of the environment of this area and the adult eel to conserve the eel resources.