Identification and growth rates comparison of divergent migratory contingents of Japanese eel ( Anguilla japonica)

The strontium (Sr) and calcium (Ca) concentrations in the otoliths of the Japanese eels Anguilla japonica collected from China, Japan and Taiwan were measured by electron probe micro-analyzer. The Sr/Ca ratios indicated that the eels beyond elver stage can be classified into three types of migratory contingents. Type 1 (seawater), the Sr/Ca ratios from approximately 150 μm from primordium to edge of the otolith maintained at the level of approximately 4–10‰, indicating that the eel after elver stage stayed in sea water until the silver eel stage. Type 2 (freshwater), the ratios were lower than 4‰, indicating that the eel stayed in freshwater from elver stage to the silver eel stags. Type 3 (estuarine), the ratios fluctuated between those of Types 1 and 2, indicating that eel migrated between freshwater and sea water before the silver stage. The estuarine contingents constituted the majority of the eel population and grew faster than the freshwater contingents.