Ecological study of the migration of eel by synchrotron radiation induced X–ray fluorescence imaging of otoliths

Synchrotron radiation–induced X–ray fluorescence imaging is suitable for determining the distribution of trace elements in fish otoliths. The Sr/Ca ratio in an otolith is an indicator of salinity changes and can be used to clarify the migration history of the eel, a catadromous fish. The otoliths of eel collected from the Tone and Elbe rivers exhibited a typical catadromous pattern, i.e. birth and breeding occurred in the ocean, but the remainder of their lives was spent in fresh water. In contrast, eels from the East China Sea and North Sea exhibited an unusual sea–locking phenomenon, as they appear to have remained in marine habitats throughout their lives.