Changes in otolith microchemistry of the Japanese eel, Anguitta japonica, during its migration from the ocean to the rivers of Taiwan

The newly recruited Japanese eel, Anguilla japonica, elvers and 1‐year‐old eels collected in estuaries and in rivers, respectively, were studied. The microstructure and chemical composition of the sagittal otolith of these eels were examined by SEM and wavelength‐dispersive spectrometer (WDS), A transition zone or‘elver mark’was observed in the otolith of the young eels. A comparison of the otoliths of elvers with those from the 1‐year‐old eels suggests that this transition zone was deposited during upstream migration, a change from a marine to freshwater environment. Strontium (Sr) content in the primordium of the otolith of both elvers and young eels was low, probably due to the maternal or freshwater origin of the oocyte. The concentration of Sr in the otolith increased gradually during marine life and reached a peak approximately 1 month before upstream migration. As the elvers entered the estuary, the Src concentration dramatically decreased and remained at a low level thereafter. These findings indicate that the history of the migratory environment of the eel can be reconstructed from a combined study of otolith microstructure and microchemistry analysis.