Life history patterns of silver eels Anguilla japonica collected in the Sanriku Coast of Japan

The migratory history in the silver eel stage of the Japanese eel, Anguilla japonica, collected in the mouth of Kozuchi River along the Sanriku Coast of Japan, was examined using the otolith microstructure and analysis of strontium (Sr) and calcium (Ca) concentrations with wavelength dispersive X-ray spectrometry by an electron microprobe. The line analysis of Sr :Ca ratios along the life history transect of each otolith showed a peak (ca 12-15×10<-3>), which corresponded to the period of their leptocephalus and early glass eel stages in the ocean. The mean Sr : Ca ratios from the elver mark to the otolith edge indicated that there were eels with several general categories of migratory history, that included sea eels that never entered freshwater (average Sr : Ca ratios, >_6.0×10<-3>) and others that had entered freshwater for brief periods, but returned to the estuary or bay. This evidence of the occurrence of sea eels in this northern area indicates that Japanese eels of the Sanriku Coast do not necessarily migrate into freshwater rivers during recruitment as glass eels at the beginning of their growth phase, and even those that do enter freshwater may later return to the marine environment