Migratory history and habitat use of Japanese eel Anguilla japonica in the Sanriku Coast of Japan

The migratory history of the Japanese eel Anguilla japonica collected in Otsuchi Bay and the adjacent stream along the Sanriku Coast of Japan was examined using otolith microchemical analysis of strontium (Sr) and calcium (Ca) concentrations. X-ray intensity maps of Sr concentration in the otoliths showed that all specimens generally had a high Sr core at the center of the otoliths, which corresponded to the period of leptocephalus and early glass eel stages in the ocean, while line analysis of Sr : Ca ratios along the life-history transect of each otolith showed a peak (approximate to 13-20 x 10(-3)) between the core and elver mark. The mean Sr : Ca ratios from the elver mark to the otolith edge indicated eels of several general categories of migratory history, including sea eels that never entered freshwater (average Sr : Ca ratios > 6.0 x 10(-3)) and estuarine eels that entered freshwater for brief periods, but returned to the estuary or bay (average Sr : Ca ratios 2.5-6.0 x 10(-3)). This evidence of the occurrence of several migratory patterns in this northern area indicates that A. japonica has a flexible pattern of migration, with an ability to adapt to various habitats and salinities. Thus, migrations of anguillid eel into freshwater are facultative and not obligatory.