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The age and migratory history of the Japanese eel, Anguilla japonica, collected along the Sanriku Coast of Japan, were examined using otolith microstructure and analysis of strontium (Sr) and calcium (Ca) concentrations. 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, including sea eels that never entered freshwater and others which had entered freshwater for brief periods but returned to the estuary or bay. This first 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.

Organotin compounds (OTs) and representative booster biocides were measured in sediment and mussels from Otsuchi Bay, Japan. The mean amounts of tributyltin (TBT) and triphenyltin (TPT) compounds in sediment were 13 microg kg(-1) dry and 3 microg kg(-1) dry, respectively. Representative booster biocides (Sea-Nine 211, Diuron, Dichlofluanid, Irgarol 1501, M1, which is a degradation compound of Irgarol 1051, and Copper pyrithione) were also detected in sediment from Otsuchi Bay. OT concentrations were higher than those of the measured booster biocides. Otsuchi Bay was divided into four parts by cluster analysis based on OT concentrations in sediment sampled from the bay. These areas included the vicinity of a shipyard, a small fishing port, the closed inner area of the bay, and the mouth of the bay. Higher concentrations of TBT and TPT and a higher ratio of TBT to total BTs were observed in the vicinity of the shipyard. A higher concentration of TPT in comparison with TBT was detected in a small fishing port. Furthermore, OT concentrations in the mouth of the bay were higher than those in the closed-off section. OT concentrations in mussels decreased with distance from the shipyard. Otsuchi Bay was then divided into three parts by cluster analysis based on the concentrations of representative booster biocides found in the bay's sediment. These areas included the vicinity of a shipyard, a small fishing port, and other sites. Concentrations of Diuron and Irgarol 1051 in the vicinity of a shipyard and a small fishing port were dramatically high in comparison with the other sites. Copper pyrithione and Dichlofluanid in addition to Diuron and Irgarol 1051 were also detected in the area of a small fishing port. The concentrations of antifouling biocides were highest in the water in front of the shipyard and showed a marked decrease with distance from the shipyard.

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 (≈ 13–20 × 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 × 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 × 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.

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