Variation in migration patterns of pond smelt, Hypomesus nippo- nensis, in Japan determined by otolith microchemical analysis

In order to investigate the migration pattern of the pond smelt, Hypomesus nipponensis, the life history trajectories of populations from various localities in Japan were estimated using the otolith microanalysis technique. The strontium:calcium (Sr : Ca) ratio of otoliths was determined for pond smelt reared in fresh, brackish and sea water and was found to show a linear increase with increasing salinity. Salinity trajectories were constructed for populations based on a regression of Sr : Ca ratio to salinity. Some individuals in Lakes Abashiri and Ogawara, and all fish in Lakes Takahoko and Shinji showed values which were consistently low, indicating residence in these lakes throughout the life history. High values were also absent in individuals from the Ishikari River, so it was concluded that the populations migrated between fresh and brackish waters. However, pond smelt with high Sr : Ca ratio profiles indicating seawater residency were found in the Hei River, off Hachinohe, off Misawa, Lake Obuchi, Oppa Bay, and also some individuals from Lakes Abashiri and Ogawara. In these cases, residence in the sea appeared to have been short, except for Hei River and Lake Ogawara. These results indicate a wide flexibility in habitat selection by pond smelt, with brackish environments possibly playing an important role in the life history of the species.