© 2019, Springer Nature B.V. A total of 30 cultured [610.6 ± 24.0 mm total length (TL)] and six wild Japanese eels Anguilla japonica collected on site (622.2 ± 133.1 mm TL) were tagged with acoustic transmitters and released into a shallow brackish water lagoon in northeastern Japan in June 2014 and July–October 2014, respectively; their post-release movements in the entire lagoon were monitored by acoustic receivers. We found that the cultured and wild eels had some common features, such as nocturnal activity and the exploitation of a wide range of brackish environments and occasionally river excursions at night from the lagoon. However, the total periods of detection and seaward movement behaviors differed between the cultured and wild eels, with 26 cultured eels no longer being detected within 6 months, whereas four wild eels were detected for >6 months. In total, 12 cultured eels moved into the outer ocean (potentially seaward migration) in October after typhoons passed and in November after the water temperature decreased (13.8 ± 1.2 °C), as opposed to the wild eels. While the contribution of the cultured eels to reproduction after the seaward movement remains unknown, these findings showed that they remained in the lagoon only for a limited time.