Atmospheric depression as the final trigger for the seaward migration of the Japanese eel Anguilla japonica

Despite the absence of rainfall or of any rise in river water level, the seaward migration of Japanese silver eels frequently occurs after the passage of a 'dry depression', suggesting that the migratory behavior of the eels is influenced by the atmospheric depression rather than by rainfall or elevated water level, as is usually thought. Our analyses showed that seaward migration of silver eels takes place mainly during the new moon in autumn to winter as water temperature decreases, suggesting the possibility that their migratory behavior depends on at least 3 independent environmental factors: water temperature, lunar phase and the passage of an atmospheric depression. However, water temperature and lunar phase were insufficient to cause the eels to start their migration, The passage of a depression was required as the final event to begin the seaward migration. These findings strongly suggest that the passage of a depression may be the only trigger that begins the seaward migration of silver eels.