Environmental factors affecting the onset of spawning migrations of Japanese eels (Anguilla japonica) in Mikawa Bay Japan

The timing of downstream migrations of anguillid silver eels in relation to environmental factors in rivers has been examined using various types of data. Less is known about the factors that affect when silver eels leave estuaries and enter the sea or about the onset of the spawning migrations of eels living in brackish-water coastal areas. The timing of collections of Japanese eels, Anguilla japonica, in shoreline set nets in Mikawa Bay, Japan, were analyzed in relation to environmental variables such as water temperature, lunar phase, rainfall and wind speed over a 10 year period from 1997 to 2006. Higher catches of silver eels occurred during new moon periods in November and December as water temperatures decreased in the bay, and when the gonadosomatic index values of the eels were highest. Hurdle model analyses for clarifying the relationship between the catch of eels and environmental factors (lunar cycle, rainfall and wind) indicated the catches of eels were correlated with the two different environmental factors of wind strength 2 days earlier and lunar cycle, while rainfall was not selected by the model. This suggests that different factors are important for determining when eels migrate in coastal waters compared to during their downstream migrations in freshwater.