Salinity and odor preferences of Japanese eel during the first year of post-recruitment growth in saline water

Freshwater eels grow both as residents of estuaries, spending the yellow eel stage in nearshore saline waters, and as residents of freshwater in rivers. In order to understand how Japanese eels diverge between estuarine and river residence, we investigated their water preferences during the first year of growth in saline water after their recruitment into growth habitats by using Y-maze behavioral trials. In the water-choice trials between natural seawater and river water, glass eels and pigmented elvers showed a significant preference for river water, while small yellow eels showed a significant preference for seawater. Subsequent experiments examining odor and salinity preferences of the eels showed that a preference for riverine odor was maintained during the growth phase. However, as the eels developed into the yellow eel stage in natural seawater, their salinity preference shifted from no preference, or a preference for freshwater, to a preference for saline water. These results suggest that Japanese eels disperse over a wide range of salinities until the elver stage, as long as water has a rich river-originated odor. When individuals grow in saline water until reaching the yellow eel stage, they are likely to become estuary residents.