Facultative amphidromy involving estuaries in an annual amphidromous fish from a subtropical marginal range

Variation in the life cycle of diadromous fishes can be explained by differential food availability between marine and freshwater habitats, since migration is often interpreted as a mechanism for exploiting food resources. Theoretically, a migration pattern of mainly remaining in freshwater occurs in tropical and subtropical habitats where fluvial productivity possibly exceeds marine productivity. However, in Yakugachi River, Amami-Oshima Island, southern Japan, low nutrient concentrations in the river suggest that food availability is limited for the subtropical Ryukyu-ayu Plecoglossus altivelis ryukyuensis. Since Ryukyu-ayu is an amphidromous fish that mainly grows in rivers after spending 2 months in the sea, limited food availability in rivers would force this species to migrate to other habitats with better food availability. Otolith increment and Sr:Ca analyses of 48 adult Ryukyu-ayu collected from the Yakugachi River revealed that all individuals visited estuaries more than three times after moving upstream. Although the specific growth rates of this species in the river had no correlation with the salinity profile in the fluvial period, this movement may be an adaptive choice because the salinity profile significantly affected the body size at maturity. Our results highlighted individual-based variations in amphidromous migration for utilizing estuaries, which could be explained by relatively higher productivity in estuarine than in freshwater and marine habitats.