Size-dependent changes in habitat use of Japanese eel Anguilla japonica during the river life stage

Japanese eel ( Anguilla japonica ) is a commercially important species; however, its population has declined in recent years. Appropriate conservation management, including habitat protection, is required to reverse this decline. However, their habitat use pattern during the riverine life stage is poorly understood. In this study, we investigated the longitudinal distribution and microhabitat of small-sized (<200 mm total length) and large-sized (≥200 mm total length) Japanese eels observed in 83 and 124 quadrats (1 m × 1 m), respectively, placed at seven stations in the Nikkeshi River in Fukushima Prefecture, Japan. Analysis using generalized linear models revealed that the eel density of both size classes decreased with increasing distance from the river mouth. In addition, the density of small-sized eels, but not large-sized ones, decreased as weir numbers increased. Moreover, analysis using generalized additive models showed that microhabitat uses differed between the size classes. The small-sized eels used near-shore habitats which had low current velocities. Their preferred habitats contained both complex substrates with smaller particle sizes and simple substrates with relatively large particle sizes. In contrast, the large-sized eels used both near-shore habitats with lower current velocities and the center of the river which had high current velocities. They preferred simple riverbed habitats with large particle-sized substrates and no underwater vegetation. These results suggest that there is a size-dependent change in the longitudinal distribution and microhabitat use of Japanese eels during their river life stage. These findings provide valuable information for the conservation and management of Japanese eels in rivers.