Summer microhabitat partitioning by different size classes of masu salmon (Oncorhynchus masou) in habitats formed by installed large wood in a large lowland river

Abstract

Different size classes of masu salmon ( Oncorhynchus masou ) were partitioned in three-dimensional space in habitats created by artificially installed large wood (LW) structures in a large river. Fish >300 mm in size (L-sized) returning from the ocean distinctly occurred in sheltered areas near the riverbed, which had a moderate current velocity and contained large root wads or tree trunks; 140–200 mm (M-sized) and 100–120 mm (S-sized) fish selected deep areas of high velocity current adjacent to LW structures; ≤80 mm fish (SS-sized) were most common in the lower depth layers throughout all LW habitats, including shallow areas with moderate currents where LW structures blocked the fast currents. Some SS-sized fish used cover areas provided by branches and leaves. Masu salmon abundance in all size classes combined was greater in habitats with LW structures than in habitats without them. Our study suggests that the natural recruitment of whole trees from the riparian zone or artificial placement of whole trees will have a profound effect on creating salmonid habitats in large rivers.