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

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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>300mm 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; 140–200mm (M-sized) and 100–120mm (S-sized) fish selected deep areas of high velocity current adjacent to LW structures; ≤80mm 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.

Les différentes classes de taille de saumons masou (Oncorhynchus masou) se répartissent dans l’espace tridimensionnel des habitats créés par l’installation d’importantes structures de bois (LW) dans une grande rivière. Les poissons >300 mm (de taille L) qui reviennent de l’océan se retrouvent de façon caractéristique dans les zones protégées près du lit de la rivière qui possèdent un courant moyen et de gros amas de racines ou des troncs d’arbres. Les poissons de 140–200 mm (de taille M) et de 100–120 mm (de taille S) choisissent les zones profondes à courant rapide adjacentes aux structures LW. Les poissons ≤80 mm (de taille SS) sont plus communs dans les couches les plus profondes dans tous les habitats de LW, mais aussi dans les zones peu profondes à courant modéré où les structures LW bloquent les courants rapides. Quelques poissons de taille SS utilisent les zones de couvert formées par les branches et les feuilles. L’abondance des saumons masou, toutes classes de taille combinées, est plus importante dans les habitats contenant des structures de LW que dans ceux qui n’en ont pas. Notre étude laisse croire que l’apport naturel d’arbres entiers provenant du rivage ou l’addition artificielle d’arbres entiers a un effet profond en créant des habitats à salmonidés dans les grandes rivières.

Document Type: Research Article

Publication date: January 1, 2009

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  • Published continuously since 1901 (under various titles), this monthly journal is the primary publishing vehicle for the multidisciplinary field of aquatic sciences. It publishes perspectives (syntheses, critiques, and re-evaluations), discussions (comments and replies), articles, and rapid communications, relating to current research on cells, organisms, populations, ecosystems, or processes that affect aquatic systems. The journal seeks to amplify, modify, question, or redirect accumulated knowledge in the field of fisheries and aquatic science. Occasional supplements are dedicated to single topics or to proceedings of international symposia.
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