Individual Differences in Resource Use, Growth and Emigration Under the Influence of a Dominance Hierarchy in Fluvial Red-Spotted Masu Salmon in a Natural Habitat

Abstract

1. The relationships between dominance status and individualdifferences in foraging behaviour, habitat use, growth and emigrationwere examined for fluvial red-spotted masu salmon, Oncorhynchus masouishikawai, in a mountain stream.2. Size-structured linear dominance hierarchies were recognized amongindividuals inhabiting the same stream pools. Observations on spaceutilization and foraging behaviour revealed fish to be eitherterritorial or nonterritorial. Within each local pool, dominant fishexclusively occupied the mid or surface layer of the pools as foragingterritories, whereas subordinates adopted nonterritorial tactics,primarily utilizing the bottom layer.3. Of the territorial fish, more dominant individuals tended to occupyfocal points nearer the pool inlet, where they had priority of access todrifting food items. These fish showed higher actual foraging rates,feeding upon larger prey than their subordinates. This foragingadvantage resulted in their having larger daily growth increments.4. The more dominant fish in each pool exhibited a more sedentarytendency than their subordinates. Population densities in the pools didnot fluctuate appreciably owing to both emigration of nonterritorialsubordinates and immigration.5. These results support the hypothesis that unequal resourcepartitioning among individuals subject to a dominance hierarchy plays animportant role in their density-dependent population regulation.