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

The relationships between dominance status and individual differences in foraging behaviour, habitat use, growth and emigration were examined for fluvial red-spotted masu salmon, Oncorhynchus masou ishikawai, in a mountain stream. Size-structured linear dominance hierarchies were recognized among individuals inhabiting the same stream pools. Observations on space utilization and foraging behaviour revealed fish to be either territorial or nonterritorial. Within each local pool, dominant fish exclusively occupied the mid or surface layer of the pools as foraging territories, whereas subordinates adopted nonterritorial tactics, primarily utilizing the bottom layer. Of the territorial fish, more dominant individuals tended to occupy focal points nearer the pool inlet, where they had priority of access to drifting food items. These fish showed higher actual foraging rates, feeding upon larger prey than their subordinates. This foraging advantage resulted in their having larger daily growth increments. The more dominant fish in each pool exhibited a more sedentary tendency than their subordinates. Population densities in the pools did not fluctuate appreciably owing to both emigration of nonterritorial subordinates and immigration. These results support the hypothesis that unequal resource partitioning among individuals subject to a dominance hierarchy plays an important role in their density-dependent population regulation.

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