The study of changes in intraspecific habitat use when closely related species coexist aids our understanding of the relationships between intra- and interspecific interactions in fishes. However, evidence of this phenomenon is few and shown in a very limited number of taxa. In particular, the stream goby [Rhinogobius flumineus (Mizuno, 1960)] is known for intraspecific variation in habitat use. We used underwater visual surveys to investigate this species’ size-dependent habitat use, both in isolation and in the presence of a sympatric congener, Rhinogobius nagoyae Jordan & Seale, 1906. A generalized linear mixed model and multiple comparison tests revealed that in the absence of R. nagoyae, R. flumineus body length was positively correlated with river flow velocity. This correlation disappeared when R. flumineus coexisted with R. nagoyae. Additionally, R. nagoyae density increased with flow velocity. Observations of interspecific territorial behavior revealed that larger individuals dominated in male–male competition regardless of species combination. Of the two species, R. nagoyae is significantly larger, so it was likely to exclude even relatively big R. flumineus individuals from fast riffles, the habitat preferred by both species. This study demonstrated that interspecific competition dominates intraspecific competition under sympatric conditions with larger related species.
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