| Summary: | To estimate the impact of weirs on large river fish communities, we applied the ecological niche space (ENS) measured quantitatively using carbon and nitrogen stable isotope ratios to the monitoring of effects of weirs constructed on four major rivers in Korea. ENS was calculated using the Bayesian stable isotope in R statistics. The ENS of fish communities showed persistent differences between upstream and downstream areas of all studied weirs. The ENSs of omnivores were larger in upstream impounded areas but the ENSs of predator species, both endemic (<i>Erythroculter erythropterus</i>) and exotic (<i>Micropterus salmoides</i>) predators had decreased ENSs in upstream areas. <i>E. erythropterus</i> showed horizontal variations in a wide range of carbon stable isotopes, whereas <i>M. salmoides</i> showed vertical variations in nitrogen stable isotopes. The results suggest that weir construction may have species-specific impact on the ENS of fish community by impoundment and increase niche overlap in upstream areas of the weir. The measured ENS of fish community was significantly correlated with the relative abundance of tolerant species negatively, whereas correlated with that of endemic species positively, suggesting that the ENS can be used as a comprehensive indicator of habitat conditions.
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