Abstract

Tropical rivers across the world are experiencing rapid degradation and loss of fish species due to flow regulation, pollution, and other anthropogenic pressures. Knowledge of how flow alterations at different scales affect tropical fish diversity remains limited, especially in terms of how resulting changes in water chemistry affect fish communities. We investigated fish guild composition and responses of guild richness to water chemistry in 120 river segments with and without local-scale water removals and pollution. This included two regulated subbasins (with flow regulation by barrages and large-scale water diversions) and two nonregulated subbasins (flows not regulated by these barriers) in India’s Western Ghats. Using multivariate ordination and regression models, we identified covariates related to water chemistry and environmental characteristics that explained differences in fish species composition and species richness of three water column position-based fish guilds in the study area. At the subbasin scale, effects of water chemistry were stronger for individual river segments. At the segment scale, no differences in water chemistry were found between segments with and without water removals or pollution in both regulated and nonregulated subbasins. Guild-wise species richness, especially of surface-dwelling fishes, was positively correlated with water temperature and total dissolved solids across regulated and nonregulated subbasins. In regulated subbasins, total alkalinity was positively correlated with species richness of bottom-dwelling and mid-column-dwelling fishes. Landscape-scale flow alterations and degree of flow regulation by river barriers significantly influenced fish composition and species richness over and above local water withdrawal or pollution. River restoration at both the local (segment) and landscape (basin) scales is crucial for fish conservation. To sustain fish guild diversity, maintenance of ecological flow regimes downstream of barriers needs to receive priority over mitigating local disturbances in regulated river catchments of the Western Ghats.