Effects of upstream cascade dams and longitudinal environmental gradients on variations in fish assemblages of the Three Gorges Reservoir

Abstract

AbstractThe construction of dams creates a series of reservoirs, causing severe environmental changes and a biodiversity crisis. We aimed to investigate the relative roles of the construction of upstream cascade dams and the longitudinal gradient in environmental conditions in the spatial and temporal variations in fish assemblages in a downstream reservoir. Fish assemblage and abiotic data from four longitudinally distributed reaches in the Three Gorges Reservoir (TGR), China—one of the largest reservoirs worldwide—were collected over 8 years (2011–2018) and analysed. The results showed that the blockage to fish passage, as well as the flow modification, caused by the upstream dams and reservoirs can significantly affect the variations in fish assemblages downstream of the TGR; fish assemblages in the upper area of the TGR were more vulnerable to upstream dam construction than those in the lower area. The longitudinal environmental gradient was more important in determining the spatial and temporal distributions of fish assemblages in the reservoir than the combined effects caused by the upstream dams and reservoirs. The spatial distribution pattern of fish assemblage structure in the TGR was mainly determined by the gradient difference in the flow regime among stations, whereas the construction of two upstream dams mainly affected the temporal variations in fish assemblages of the TGR by altering the downstream flow and thermal regimes and blocking fish movement down to the TGR.