Potential effects of dam cascade on fish: lessons from the Yangtze River

Construction of hydroelectric dams affect river ecosystems, fish diversity, and fisheries yields. However, there are no studies assessing the combined effects on fish caused by several adjacent dams and their reservoirs, as in a 'dam cascade'. This study predicts the potential effects that a cascade of ten dams currently under construction in the upper Yangtze River in China will have on local fishes, and uses such predictions to assess the effectiveness of possible fish conservation measures. We found that the dam cascade will have serious combined effects on fishes mainly due to impoundment, habitat fragmentation and blocking, flow regime modification, and hypolimnetic discharges. The impoundments will cause loss of critical habitats for 46 endemic species. The dams will fragment the populations of 134 species and will block migration routes for 35 potamodromous fishes. Corieus guichenoti will have a high risk of extinction due to the combined effects of impoundment and blocking. Modification of the flow regime will adversely affect the recruitment of 26 species that produce drifting eggs. The start of annual spawning for 13 fishes will be postponed by more than 1 month, and fish spawning and growth opportunities will be reduced due to low water temperatures associated with hypolimnetic discharges. Combined dam effects will further reduce the likelihood of successful recruitment of some endangered species, such as Acipenser dabryanus and Psephurus gladius. Three countermeasures hold promise to mitigate the near-term effects of the dam cascade, including preservation and rehabilitation of critical habitat, restoration of a semi-natural flow regime, and stock enhancement that respects genetic integrity. These conclusions can guide the development of protection plans for fishes in the upper Yangtze River. The approach undertaken in this study-by which the known and likely effects of present and future dams were simultaneously considered in light of the biology of the species-highlights the usefulness of ichthyology for fish conservation.