Assessing existing and future dam impacts on the connectivity of freshwater fish ranges worldwide

Dams are vital for water management and energy generation, but also fragment river systems, with important consequences for freshwater biodiversity and ecosystem services. Yet, there is so far no global assessment of dam-induced habitat fragmentation nor are there any forecasts for how future dams will increase habitat fragmentation on the species level and the resulting impacts on biodiversity. Here, we assessed the degree of fragmentation for the occurrence ranges of ~5,700 fish species in rivers worldwide due to ~40,000 existing dams and forecasted how fragmentation will increase with ~3,700 additional future dams. Per river basin, we quantified a connectivity index (CI) for each fish species by combining its occurrence range with a high-resolution hydrography and the locations of the dams. Ranges of fish that complete their life cycle in fresh water were more fragmented (CI = 69 ± 30%; mean ± standard deviation across species) than ranges of fish that migrate between fresh water and the ocean (CI = 82 ± 22%). Hotspots of future fragmentation were in the tropics, with declines in mean CI of ~20-30% in the Amazon, Niger, Congo and Mekong basins, potentially having socio-economic repercussions on local communities highly dependent on inland fisheries. Our assessment can guide river management at multiple scales and in various domains, including strategic hydropower planning, identification of species at risk, and prioritization of restoration measures, such as dam removal and bypass construction.