Impacts of current and future large dams on the geographic range connectivity of freshwater fish worldwide

Abstract

Dams contribute to water security, energy supply, and flood protection but also fragment habitats of freshwater species. Yet, a global species-level assessment of dam-induced fragmentation is lacking. Here, we assessed the degree of fragmentation of the occurrence ranges of ∼10,000 lotic fish species worldwide due to ∼40,000 existing large dams and ∼3,700 additional future large hydropower 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 nondiadromous fish species were more fragmented (less connected) (CI = 73 ± 28%; mean ± SD) than ranges of diadromous species (CI = 86 ± 19%). Current levels of fragmentation were highest in the United States, Europe, South Africa, India, and China. Increases in fragmentation due to future dams were especially high in the tropics, with declines in CI of ∼20 to 40 percentage points on average across the species in the Amazon, Niger, Congo, Salween, and Mekong basins. Our assessment can guide river management at multiple scales and in various domains, including strategic hydropower planning, identification of species and basins at risk, and prioritization of restoration measures, such as dam removal and construction of fish bypasses.

Keywords: biodiversity; habitat fragmentation; hydropower; migratory fish; river management.

Fig. 1.

Connectivity index (CI) across species (Left) and main hydrologic basins of different size (Right) for nondiadromous (Top) and diadromous (Bottom) fish species. Values are shown for present dams (purple) as well as present and future dams together (light blue). Main hydrologic basins are defined as having an outlet to the sea or internal sink. For species occurring in multiple main basins, the area-weighted mean of the basin-specific CI values was calculated (Top). The basin-level CI (Right) represents the mean of the CI values across the species occurring within the basin. Boxes represent the interquartile range and the median, and whiskers the 95% interval. Colored violins around the boxes show the values distribution. Diamonds represent the mean.

Fig. 2.

Mean Connectivity Index (CI in percent) per subbasin (∼1 million units) for present situation (Top), future projection (Center), and the difference between them (Bottom) for nondiadromous (Left) and diadromous (Right) fish species. Gray represents areas without species range data.

Fig. 3.

Mean Connectivity Index (CI in percent) across nondiadromous species in four exemplary main hydrologic basins. The maps show the basin hydrography with the locations of dams (Left) and the CI at the subbasin level for the present situation and future projection (Center). The species-specific CI values are summarized as boxplots, with diamonds representing the mean (Right). Numbers in brackets next to each basin name represent the number of fish species considered. Locations of the selected basins are shown in SI Appendix, Fig. S7.

Fig. 4.

Mean connectivity index (CI in percent) across diadromous species for four exemplary main hydrologic basins. The maps show the basin hydrography with the locations of dams (Left) and the CI at the subbasin level for the present situation and future projection (Center). The species-specific CI values are summarized as boxplots, with diamonds representing the mean (Right). Numbers in brackets next to each basin name represent the number of fish species considered. Locations of the selected basins are shown in SI Appendix, Fig. S7.

Fig. 5.

Mean of species-specific connectivity index (CI in percent) values by different traits and categories for present and future, including (A) Köppen–Geiger climate zones, where A = equatorial, B = arid, C = warm temperate, D = snow, and E = polar. (B) Geographic range area (log [base 10]-transformed square kilometers). (C) Body length (centimeters). (D) IUCN threat status, where CR = critically endangered, VU = vulnerable, NT = near threatened, LC = least concern, EN = endangered, and DD = data deficient. (E) Commercial relevance, where Com. = commercial, HCom. = highly commercial, MCom. = minor commercial, NonInt. = of no interest, and Subs. = subsistence fisheries. (F) Species order, where groups with less than 20 species are grouped together in the “other” category (the full list of order names is provided in SI Appendix, Table S2). For species occurring in multiple main hydrologic basins, an area-weighted mean of the basin-specific CI values was calculated before averaging across species. Each panel shows a subset of the analyzed 9,794 species in this study (underneath each category the number of species is reported in brackets), as metadata for species traits and categories were not available for all of the species.

Fig. 6.

Effect of small dams on the CI values across freshwater fish species in Brazil, the greater Mekong region, and the United States. The comparison is made for each region by considering only large dams (consisting of dams employed in this study complemented with large dams [height >15 m] from national datasets; left) and by adding small dams from national datasets (right). Boxes represent the interquartile range and the median and whiskers the 95% interval. Gray areas around the boxes show the values distribution. The red diamonds represent the mean.