Freshwater habitats like rivers, creeks, lakes, ponds, streams, swamps and marshes account for only three percent of the world’s water and cover about 0.8 percent of the Earth’s surface, but harbour incredible diversity and are homes to more than 100, 000 species of plants and animals.
Freshwater biodiversity is being disproportionately threatened in recent years and damming of rivers is one of the important threats to freshwater biodiversity. While big dams are claimed to provide direct economic benefits like water security, flood protection, and renewable energy, they can lead to negative effects on freshwater ecosystems like flooding, hydrologic alteration, and fragmentation (interruption of the rivers natural flow).
As high as 40 percent of fish species are found in freshwater environments and fragmentation of the freshwater environment has major implications for fish as dams obstruct migration routes of fish that are essential for spawning, feeding and dispersal. It has been feared that the near-future expansion of hydropower facilities throughout the world will further threaten freshwater fish biodiversity.
Global studies show that an estimated 48 percent of the river volumes are currently being altered because of flow regulation or fragmentation and the pending construction of around 3,700 major hydropower dams is expected to increase this percentage to 93 percent. While studies have looked at the impacts of dams on habitat connectivity, few studies have looked at the impact of loss in connectivity of rivers (hindrances in the continuous flow of the rivers) induced due to damming on freshwater biodiversity.
This study 'Impacts of current and future large dams on the geographic range connectivity of freshwater fish worldwide' published in PNAS assessed the impacts of current and future large dams on the geographic range connectivity of 10,000 fish species living in partially or exclusively flowing freshwater bodies worldwide.
The impact of around 40,000 existing large dams and 3,700 large hydropower dams currently under construction or planned on fish that migrate between freshwater and marine environments (classified as diadromous fish) and fish that complete their lifecycle in freshwater (non-diadromous fish) was assessed. The study used ‘connectivity index’ to assess impacts on fish that migrate between fresh and marine waters (diadromous) and those that are found only in freshwater (non-diadromous).
The findings revealed that:
The current study, is of great importance for India, having one of the most fragmented fish habitats in the world according to this study, and which as has about 5,264 large existing dams while 437 are under construction.
Evidence from India shows that dams have had a major impact on downstream ecosystems and fisheries due to hydrological modification, absence of water in rivers, obstacles in migration, changes in salinity and sediment, loss of riparian areas and floodplains.
According to a SANDRP report,
A study in the Western Ghats showed that fish populations were severely affected just downstream of dams and barrages in the Malaprabha basin. Another study in the Godavari basin also shows that dams led to habitat fragmentation.
The impacts of construction of large dams on fish species can be huge and demands immediate attention. The study calls for an urgent need to have a relook at the current ways in which we manage our rivers, including the narrow ways in which we plan our hydropower projects without consideration for the health of the river and its ecosystem, pay more attention to identifying types of fish and the river basins at risk and focus on restoration efforts such as dam removal and construction of fish bypasses.
The paper can be accessed here