Dams threaten freshwater biodiversity

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.

Shrinking rivers, growing dams

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).

Freshwater fish suffer more due to damming

The findings revealed that:

• Freshwater fish suffered more due to fragmentation and loss of connectivity of rivers due to building of dams. The connectivity values were better for fish that moved between freshwater and marine environments (86 percent) than for freshwater species (73 percent).
• Fish habitats that were most impacted and disconnected were found in the rivers in United States, Europe, South Africa, India, and China and these were predicted to increase further with more construction of dams. The largest differences between impacts of present and future dams were found in South America, Africa, and Southeast Asia.
• Decreases in connectivity due to future dams were higher for freshwater species in large tropical and subtropical rivers such as the Amazon, Congo, Niger, Salween, and Mekong. The connectivity dropped by 40 percent in larger river basins like the Danube and the Niger.
• While the current connectivity of fish in the tropics was better than that in the temperate regions, the proposed dams were predicted to affect the fish population in the tropics the most.

Fragmented rivers, disappearing fish

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,

• Dams constructed in the upstream and the Prakassam Barrage on the Krishna estuary in Andhra Pradesh have diverted all the water away from the river leading to drying up of the estuary in the summer season, increase in salinity leading to the disappearance of a number of freshwater fish such as carps, catfishes, murrels, feather backs etc.
• Fisheries in dammed Krishna, Godavari, Mahanadi, Pennar, Narmada, Tapi, Sabarmati, Mahi and Cauvery estuary are rapidly declining because of absence of freshwater in the estuaries all round the year and destruction of the mangrove forests. Riverine and estuarine fisheries in Narmada are also already affected by the Sardar Sarovar, Narmada Sagar, Omkareshwar, Maheshwar, Tawa and Bargi dams.
• Hilsa fisheries in Cauvery collapsed in the upstream after Mettur Dam while Hilsa that migrated from the Bay of Bengal upto Allahabad prior to commissioning Farakka Barrage, showed an increasing decline in production post Farraka. Puntius species was also found to have disappeared in Cauvery post dam, which formed 28 percent of the landings prior to dam construction. Mahseer, is now an endangered fish and has literally disappeared from all Indian rivers due to dams and barrages. Upcoming dams in North East and Himalayas are also feared to threaten the survival of this fish further. Tehri Dam on Bhagirathi has already impacted Mahseer migration to a great extent. The 330 dams planned in Uttarakhand are also feared to impact the survival of fish in the region.
• Narmada River system on the west coast saw a decline in Hilsa catch following the construction of the Tawa Dam that led to reduction of water depth and loss of carp breeding and feeding grounds in the central 240 km stretch of the Narmada Basin. Embankments constructed along the Brahmaputra as a flood control measure have led to shrinkage of feeding and spawning grounds of many fish species. Plans are being made to construct more than 135 dams on tributaries of Brahmaputra in Arunachal Pradesh namely, Siang, Dibang, Subansiri, Kameng, Tawang and Lohit. These are also feared to block the migratory routes of trouts and mahseer. 

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