Dams spell doom for freshwater fish

India’s fish habitats are one of the most fragmented in the world due to the presence of dams, write Alakananda Batni and Spoorthy Raman

The picturesque mountains of the Western Ghats, with its pristine rivers and lush green forests, are a treat to the eyes. Innumerable plants and animals, some of which are found nowhere else in the world, call these mountains home. Most rivers that flow in South India, including the Godavari, Krishna, Tunga, Bhadra, Cauvery and Netravati originate in these mountains. Over the last several decades, this biodiversity hotspot, like many others around the world, has been plagued by habitat loss, erratic rainfall patterns, landslides, floods and droughts. A recent addition to this list is the numerous dams—about 50 of them and counting—constructed across most rivers here.

Dams are indeed marvels of engineering. They store water, and in most cases, also help generate electricity, by taming a wild river’s flow. And that, unfortunately, is the problem for the free-swimming living forms in the river waters. Many species of fish move to different regions along the river to feed, find mates and lay their eggs. Some even swim to estuaries, where the cold, sweet river water meets the warm, salty seawater, to breed. When dams are built, the flow of water is blocked or altered, and the fish cannot reach their breeding or feeding grounds.

Obstacle to migration

“Dams can have a huge impact on fish communities as several species of freshwater fish are known to migrate upstream and downstream during their life cycle,” says Prof Anuradha Bhat, an associate professor at the Indian Institute of Science Education and Research (IISER), Kolkata. She has extensively studied the ecology and behaviour of fish communities across India.

In the Western Ghats, which has more than two-thirds of endemic fish found in India, the increasing numbers of small hydroelectric projects are hurting fish diversity. In a 2017 study, Dr Vidyadhar Atkore from the Ashoka Trust for Research in Ecology (ATREE), Bengaluru, and his collaborators studied the Malaprabha river basin to understand how small barriers to the river affect fish. They found that in the immediate downstream of a barrage or a dam, the number of fish species found reduced drastically. However, further down, the number of fish species picked up.

In another recently-published study, Vidyadhar and his collaborators compared dammed and undammed rivers to understand how dams affect fish migration and water quality. They chose stretches of the Tunga and Bhadra without barriers and contrasted the water flow in these rivers to the Mahadayi and Malaprabha rivers, which have dams. They found that the Tunga and Bhadra rivers had more fish species that were either endemic or preferred specialised habitats, than those in Mahadayi and Malaprabha.

Interestingly, they found a few individual fish (50 out of about 13,000) that were physically deformed or injured from the Malaprabha and Mahadayi rivers. The researchers believe that destructive fishing practices, exposure to chemical pollutants or injury due to the release of dam water may be why they have deformities.

Loss of water quality

An international study, published early this year, found that India’s fish habitats were one of the most fragmented in the world due to the construction of dams. For a country that has over 5,000 large dams constructed across most of its rivers, this should not be surprising. The other worst-affected regions included the United States of America, Europe, South Africa and China. With more dams proposed in the hitherto-undisturbed waters of the Amazon, Congo, Mekong and Salween, the connectivity of fish habitats is projected to worsen in the tropics.

Vidyadhar’s work has also shown that hydrological barriers, like dams, also affect water quality. In waters that are just downstream of such barriers, the dissolved oxygen content and the salt content (alkalinity) were low.

Dams also affect the content of alkanes, calcium, carbon dioxide, nitrates and the temperature of the water — factors that decide which fishes survive and how they are distributed. “Small, isolated, and stagnant pools quickly become warm, which influence the fish movement,” explains Vidyadhar. “In such pools, if the temperature crosses an optimum threshold, it may result in the death of fish.” The researchers of the study propose regularly monitoring these factors in the waters. “It will be great if we can install water testing stations in suitable sites to measure the water level and water flow of the rivers,” he recommends.

Dams are versatile structures necessary for generating electricity, irrigation and drinking water. But, we need to understand the biodiversity of the river and not just its surroundings while building them. Some scientists advocate the use of fish ladders — structures that allow migrating fish to pass over or around an obstacle — to minimise the inconvenience. In a few European countries, dams have been removed altogether to restore the connectivity of fish habitats and improve their populations.

Undammed river tributaries, which bring fresh water and nutrients into the main river, could help the fish. “They also serve as a potential refuge for fish to hide or find breeding grounds, allowing these fish to move upstream or downstream as they are connected to the main river,” says Vidyadhar. Identifying such tributaries in a hydrologically modified river basin is vital for restoring river habitat and fish population.

There is, of course, no silver bullet. “Instead of following a single-discipline approach, it is time that we use approaches from ecology, social sciences, engineering and natural sciences to safeguard our freshwater heritage,” signs off Vidyadhar.

(The authors are with Research Matters)