An aerial view of the Tungabhadra dam in Karnataka [Image Credits: Bishnu Sarangi from Pixabay]
Over the years, we have marvelled at the engineering feat: how good are the dams built across rivers? The massive structures, capable of taming a wild river’s flow, store water, and in most cases, generate electricity. It is known that fish in India’s rivers are threatened by water withdrawals for agriculture, pollution and excessive fishing. But how do dams, about 5000 and counting in the country, affect fish in the rivers? A recent study by researchers at the Ashoka Trust for Research in Ecology (ATREE), Bengaluru, Manipal Academy of Higher Education has some answers.
The study, published in the journal Transactions of the American Fisheries Society, early 2020 looked at how dams in the Western Ghats, a biodiversity hotspot, affect fish movement and migration in the rivers there. It examined the differences in the river flow with and without dams, and whether these differences affected fish populations. The study was a part of Dr Vidyadhar's PhD work at ATREE, Bengaluru, funded by a few International and national institutes and partially funded by the Ministry of Earth Sciences, Government of India.
The researchers chose the Tunga and Bhadra rivers for their near-natural, (river stretches without dam) undisturbed water flow, and the sub-basins of Mhadei and Malaprabha in northern Karnataka for their water flow regulated by dams. They conducted various water quality tests along with mapping river habitat to investigate what factors affect the quality of water and the ecosystems of fishes that dwell in them.
The study also examined the fish population in the four rivers and recorded a total of 12,840 fish individuals belonging to 79 species across wet and dry seasons during 2011–2014. Among these, as many as 31 species were endemic to the Western Ghats and found nowhere else. The Tunga and Bhadra rivers come from the headwater region and have more endemic and habitat specialist species than Mhadei and Malaprabha.
Among the fish observed, the researchers found nearly 50 individuals with physical deformation due to destructive fishing practices and all of them came from the dammed river basins of Malaprabha and Mhadei. Exposure to chemical pollutants or injury due to the release of dam water may be other reasons for these deformities, say the researchers.
“We observed a few fishes that were alive but severely damaged,” recollects Dr. Vidyadhar Atkore, a lead author of this study.
The researchers used data from the near-natural Tunga river basin to explain how atmospheric oxygen penetrates the surface of the water and reaches levels that are conducive for the fish and other creatures that live on the surface of the water. In contrast, the restricted water flows of Malaprabha and Mhadei rivers resulted in lower oxygen levels, which is perfect for certain type of fishes that dwell in the middle and bottom layers of the rivers. At such middle and bottom layers many aquatic plants grow with high nitrogen and phosphorus levels, benefitting plant-eating fish.
Besides, other factors like the content of alkanes, calcium, carbon dioxide, nitrates and the temperature of the water decide which fishes survive and how they are distributed. “Dams obstruct the flow of water downstream. Small, isolated, and stagnant pools quickly become warm, which influence the fish movement,” explains Dr. Vidyadhar. “In such pools, if the temperature crosses a optimum threshold, it may result in the death of fish.”
It is well-known that dams affect the migration of fish and India is one of the worst affected countries in the world, thanks to the number of dammed rivers we have. While there have been steps proposed to make dams fish-friendly, like constructing fish ladders, more factors need to be considered, says the study. The researchers urge to focus on the quality of water of rivers that are dammed.
“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,” recommends Dr. Vidyadhar. While dams have helped humans thrive at the cost of displacing others, they also severely affect creatures that live in the water. How do we strike a balance is a question worth pondering.
This article has been run past the researchers, whose work is covered, to ensure accuracy.