Science

Damming river water impacts fish diversity

The dam Close to the barrier , downstream Malaprabha river the number of fish dipped and fish recovery was low.  

A new study has found that dams and other barriers across rivers in the Western Ghats do affect fish species and their recovery downstream. However, barrier-free tributaries that drain in to these rivers can help fish recover even in dammed stretches; protecting such tributaries could be crucial to maintaining fish diversity in the Western Ghats.

The Western Ghats is home to 290 freshwater fish species, more than half of which are endemic. While other studies have shown that river barriers such as barrages and dams can affect fish diversity, there is no hard evidence to prove this in the Western Ghats.

To test if barriers across a 72-km stretch of Karnataka's Malaprabha River in the central Western Ghats could affect fish diversity, scientists from the Asoka Trust for Research in Ecology and the Environment (ATREE) compared fish diversity both upstream and downstream of barriers.

Endemic species

Studies in the upstream, barrier-free stretches confirmed the presence of 28 fish species, including the Deccan mahseer, an endemic carp. However, this number dipped immediately downstream of barriers; fish recovery was low here. But it picked up further downstream, as distance from the barrier increased. The scientists also noticed that the more the number of barriers upstream, the more diminished was fish recovery downstream.

But there was a silver lining: as long as barrier-free tributaries merged with the Malaprabha River, fish — including endemics — recovered well, even in dammed stretches of the river. These tributaries, along with other environmental factors, increased dissolved oxygen content and reduced the hardness of water (alkalinity) in the Malaprabha, helping fish recover better. Fish diversity recovered to the highest level after a distance of 2 km downstream of barriers.

The finding that barrier-free tributaries are crucial to mitigate the impacts of already-existing barriers could be important to consider while implementing potentially high-impact river projects like Kerala's Athirapilly hydro-electric project in the southern Western Ghats, an area that has the highest endemicity of freshwater species in the entire range.

“The study helps classify stretches of rivers or tributaries in an already developed basin that should be spared from future (hydrologic) barriers and regulation,” says co-author Jagdish Krishnaswamy from the Suri Sehgal Centre for Biodiversity and Conservation, ATREE.

It could also help policy makers and managers redesign hydrological barriers, and develop recovery plans for endemic species like the mahseer, says Vidyadhar Atkore, lead author of the study published in the journal River Research and Applications.

“The efficacy of all existing hydrological barriers (small or large) needs proper evaluation to assess its impact on aquatic biodiversity,” he says, adding that they hope to test the effects of river barriers in another regulated river basin of the Western Ghats as well.

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Printable version | Oct 27, 2020 12:04:32 AM | https://www.thehindu.com/sci-tech/science/damming-river-water-impacts-fish-diversity/article19140868.ece

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