‘River pits,’ caused by unregulated release of water from dams, pose threat to endangered otters along Moyar River in Nilgiris

Deaths of Asian small-clawed otters in river pits due to dam water flow highlights conservation risks.

The deaths of a group of Asian small-clawed otters (Aonyx cinereus), inside a “river pit” formed by high water flows from the Pykara Dam, has highlighted an often overlooked risk faced by river otters along the Moyar River due to the unregulated release and withholding of water from the district’s major dams.

The Asian small-clawed otter, classified as ‘vulnerable’ in the International Union for the Conservation of Nature’s Red List, is among two otter species found in the Moyar Valley, the other being the Smooth-coated otter (Lutrogale perspicillata).

The deaths of the three Asian small-clawed otters in a river pit, around eight kilometers from the Pykara Dam were observed by researchers, K. Narasimmarajan, H.S.Palei and M.T. Mathai and published in the IUCN Otter Specialist Group bulletin, titled “Do natural river pits pose a danger to otters? A field report from the Moyar Valley, Western Ghats, India.”

Describing a river pit in the bulletin, the researchers state, “River pits are formed naturally when high water flows crack rock surfaces and creates [sic] a deep hole, or pit, in the bed of the river, about 1 to 3 meters deep. These pits fill with water during a discharge from upstream hydroelectric dams, but dry up during the non-pumping time.” The mortality report detailed in the bulletin showed that three of the otters had become trapped in the pit and were unable to escape when water levels dropped due to water release being stopped upstream in the Pykara Dam.

“From 1947-54, the Tamil Nadu Electricity Board (TNEB) constructed a dam in Pykara on the Moyar River, with a hydro-electric power generation capacity of 28 MW/day. Unregulated water pumping during electricity generation, controls the entire downstream river water flow, restricting the free movement of Small-clawed otters. Otters can become trapped in the deep river pits when the pumping of water is suddenly stopped. The otters cannot escape from the bottom of the pits when the water level falls too far below the surface level. Unregulated or sudden stoppage of water pumping from the dam can lead to death of a trapped otter due to starvation,” the authors note.

The researchers, apart from noting the death of the three otters in the river pit, also rescued a rat snake from within it, indicating that a number of species could be at risk from falling in and being unable to escape. “Asian small-clawed otters, having small paws, might face challenges climbing slippery surfaces like deep rocky pits,” the researchers note.

Speaking to The Hindu, K. Narasimmarajan, one of the authors of the paper and a PhD scholar at the Department of Zoology, Madras Christian College, said river pits attract food sources such as fish and crustaceans, which draw in the otters. “This observation has highlighted the importance of regulating water flow and ensuring the regular, periodic release of water along river systems supporting endemic wildlife,” said Mr. Narasimmarajan.