Ganga has become so noisy that river dolphins are struggling to talk to each other

    20-Nov-2019
Shreya Dasgupta
India’s Ganga is a noisy river. There’s the churning of sediment, the hums, grunts and growls of fishes and turtles. There are the cacophonous stretches of cities and industries breathing and dumping their waste into the river. Then there’s the constant din of boats and ships, and the clamour of heavy machinery dredging the riverbed.
These noises are only getting louder. And that’s stressing the Ganga River’s iconic dolphins (Platanista gangetica) and changing how they communicate, a new study has found. This is a matter for concern, researchers say, since the Indian government has plans to expand the Ganga waterway, increasing the number and frequency of the ships that ply the river.
“Over the last few years, there have been debates about the impacts of vessel traffic, noise, and dredging on endangered Ganges river dolphins,” Nachiket Kelkar, a researcher at the Ashoka Trust for Research, Ecology and the Environment in Bengaluru and co-author of the study, said in an email. “In these debates, many of the anticipated impacts, even those suggested by ecologists, were based on some knowledge of the system, but still they were speculative and lacked strong evidence from field studies. This study was important to objectively demonstrate how underwater noise affected river dolphin behaviour and what implications it had for conserving them.”
Ganga River dolphins  alter their vocal activities in response to ship traffic. Photo credit: Mayukh Dey.
The Gangetic river dolphins are effectively blind; they don’t really have use for eyesight in the shallow, sediment-rich, murky waters of the rivers they inhabit. Instead, the mammals see with sound. They produce ultrasonic or high-frequency clicks in the 20- to 160-kilohertz range, and use this echolocation to find food, avoid ships and chart their way around the waters. They also modulate their clicks to talk to each other. But what does a dolphin do when its underwater home gets increasingly cacophonous?
Studies that have previously tried to answer this have focused on marine mammals like the bottlenose dolphins and whales. But unlike oceans, where space isn’t really a constraint, Gangetic dolphins live in shallow, often narrow stretches of rivers. There, the impacts of underwater noise pollution are poorly understood.
What’s certain is that ship traffic on the Ganga is increasing, and these ships have propellers that produce high-frequency sounds. Similarly, while the act of dredging doesn’t necessarily produce sounds that are audible to dolphins – although the act of dredging disrupts the dolphin’s habitat considerably – ships associated with dredgers have propellers and onboard sonars that create high-frequency sounds. How are dolphins responding to these noises? Do they call more loudly or more frequently, or change their calls? Do they stop calling altogether?
To find out, lead author Mayukh Dey monitored four sites along the Ganga in the state of Bihar between November 2017 and April 2018. Bihar is home to around 1,200 dolphins, and Dey’s selected sites – Kahalgaon, Barari, Janghira and Doriganj – each has about three dolphins per kilometre, among the highest known densities of these endangered mammals.
(To be contd)