- India’s Ganga River is getting noisier with increased ship traffic and dredging, and that’s stressing the river’s iconic dolphins and changing how they communicate, a new study has found.
- When fewer than five vessels moving on the river per hour, the dolphins seem to enhance their vocal activities to compensate for the high-frequency noise generated by the propellers.
- But as vessel traffic increases and water levels fall during the dry season, leading to more intense and sustained noise pollution, the dolphins don’t seem to alter their clicks much compared to baseline levels, the researchers found.
- This is likely because having to continuously emit clicks in a persistently noisy world can be physically taxing, forcing the endangered mammals to “either call at baseline levels or shut up,” according to the researchers.
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 clamor 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 concerning, 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 (ATREE) 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.”
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 have about three dolphins per kilometer, among the highest known densities of these endangered mammals.
Dey, who was studying for his master’s degree at the National Centre for Biological Science in Bengaluru at the time, spent several days and nights on a boat, monitoring and recording ship traffic, characteristics of the river, including its depth and the volume of water flowing through it, and fishing intensity. He also used hydrophones to record both ambient underwater noise levels and changes in the dolphins’ acoustic activities.
“The dolphins only surface for like less than a second then they go back, and because they live in sentiment-laden river, visual behavioral observations are hard to come by,” Dey said. “That’s why you rely on acoustics which is a much truer indication of potential stress in dolphin.”
Dey found that the propellers of all the boats and ships he monitored produced sound frequencies that overlapped completely with the high frequencies of the dolphins’ clicks. This suggests the dolphins do hear the vessels loud and clear.
The researchers also found that as the dry season progressed from November to March, water levels went down, while boat traffic increased. And with more boats passing through a smaller volume of water, underwater ambient noise levels increased. This combination of higher vessel traffic and low water level was particularly bad for the dolphins. Take Doriganj, for instance, where the river is shallow and the ambient sound is really loud. There, Dey and his colleagues observed adult dolphins jumping out of the river and diving in headfirst while splashing their tail.
“Studies have shown that this is a severe sign of stress,” Dey said.
The dolphins’ calls also changed in response to increasing noise, but the patterns weren’t quite straightforward.
During the initial months when the river had more water and there were fewer than five vessels per hour moving on the river, the dolphins enhanced their vocal activities whenever the boats moved past them. To compensate for the intermittent, loud noise from vessels, they were calling louder, calling for longer, and producing more clicks, compared to their baseline vocal levels in a “quiet” river.
But over time, water levels receded in the study sites, and vessel traffic increased to more than seven boats plying every hour. This led to long-lasting chronic noise pollution that did not leave a lot of quiet time in the river. During these drier, noisier months, the dolphins didn’t alter their clicks much compared to the baseline levels, the researchers found.
This could be because emitting clicks in a noisy world can be physically exhausting. Gangetic dolphins emit clicks almost continuously to see and sense their surroundings, and this activity consumes energy. Dey and his colleagues modeled their observations and found that if the dolphins were to enhance their vocal activities to compensate for the doubling or quadrupling of ambient underwater noise levels, they would have to consume two to four times more prey per day. But there’s only so much a dolphin can eat, and the amount of prey isn’t increasing either; finding prey using clicks in noisier water is also harder.
“If the dolphins were to use enhanced acoustic activity for 12 hours of the day, they’d be unable to sustain such high levels of activity simply because of how energetically costly it is,” Dey said. “So they essentially have to not manipulate their calls, and either call at baseline levels or shut up.”
For the Ganga River dolphins, chronic noise pollution is yet another threat they’re learning to deal with. But there are some solutions that could help tackle the problem, the researchers say.
Limiting the number of vessels that ply the river is an obvious measure. The study also shows that shallower water levels during the dry season can considerably aggravate the impacts of noise on river dolphins — but water levels can be modulated. “It is critical to recognize that rivers need more water than they get in the dry season, and to provide it by modifying dam/barrage operations to allow for ‘ecologically adequate’ flows,” Kelkar said.
Boats and ships could also lower the noise they produce with just a small change in the propeller design, Dey said.
“With just a simple modification in the propeller, noise in the high-frequency range, and noise in general, can be cut down significantly,” Dey said. “And it’s not just noise that gets cut down, even fuel efficiency increases. So it’s a win-win scenario for both ecologists and economists. It’s only the political will to do it, and that’s the hold up.”
Citation:
Dey, M., Krishnaswamy, J., Morisaka, T., & Kelkar, N. (2019). Interacting effects of vessel noise and shallow river depth elevate metabolic stress in Ganges river dolphins. Scientific Reports, 9(1). doi:10.1038/s41598-019-51664-1
