-Shreya Dasgupta
Contd from previous issue
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 less than a second, then they go back, and because they live in sediment-laden river, visual behavioural 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.”
Mayukh Dey recorded Ganga River characteristics, underwater noise levels and dolphin clicks. Photo credit: Mayukh Dey.
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 modelled 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.”
Vessel traffic on the Ganga is expected to increase. Photo credit: Mayukh Dey.
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 recognise 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.”
This article first appeared on Mongabay.