![]() ![]() ![]() In response, when the water levels were low, the dolphins called to each other more softly and at an even higher frequency. When 7-11 vessels passed by every hour, their noise completely drowned out the dolphins’ clicks. Finally, they emitted louder clicks for a longer duration than in quiet conditions, according to Dey. They used 30-40% more clicks per second to communicate over this noise. The dolphins could only increase their echolocation activity to cope with the noise produced by four or five vessels plying over the surface every hour. The more boats moved over the surface, the louder underwater noise got. They also recorded the river dolphins’ clicks using hydrophones. To ascertain this, a team including Mayukh Dey, a researcher at the National Centre for Biological Sciences, Bengaluru, studied various aspects of the river at four sites between Doriganj and Kahalgaon in Bihar and quantified vessel traffic and ambient underwater noise levels before and during vessel passage. And the vessels’ propeller blades and underwater rakes produce high-frequency sounds that interfere with the dolphins’ clicks. Then again, though a part of the Ganga is now protected as the Vikramshila Gangetic Dolphin Sanctuary, it’s also a national waterway with lots of vessel traffic. Indeed, the Ganga between Buxar and Manihari in Bihar has the highest population density of such dolphins in the world.Ī 19th century illustration of a Gangetic river dolphin. This species is largely restricted to the Ganga and Brahmaputra rivers (and their large tributaries) in Nepal, India and Bangladesh. For example, bottlenose dolphins near Florida increased the frequency of their whistles as underwater noise got louder.īut while marine mammals can often move away from such soundscapes, the Ganges river dolphin doesn’t have much wiggle room. Studies also show some species trying to alter their calls in order to be heard. It disrupts disrupts foraging and communication. Underwater noise from ships has been of growing concern in the open seas, where it interferes with the echolocation of animals that call at similar frequencies, such as dolphins and toothed whales. Now, a study published on October 28 leaves little room for doubt: these sounds are indeed drowning out the clicks and altering the dolphins’ behaviour. Scientists have long suspected that they could be affecting the unique Ganges river dolphins as well. However, noise from boat engines and propellers now frequently override this aquatic concert. These endangered mammals depend entirely on such high-frequency echolocating clicks to catch prey, communicate and move around in the river’s muddy depths. A series of steady, clicking sounds – like switches being flipped on and off – is a sign that the almost-blind Ganges river dolphin is nearby. Tiny mayflies let out shrill, pulsating calls. Quiet flows the Ganga as it snakes its way through Bihar, but under its surface, there is a symphony.
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