In an on-going effort to convert certain inland waterways into national waterways, many rivers are undergoing commercialization, including the Ganges. This has critically affected the habitat and survival of one of its flagship species, the endangered Ganges river dolphin. A recent study provides empirical scientific data to understand how anthropogenic interventions are impacting the already dwindling population of aquatic animals.
The National Waterways Act (2016) which declared 111 rivers as national waterways was a major motivating factor for the researchers to conduct this study. The Ganges will be one of the first rivers to be developed for commercial and industrial use with vessel traffic predicted to increase in the coming years. “This means that the dolphins will share 80 – 90% of their natural habitat with vessels. That is obviously a cause for concern with impacts on biodiversity that are likely to be serious,” says Nachiket Kelkar, one of the authors of the paper and a PhD student at ATREE.
Dolphins ‘see’ by echolocation, a phenomenon where they emit high-frequency sounds or clicks to communicate, navigate, and detect threats, preys, or mates. Any threat to this mode of communication is a threat to their existence. In the natural underwater soundscape, no other sound overlaps with the dolphin’s call frequency. But noise frequencies from the propellers of different vessels overlap completely with these clicks, thus interfering with activities essential for the dolphins’ survival.
This prompted the researchers to examine the effect of vessel noise and river depth on the well-being of dolphins. Vessel noise is amplified during the dry season due to low river depths. The researchers used sound production by the dolphins as an indicator of their well-being, since this is an energy-demanding activity. Changes in sound production are a signature of disrupted survival activities of dolphins.
Mayukh Dey, the first author of this paper and a postgraduate student at Wildlife Biology and Conservation, NCBS, points out that previous studies on the impact of noise on marine mammals have been carried out in oceans, where there is more space and animals can choose to move away from regions of disturbance. “But no study has shown the impacts of noise and water levels on mammals in constrained environments like a river,” he says.
The researchers conducted their study over six months (November 2017 to April 2018), twelve hours per day, at four sites in Bihar with different river depths and noise levels to understand which combination had the greatest impact on echolocation by the dolphins. The sites studied were Kahalgaon (deep, noisy), Janghira (deep, quiet), Barari (shallow, quiet) and Doriganj (shallow, noisy). The researchers also took into consideration any changes in energy intake due to reduction in food levels by human fishing activities.
The researchers found that an increase in vessel noise and a decrease in water levels skewed the dolphin’s sound production. To compensate for interference from vessel noise, dolphins increased the volume of their calls/clicks but did not go above a certain noise level, to conserve energy. As a result, their ability to hunt fish reduced by 40% in noisy conditions which reduced their energy intake over time. According to Kelkar, constantly having to modify their behaviour to adjust to the surrounding noise levels can increase the dolphins’ stress levels, which can impact their health in the long-term.
Apart from vessel noise, riverbed excavation, capture by fishing nets, physical injury from vessels, altered river flows, poaching, and pollution are other serious threats to the ecology, behaviour, and existence of dolphins.
“This study could serve as an example for future initiatives to understand the impacts of other anthropogenic interventions on different river animals. One future direction would be to replicate this study across the habitat range of river dolphins. It would also be interesting to extend this study to fishes, which also use sound for communication,” says Dey.
Ravindra Kumar Sinha, Padma Shri awardee and vice-chancellor of Shri Mata Vaishno Devi University, Jammu and Kashmir, who was not associated with this research, says that there have been discussions regarding the impact of underwater noise on Ganges river dolphins since the last three decades without any scientific evidence. This study provides a scientific database which not only highlights and creates awareness about the impacts of underwater noise and declining flow in our rivers, but would be useful for convincing concerned authorities to take necessary actions against underwater noise pollution.
Sinha says, “One of the causes of extinction of Yangtze river dolphin in China was supposed to be underwater noise due to high river traffic. This study raises concerns of impacts of the same on the Ganges river dolphin.”