Ganges River dolphins strongly alter their acoustic behaviour in response to underwater noise, finds study from India

Article written by Nachiket Kelkar1,2
 1Ashoka Trust for Research in Ecology and the Environment, Bangalore, India.
2 Member, IUCN Cetacean Specialist Group.
Email: nachiket.kelkar@atree.org

 

The impacts of underwater noise on marine cetaceans are starting to be relatively well understood across the world’s oceans. Noise can trigger responses ranging from avoidance to chronic stress, to permanent hearing loss and sometimes even stranding, injury, and mortality (see here for a recent review of impacts: Erbe et al. 2019; https://doi.org/10.3389/fmars.2019.00606). Direct investigations of the effects of noise on freshwater cetaceans, however, remain very limited.

 

Ganges River dolphins Platanista gangetica gangetica are listed as Endangered on the IUCN Red List, and are found only in the shallow, turbid rivers of the Indian subcontinent (India, Bangladesh and Nepal). They are almost blind and continuously emit high-frequency echolocation clicks to sense their environment, navigate, and locate prey (fish and shrimp). Importantly, Ganges River dolphins do not produce whistles, and their acoustic repertoire is restricted to modulations of their clicks. A belief even among some experts has been that the mostly low-frequency underwater noise from vessel engines does not affect these river dolphins. However, riverine environments have different background conditions that can complicate assessments of noise impacts on dolphins. Reverberation and reflection effects in shallow river habitats are complex. Further, space is naturally restricted for river dolphins, limiting their ability to avoid vessel traffic and noise as both vessels and river dolphins need adequate depth.

 

A study published in Scientific Reports https://www.nature.com/articles/s41598-019-51664-1 by Mayukh Dey, Jagdish Krishnaswamy, Tadamichi Morisaka, and Nachiket Kelkar shows that underwater noise, especially that produced by propeller-induced cavitation, can alter the acoustic behaviour of dolphins in the Ganga River in India. At a point in March when vessel traffic peaked and recorded flows in the river were at their lowest, the dolphins actually suppressed their echolocation activity (click rate) and  also showed shifts in the peak frequency and sound pressure level of their clicks, indicating fairly significant impacts of ambient noise on echolocation. The noise also masked their communication clicks, and caused increases in metabolic stress.

 

The study also found that at shallow river depths, the impacts of underwater noise were even more severe. It thus highlighted the importance of considering underwater noise pollution as a factor that should be considered in the management of ecological flows in the Ganga mainstem and in tributaries where waterways for ship navigation are planned.

 

The study attains particular importance in today’s conservation context. Many large rivers in the Ganga-Brahmaputra basin, where river dolphins occur, are to be transformed into major waterways by the Government of India. In 2016, a letter jointly signed by the IUCN Director-General and the Chair of the Species Survival Commission, expressed concern about the potential impacts of waterways development on river ecology and dolphin conservation: https://iucn-csg.org/iucn-letter-to-the-indian-minister-of-environment-concern-over-the-impact-of-indias-national-waterways-act-on-ganges-river-dolphins/. Between 2016 and 2019, the issue has been discussed often in environmental policy debates in India. Indian waterways authorities are also in the process of conducting studies on the impacts of dredging and noise on the dolphins. It is expected that the findings of this recent study by Dey et al. will inform the species recovery programs and conservation initiatives being carried out for the species at national and state levels and by civil society (NGOs).

 

Spectrogram showing the echolocation clicks of a Ganges river dolphin (top), and another spectrogram of clicks (bottom) when a motorised vessel passes. The clicks that are seen to be masked, are masked by cavitation noise. The high-frequency range of such noise clearly interferes with the acoustic signalling of Ganges river dolphins. Graphs: courtesy of Dey et al. (2019).

Citation: Dey, M., Krishnaswamy, J., Morisaka, T., & Kelkar, N. Interacting effects of vessel noise and shallow river depth elevate metabolic stress in Ganges river dolphins. Scientific Reports 9, 15426 (2019).

New Cetacean Specialist Group Logo

We decided that it was time for an upgrade of the Cetacean Specialist Group Logo. The importance of a professional and attractive logo has increased in recent years; a strong logo has the ability to convey to the world what the Cetacean Specialist Group is, and it will help us with our social media and online communication.  Uko Gorter offered his impressive design services for this effort, and we are extremely grateful.  The new logo depicts three of the most endangered cetaceans, the vaquita, the South Asian river dolphin and the North Atlantic Right whale, all species that are the target of much of the work of the CSG.

These are already displayed on our website, facebook page, and our Twitter profile.

Critically Endangered Taiwanese white dolphin: International Collaboration for Recovery Plan

In August of 2019, Wild at Heart Legal Defense Association (Wild at Heart) and Matsu Fish Conservation Union (MCFU), both based in Taiwan, organized and sponsored an international workshop for the purpose of producing a recovery plan to identify actions needed to stop the decline of Taiwanese white dolphins (Sousa chinensis taiwanensis) and promote their recovery. Sixteen experts, including experts in science and conservation policy, along with representatives of Taiwanese NGOs and the Taiwan Ocean Conservation Administration, gathered at the University of Western Ontario in London, Ontario, Canada for a 4-day workshop to produce the plan.   

 

Participants at the Workshop to Develop a Recovery Plan for the Taiwanese White dolphin

The subspecies was discovered in 2002 and described and classified as a subspecies of the Indo-pacific Humpback dolphin (Sousa chinensis) in 2015.  With no more than 75 individuals existing, the subspecies, has been red-listed since 2008 as Critically Endangered. It was also listed in 2018  as Endangered under the US Endangered Species Act. There is evidence that the numbers have declined to below 65 individuals since the times of those assessments and listings.

 

The massive development of windfarms along Taiwan’s densely populated west coast represents a recent and growing threat to Taiwanese white dolphins, since it involves a substantial increase in boat traffic and construction noise, as well as functionally reducing the extent of their habitat. The proliferation of windfarms could also result in fishing efforts becoming more concentrated in the dolphins’ nearshore habitat. These negative effects on a critically endangered subspecies could be at least partially offset if the greater and overarching threat of entanglement in fishing nets was eliminated.

 

Immediate Actions Recommended

 

A ban on gill and trammel nets is the single most urgently needed action, according to the recovery plan. If effectively enforced, such a ban would go a long way towards halting the dolphins’ decline. The workshop proposed a creative solution: Companies and financial institutions involved in ongoing offshore windfarm development should help finance government programs to eliminate gill and trammel nets from dolphin habitat.

This was seen as a ‘win-win-win’ solution but the windfarm developers, government agencies and fishermen would all have to adopt and implement such a solution immediately. Ideally, the developers would benefit from having their conflicts with the fishing community resolved while at the same time blunting some of the criticism that has been directed their way. The fishing community would benefit by receiving adequate compensation. And most importantly, the Taiwanese white dolphins would no longer die in gill and trammel nets.

 

The workshop also identified five other actions that may not have such immediate effects, but need to be implemented quickly for sustained dolphin recovery:

  • Locate any new development projects and related impacts away from the dolphins’ habitat;
  • Establish mandatory routes and speed limits for vessels to reduce both noise and the risk of vessel strikes;
  • Reduce air, water, and soil pollution;
  • Increase natural river flows; and
  • Establish regulations to limit human-caused underwater noise levels in dolphin habitat.

 

The recovery plan is available at here.

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