Ganges river dolphin

(Platanista gangetica gangetica)

Common or local names: South Asian river dolphin, Ganga dolphin, Gangetic dolphin. Many names in local languages are reminiscent of the noise the dolphin makes when it breathes, such as susu, soos, shushuk, socho, shus and suongsu.

Introduction

IUCN Red List Conservation status: Endangered

The Ganges river dolphin is known as the “Tiger of the Ganges” for the role it plays as a top predator, and because it is an ecosystem indicator species – much like a tiger is in a forest. It is legally protected in all countries within which it is found and is the National Aquatic Animal of India. Both Indus and Ganges river dolphins are considered to be living fossils, as they are the most ancient dolphin species still alive.

Distribution and habitat

The South Asian river dolphin consisted of two sub-species, the Ganges river dolphin and the Indus river dolphin. The taxonomy of the group is in the process of being revised so that they are recognised as two separate species during the course of 20211. The distributional range of the Ganges river dolphin includes over 10,000 km of the Ganges-Brahmaputra-Meghna, and the Sangu-Karnaphuli River systems in Bangladesh, India and Nepal. Ganges river dolphins are commonly found in deeper sections of the river, and they prefer eddies around islands, river bends and confluences which are often also the places where people prefer to fish2, 3. Now dams and barrages restrict dolphin movement, but in the past during the monsoon season, the dolphins would have moved upstream into smaller rivers, and then back downstream to larger river channels in the low water winter season4.

Abundance

Surveys to estimate abundance or the number of the Ganges river dolphins have been conducted in many places in India, Nepal and the Sundarbans of Bangladesh. The total of all dolphin counts throughout the range is approximately 5,000 individuals. However, some significant parts of the range of the dolphin have not been surveyed, especially in Bangladesh. Only very small numbers of dolphins (<100) are present in Nepal5. The largest numbers of dolphins documented are in the main stem of the Ganges River in India and the Brahmaputra River in Bangladesh6. As of 2018, a total of about 1,340 dolphins were recorded between the Uttar Pradesh-Bihar border and the Farakka barrage (625 km) on the Ganges mainstem6, and this area also had the highest recorded encounter rate/ linear km2, 7. There were an estimated 225 Ganges river dolphins in the Bangladesh Sundarbans in 20028.

Threats

The major threats to the Ganges river dolphin include water-related infrastructure such as dams, barrages, diversions, and embankments which cause flow regulation and habitat fragmentation, mortality from entanglement in fishing nets, hunting of dolphins for oil and meat, pollution from domestic, industrial and agricultural sources, and disturbance from human activities including boat traffic, dredging and underwater noise7, 9, 10. Dams and barrages fragment the dolphins habitat, reducing population connectivity and isolating animals in river sections. They also affect river habitats by reducing water availability and habitat quality, and blocking passage for migratory fish11, 12. Ganges river dolphins, as with many other cetacean species, frequently become entangled in fishing nets and then drown. As long-lived top predators, Ganges river dolphins are vulnerable to negative health effects from living in waterways contaminated with heavy metals, industrial pollution and agricultural run-off13, 14. River cetaceans, including the Ganges river dolphin, are considered to be indicators of river health15.

Biology and ecology

Ganges river dolphins are usually a grey or light brown colour, but may also have a pinkish tone to the belly1. The dorsal fin is very small, and is a low fleshy lump on the back, usually just a few centimetres in height. The dolphins have a steep forehead, and flexible necks with unfused vertebrae which permit them to turn their heads from side to side. They have an elongated snout that can bend slightly upward; it can reach up to 20% of total body length in females. They have long, sharp teeth which are visible even when the mouth is closed. The pectoral flippers on the side of the body are large and paddle shaped.

This species is functionally blind, and therefore relies heavily on echolocation to navigate and hunt4, 16. Externally, the eye appears to be barely larger than a pinhole and therefore restricting light from reaching the retina, there is no lens, and the optic nerve is very narrow, leading scientists to conclude that their eyes are incapable of forming clear images, but that they may still serve as light receptors4, 16.

Ganges River Dolphin Illustration by Uko Gorter

Reproduction and growth

Not much is known about the mating and reproduction of Ganges river dolphins except that they breed year-round and most births are thought to occur between October-March17. Gestation is thought to be 8 to 9 months4, 18. At birth, calves are about 70 cm long, but adults can reach about 2.60m and weigh up to 90 kg as adults. Females are generally longer than males due to a longer rostrum1, 18.

Ganges river dolphin females give birth to a single offspring, usually weaning them within a few months. Males and females typically reach sexual maturity at approximately 10 years of age18. The oldest known Ganges river dolphin was a 30 year old male1.

Diet

Ganges river dolphins eat a large variety of small and medium sized fish and crustaceans19. It is thought they use echolocation clicks to scan and detect prey at distances of about 20 m but that they may use electroreception to detect prey that occurs near to the river bottom20.

Behaviour

Like Indus river dolphins, Ganges river dolphins have developed a unique side swimming behaviour which is an adaptation to help them navigate through shallow waters16. Dives are typically short, lasting between 70-100 seconds. Like other dolphin species, they navigate using echolocation, but reflecting the complexity of the river environment Ganges river dolphins emit sound almost constantly. Ganges river dolphins are most frequently found alone, but they also gather in small groups, sometimes with more than 10 individuals, at the confluence of tributaries19. Generally, these animals are shy towards humans.

References

  1. Braulik, G. T., Archer, F., Khan, U., Imran, M., Sinha, R. K., Jefferson, T. A., Graves, J.A. (in press). Taxonomic revision of the South Asian River dolphins (Platanista): Indus and Ganges River dolphins are separate species. Marine Mammal Science.
  2. Kelkar, N., Krishnaswamy, J., Choudhary, S., & Sutaria, D. (2010). Coexistence of fisheries with river dolphin conservation. Conservation Biology, 24(4), 1130-1140.
  3. Smith, B. D. (1993). 1990 Status and conservation of the Ganges River dolphin Platanista gangetica in the Karnali River, Nepal. Biological Conservation, 66, 159-169.
  4. Anderson, J. (1879). Anatomical and zoological researches: comprising an account of the zoological results of the two expeditions to western Yunnan in 1868 and 1875 and a monograph of the two cetacean genera Platanista and Orcella (Vol. 1 – Text and Vol. 2 – Plates): Bernard Quaritch, Piccadilly, London.
  5. Paudel, S., Pal, P. C., M.V., Jnawali, S. R., Abel, G., Koprowski, J. L., & Ranabhat, R. (2015). The Endangered Ganges River dolphin Platanista gangetica gangetica in Nepal: abundance, habitat and conservation threats. Endangered Species Research, 29(1), 59.
  6. Convention on Migratory Species (CMS) 2020. Proposal for a Concerted Action for the Ganges River Dolphin (Platanista gangetica gangetica) already listed on Appendix I and II of the Convention. Prepared by the Government of India. Agenda Item 28.2 UNEP/CMS/COP13/Doc.28.2.6/Rev.2 presented at the 13th Meeting of the Conference of the Parties, Gandhinagar, India, 17-22 February 2020.
  7. Sinha, R. K., & Kannan, K. (2014). Ganges River Dolphin: An Overview of Biology, Ecology, and Conservation Status in India. Ambio, 43(8), 1029-1046.
  8. Smith, B. D., Braulik, G., Strindberg, S., Ahmed, B., & Mansur, R. (2006). Abundance of Irrawaddy dolphins (Orcaella brevirostris) and Ganges River dolphin (Platanista gangetica gangetica) estimated using concurrent counts made by independent teams in waterways of the Sundarbans mangrove forest in Bangladesh. Marine Mammal Science, 22(2), 527-547.
  9. Reeves, R.R., Smith, B.D. and Kasuya, T., eds., 2000. Biology and Conservation of Freshwater Cetaceans in Asia. IUCN, Gland, Switzerland and Cambridge, UK, viii + 152 pp.
  10. Braulik, G.T., Smith, B.D., 2017. Platanista gangetica. The IUCN Red List of Threatened Species 2017: e.T41758A50383612. http://dx.doi.org/10.2305/IUCN.UK.2017-3.RLTS.T41758A50383612.en. Accessed 05 September 2018.
  11. Smith, B. D., & Reeves, R. R. (2000). Report of the Workshop on the Effects of Water Development on River Cetaceans, 26-28 February 1997, Rajendrapur, Bangladesh. In R. R. Reeves, B. D. Smith & T. Kasuya (Eds.), Biology and Conservation of Freshwater Cetaceans in Asia (pp. 15-21). Gland, Switzerland & Cambridge, U.K.: IUCN.
  12. Braulik, G. T., Arshad, M., Noureen, U., & Northridge, S. (2014). Habitat fragmentation and species extirpation in freshwater ecosystems: causes of range decline of the Indus River dolphin (Platanista gangetica minor). PLoS ONE, 9(7), e101657. https://doi.org/101610.101371/journal.pone.0101657.
  13. Senthilkumar, K., Kannan, K., & Sinha, R. K. (1999). Bioaccumulation profiles of polychlorinated biphenyls congeners and organochlorine pesticides in Ganges River dolphins. Environmental Toxicology and Chemistry, 18(7), 1511-1520.
  14. Kannan, K., Sinha, R. K., Tanabe, S., Ichihashi, H., & Tatsukawa, R. (1993). Heavy metals and organochlorines residues from Ganges River dolphins from India. Marine Pollution Bulletin, 24(124), 159-162.
  15. Turvey, S. T., Risley, C. L., Barrett, L. A., Yujiang, H., & Ding, W. (2012). River dolphins can act as indicator species in degraded freshwater systems. PLoS ONE. 7(5): e37902. doi:10.1371/journal.pone.0037902
  16. Herald, E. S., Brownell, J. R. L., Frye, F. L., Morris, E. J., Evans, W., E., & Scott, A. B. (1969). Blind river dolphin: first side-swimming cetacean. Science, 166, 1408-1410.
  17. Sinha, R. K., Behera, S., & Choudhary, B. C. (2010). Conservation Action Plan for the Gangetic River dolphin 2012-2020. India: National Ganga River Basin Authority, Ministry of Environment and Forests.
  18. Brownell, R. L. (1984). Review of reproduction in Platanistid dolphins. Report of the International Whaling Commission(Special Issue 6), 149-158.
  19. Sinha, R. K., Das, N. K., Singh, N. K., Sharma, G., & Ahsan, S. N. (1993). Gut-content of the Gangetic dolphin, Platanista gangetica. Investigations on Cetacea, 24, 317-321.
  20. Kelkar, N., Dey, S., Deshpande, K., Choudhary, S. K., Dey, S., & Morisaka, T. (2018). Foraging and feeding ecology of Platanista: an integrative review. Mammal Review, 48(3), 194-208.

Also in this section

Amazon river dolphin

The Amazon River dolphin is a unique freshwater species found only in the rivers of South America.

Indus river dolphin

Indus river dolphins are also known as the blind dolphins because over millions of years they have lost the use of their eyes.

Irrawaddy dolphin

The Irrawaddy dolphin is found in marine, brackish, and freshwater environments throughout southeast Asia. The name is derived from the first specimens that were described from the Ayeyarwady river in Myanmar.