Species

Ganges River dolphin

April 19, 2012, 4:42 pm
Content Cover Image

Antique engraving of the Ganges River dolphin. Source: Lydekker, 1894 Royal Natural History. Vol 3

The Ganges River dolphin (scientific name: Platanista gangetica) is s freshwater cetacean closely related to the Indus river dolphin (Platanista minor). These two endangered dolphins were long regarded as a single species and some regard them as two subspecies rather than distinct and separate species.Though Platanista minor and Platanista gangetica barely differ physically except for slight differences in tail lengths, the two species are distinguishable by their ranges. P. minor lives only in the Indus River system, while P. gangetica only inhabits the Ganges River system.

The close connection of these species is likely related to the fact that "until the late Pliocene, the present-day Indus, Ganges and Brahmaputra (except for the upper reach, the Yarlung Zangpo Jiang) Rivers constituted a single westward-flowing river called the Indobrahm (Hora 1950, 1953). Even up until historical times there was probably sporadic faunal exchange between the Indus and Ganges drainages by way of head-stream capture on the low Indo-Gangetic plains, between the Sutlej (Indus) and Yamuna (Ganges) Rivers (Dey 1968)." 

caption Ganges river dolphin. Source: S K Sinha
caption Size comparison of an average human against the Finless porpoise. Source: Chris Huh

Conservation Status:

Scientific Classification

Kingdom: Animalia
Phylum:--- Chordata
Class:------ Mammalia
Order:-------- Cetacea
Family:-------- Platanistidae
Genus:--------- Platanista
Species:--------Platanista gangetica (Roxburgh, 1801)

Common Names:
Blind river dolphin
Ganges River dolphin
Ganges river-dolphin
Ganges Dolphin
Ganges Susu
Gangetic dolphin
Indian river-dolphin
South Asian river dolphin
Susu
Bhulan

Physical Description

Overview physical characteristics include endothermic metabolisma and bilateral symmetry. The Ganges River dolphin is gray or brown in colour, though sometimes a lighter colouration is exhibited on the underside.

Their beaks are distinctively swollen at the tip and are highly elongated, attaining 20 percent of the length of their bodies, with large visible teeth.

In contrast to the beak, the dorsal fin is rather small and reduced compared to other river dolphin species. Large flippers and flukes, combined with long and remarkably flexible necks, assist this cetacean in effective navigation. Platanista minor has external ears located below the eyes, which are diminutive and likely can only resolve shadowy, unclear images.

Though Platanista minor and Platanista gangetica barely differ physically except for slight differences in tail lengths, the two species are distinguishable by their ranges. Platanista minor lives only in the Indus River system, while Platanista gangetica only inhabits the Ganges River system.

There is a species sexual dimorphism, with females being larger than males. (Moreno, 2004)

Behaviour

The Ganges River dolphin is a solitary marine mammal, but occasionally congregates in groups of three to ten individuals; moreover, groups of up to 30 animals have been reported. Mothers and calves stay together until the infants are weaned. Despite their mostly solitary nature, these river dolphins are found in loose aggregations, especially at tributary junctions where prey congregate. Some consider Ganges River dolphins semi-gregarious.

There are some indications of territoriality, as chasing behaviors in males have been observed. Generally, these animals are shy towards humans, even in captivity. Their elusive nature has made them difficult subjects to study. (Moreno, 2003; Nowak, 2003; Perrin, Wursig, and Thewissen, 2002; Reeves and Brownell Jr., 1989)

Ganges River dolphins have a peculiar method of swimming, in that they swim on their sides when submerged. At roughly a ten degree angle, they swim a few centimeters (cm) from the bottom, constantly nodding the head, allowing it to sweep the bottom in search of food. For unknown reasons, most individuals swim on their right sides. Side swimming also positions the eye at an angle appropriate to sensing sunlight, perhaps providing orientation for the animal while diving. Their tails are always positioned higher than the head in side swimming, allowing them to swim in water as shallow as 30 cm. When they surface for air, they level out and swim laterally.

In captivity, measured swimming speed reached 5.4 km/hr, but this may not be representative of possible speeds in the wild, where habitats are more open. They have been recorded swimming upwards of 27 km/hr in the wild. Captive animals swim and vocalize continuously over a 24 hour period, with only brief interruptions lasting a few seconds. When swimming intensity relaxed, for example in drifting locomotion, the intensity of vocalizations emitted also relaxed.

Dives are typically short, the longest wild dive was three minutes. Average dive times in the wild are between 70 seconds and 100 seconds. Dives in captivity are shorter than wild dive times, with the longest being 95 seconds. In many cetaceans, myoglobin concentrations are generally high to cope with the stress of diving, but due to their relatively shallow river habitats and short dive periods, myoglobin concentrations in heart and muscle tissues are considerably lower.

Ganges River dolphins have poor vision. They lack lenses in their eyes, making it impossible for them to resolve images, they are likely to only be able to detect the presence or absence of light. Ganges River dolphins have highly developed sonar systems. They use pulse sounds not whistles to navigate. This allows them to perceive objects, specifically prey, in murky water. Over a 24-hour period there is almost always a constant emission of sound, 87% of these sounds are clicks for echolocation, the remaining sounds are sounds used in communication. There have not been enough studies to determine what the significance is of these communicative sounds. (MacDonald and Norris, 2001; Moreno, 2003; Nowak, 2003; Reeves and Brownell Jr., 1989)

Reproduction

The IUCN Red List reports: "Only very limited data are available on life history. Age at first reproduction is probably between six and ten years and maximum longevity may be close to 30. Therefore, generation time is probably well over 10 but possibly less than 20 years, which would mean that three generations equals at least 30 years (i.e., from 1974 counting backwards or until 2034 counting forwards) but less than 60 (i.e., from 1944 counting backwards or until 2064 counting forwards)."

Not much is known about mating systems in Ganges River dolphins. Further studies must be done to provide information regarding their mating behavior.  Difficulty studying these species can in part be attributed to environmental conditions in their habitat due to the monsoon season. In addition the political and socioeconomic state of the area where these dolphins are found is not conducive to research. (Moreno, 2003; Reeves et al., 2002)

Breeding in Ganges River dolphins occurs year around, as does birthing. Most births are from October to March, with a peak in December and January, preceding the beginning of the dry season.

Gestation is typically about ten months but can range from eight to twelve months. Ganges River dolphins bear a single offspring from 70 to 90 cm long. Weaning can begin as early as two months or as late as 12 months, typical time to weaning is at age nine months. Once offspring have been weaned, they disperse and become independent. (Jefferson, Webber, and Pitman, 2008; Moreno, 2003; Nowak, 2003; Reeves et al., 2002) 

Distribution and Movements

Ganges River dolphins occupy freshwater river systems in southern Asia. They inhabit the Ganges River system and their many tributaries, streams, and connecting lakes. They are found in tributaries that run through the hills and lowlands in Nepal (roughly 250 meters above sea level) and sometimes in flood plains and areas of rivers with heavy currents.

These river dolphins prefer areas that create eddy countercurrents, such as small islands, river bends, and convergent tributaries. Since these animals occupy a vast area of river systems, they can tolerate a wide variance of temperatures; some as cold eight degrees Celsius to warm waters above 33 degrees Celsius (46.4F to 91.4F). They inhabit depths from three to nine meters and must surface every few minutes for air. In the monsoon season, Ganges River dolphins locally migrate to tributaries and then back to larger river channels in the dry, winter season. They also move along the coast of the Bay of Bengal when monsoons flush freshwater out along the southeastern coast of India.(Moreno, 2003)

The IUCN Red List provides additional details:

This species occurs in the Ganges-Brahmaputra-Megna, and Karnaphuli-Sangu river systems of the South Asia, from the deltas upstream to where they are blocked by rocky barriers, shallow water, fast currents, dams, or barrages (low, gated diversion dams). The two river systems are disjunct and therefore so are their respective dolphin subpopulations, although there may be occasional demographic interaction between the latter two during the high-water season if the freshwater plumes of the two systems meet. There is further subpopulation separation within all three systems, some of it natural but much of it caused by physical barriers constructed within the last 100 years.

. . . Highest "densities" of Ganges River dolphins (defined as animals per linear river kilometer) have been observed in the Ganges mainstem between Maniharighat and Buxar (1.5/km) (Sinha, unpublished) - and within this segment particularly in the Vikramshila Gangetic Dolphin Sanctuary (2.8/km) (Smith, unpublished) and just downstream between Kahalgaon and Manihari Ghat (near Katihar) (3.4/km) (Sinha, unpublished) - and the lower Sangu River, Bangladesh (1.4/km) (Smith et al. 2001). A few Ganges River dolphins were still present during the mid 1990s as far downstream in the Hoogly River as Kakdwip (Sinha 1997). In the Sundarbans of Bangladesh a minimum of 134 Ganges River dolphins were counted resulting in a "density" estimate of 0.09 individuals/km, with Irrawaddy dolphins Orcaella brevirostris replacing Platanista in higher salinity waters of the southern and western portions (Smith, unpublished). No information is available on the status of Ganges River dolphins in the Indian Sundarbans, except for historical reports of occurrence (Anderson 1879, Jones 1982).. . .

The species range . . . has declined progressively since the nineteenth century when it was mapped by Anderson (1879). No dolphins have been reported in recent years between the Madhya Ganga Barrage at Bijnor and the Bhimgoda Barrage near Haridwar, at the upstream limit of their historical range in the Ganges (Sinha et al. 2000). This suggests a roughly 100 km decline in their range in the Ganges River since the late 1800s. In recent years, dolphins have not been reported in the Yamuna River above the Chambal River confluence during the dry season because upstream channels have become too shallow and polluted to support dolphins, but the segment may still be occupied during the monsoon (Sinha et al. 2000). Historically, they were found year-round in the Yamuna River approximately 400 km upstream to Delhi (Anderson 1879).

Elsewhere in the Ganges mainstem, four extant subpopulations are isolated by barrages, including Farakka Barrage located approximately at the center of the subspecies range. In the northern Ganges tributaries, of the six subpopulations that were isolated above or between barrages, three have been extirpated (in the Gandak River above the Gandak Barrage and in the Sarda River above the upper and lower Sarda barrages) (Sinha et al. 2000) and one reduced to insignificant numbers (in the Kosi River above the Kosi Barrage) (Smith et al. 1994). Ganges River dolphins have apparently been extirpated from the Son River (at least during the dry season) based on a survey covering approximately 300 km upstream of the Ganges confluence, above and below the Indrapuri Barrage (Sinha and Sharma 2003).

Occasional reports of dolphins in the reservoir behind Kaptai Dam (built in 1961) of the KS system occurred until the mid-1990s (Ahmed 2000), but recent surveys have found no evidence that the subspecies survives there (Smith et al. 2001). Thus, the dam's construction is likely to have caused a substantial reduction in the subspecies’ range in southeastern Bangladesh, but in the absence of any historical information on occurrence in the upper Karnaphuli no quantitative estimate of range reduction is possible.

No surveys have been conducted in the Damodar River system but a single dolphin was rescued after becoming stranded in a deep pool after flow was diverted during the dry season by an upstream barrage. The downstream effects of at least ten dams and barrages constructed in its mainstem and tributaries has probably severely reduced and fragmented dolphin habitat (Smith et al. 2000).

Habitat

 The IUCN Red List reports:

Ganges River dolphins are generally concentrated in counter-current pools below channel convergences and sharp meanders (Kasuya and Haque 1972, Smith 1993, Smith et al. 1998) and above and below mid-channel islands, bridge pilings, and other engineering structures that cause scouring (Smith, unpublished data). Their fidelity to counter-current pools is probably greatest in fast-flowing channels (Smith et al. 1998). Annual monsoon-driven floods cause great variability in the dolphins’ access to large parts of their range. Isolation in seasonal lakes sometimes occurs (especially in the Brahmaputra basin), as does "escapement" from the river channels into artificial water bodies such as canals and reservoirs. Deltaic (brackish) waters are a major component of the total range, but Ganges River dolphins are not generally known to occur in salinities greater than ten parts per thousand (ppt), although they have been recorded in waters as saline as 23 ppt (Smith and Braulik, unpublished data)

Feeding Habits

Ganges River dolphins are top predators in their river ecosystems. Side swimming and a flexible neck allow them to search river bottoms to stir up hiding prey. Their formidable speed and ability to swim in shallow water allows them to chase and herd schools of fish. They feed on a variety of aquatic animals. Their physical appearance demonstrates how well equipped they are to catch fish and crustaceans. They are strictly carnivorous, although some vegetation has been found in their stomachs, most likely as a result of messy foraging in the river bed or left over plant remains inside the fish the dolphins have consumed. Their teeth and long snouts are designed to catch and hold fish. They have been observed shaking prey in their jaws and manipulating it to be swallowed head first so that the scales on the fish do not move against the animals throat. As these dolphins do not use vision as a sensory system to catch prey, they rely on echolocation to find food hidden in the mud and river bottom. Once prey are located, they grab it with their long snouts. (Reeves and Brownell Jr., 1989)

Threats and Conservation Status

The IUCN Red List reports:

Although the aggregate range-wide abundance of Ganges River dolphins was estimated by Jones (1982) as 4000–5000 individuals and more recently by Mohan et al. (1997) as fewer than 2000, these were only guesses. . . [in the regions studied] the total of about 1200–1800 animals provides a reasonable lower range for the total metapopulation abundance. However, considering that areas with potentially large numbers of animals have not been accounted for at all (e.g., Indian portion of the Sundarbans Delta) and that at least some of the counts and estimates are known to be negatively biased (e.g., see Smith et al. 2001), the true number could be several times as high.

Numerical Declines: Although no credible time series of abundance estimates are available for most of the subspecies' range, [studies] imply downward trends in a number of upstream tributaries.

Further, major threats include: 

Water Development Projects

Construction of at least 50 dams and dams within the known or suspected historical range of the subspecies (Smith et al. 2000) has dramatically affected its habitat, abundance, and population structure. The subspecies exists as a metapopulation, with numerous subpopulations isolated to varying degrees by mostly manmade but also natural barriers, as outlined in the preceding section.

In addition to fragmenting dolphin populations, dams and barrages degrade downstream habitat and create reservoirs (known as head ponds (or pondage in India) in the case of barrages) with high sedimentation and altered assemblages of fish and invertebrate species. For example, luxuriant growth of macrophytes and excessive siltation have eliminated suitable habitat immediately above Farakka Barrage (Sinha 2000). Moreover, the insufficiency of water released downstream of this barrage has eliminated dry-season habitat for more than 300 km, or until the Ganges (Padma)-Brahmaputra confluence (Smith et al. 1998). It has also allowed salt water to intrude an additional 160 km into the Sundarbans Delta (Rahman 1986), further decreasing the amount of suitable habitat for this obligate freshwater dolphin (Reeves et al. 1993).

A high dam has been planned for some time just upstream of the species current (or at least recent) range in the Karnali River, Nepal. If built, this structure would almost certainly eliminate the small amount of dolphin habitat in Nepal’s last river with a potentially viable dolphin population (Smith and Reeves 2000). Disturbance and environmental degradation associated with geotechnical feasibility studies and bridge and road construction for the dam already may have contributed to a decline in the number and range of dolphins above the Nepal-India border (Smith 1993, Smith et al. 1994). Another high dam has been proposed for the Surma River in Cachar, India, which would certainly affect dolphins downstream in the Kalni-Kushiyara distributary (Smith et al. 1994).

Since the 1980s, momentum has been growing within India to proceed with large-scale inter-basin water transfer projects, which will involve additional dam construction and diversion of water from rivers inhabited by dolphins. Although no final decision has been taken to proceed with construction, feasibility studies are to be completed in December 2005 and detailed project reports in 2006. It was anticipated in 2004 that, if built, the entire project would be finished by 2016. During the May 2004 national elections in India all political parties supported the construction of inter-basin water transfer projects and promised to accelerate the construction process. Several key categories of potential threat are: (a) further fragmentation of the dolphin metapopulation, (b) reduction or elimination of habitat simply in terms of dry-season flow, (c) "escapement" of dolphins into canals where they are unlikely to be able to get back into rivers and are therefore doomed, (d) cascading effects from interrupted migrations of prey organisms, degradation of prey spawning habitat etc., (e) contaminant flux leading to significant changes in chronic and/or acute exposure to toxins, (f) loss of complexity (channelization, sediment entrapment upstream of dams, etc.) making the rivers less habitable for dolphins, etc, and (g) downstream effects on the ecology of the delta (e.g., saline encroachment, loss of sediment).

Embankments cause sediments to be deposited in the riverbed instead of on the floodplain, thereby eliminating or reducing the extent of the eddy-counter currents where dolphins are generally found. (Smith et al. 1998). They also restrict access to floodplain habitat critical to the reproduction and growth of riverine fish species (Boyce 1990). Approximately 3,500 km of embankments have been constructed in the Ganges mainstem and Gandak, Buri Gandak, Bagmati, Kamala, Yamuna, and Son tributaries (Mishra 1999). Dolphins were apparently extirpated from at least 35 km of the Punpun tributary of the Ganges after embankments were constructed in 1975 (Sinha et al. 2000). Although plans for constructing an extensive system of embankments in the rivers of Bangladesh under the Flood Action Plan (FAP) coordinated by the World Bank (see World Bank 1990) have been drastically scaled-down, several projects are currently planned or being constructed that will have adverse effects on dolphin habitat. These include the Bank Protection and River Training Project (FAP 21/22), Brahmaputra River Bank Priority Works, and Jamalpur Priority Project (FAP 3.1) (Smith et al. 1998). Environmental assessments of these projects have not considered river dolphins, nor have they acknowledged the cumulative impacts of planned embankments, and others built before the FAP, on the fish and crustacean species eaten by river dolphins. Other sources of habitat degradation in the GBM system include dredging (Smith et al. 1998) and the removal of stones (Shrestha 1989), sand (Mohanet al. 1998), and woody debris (Smith 1993). These activities compromise the ecological integrity of the riverine environments, especially small tributaries where suitable habitat is limited and disproportionately vulnerable to local disturbance. Dolphin habitat is also threatened by water abstraction from surface pumps and tube wells, especially in the Ganges where the mean dry-season water depth has declined dramatically in recent years (Sinha, unpublished). Although the long-term implications of reduced dry-season flows in the Ganges are catastrophic, both for the survival of river dolphins and a major portion of the world’s human population that inhabits the Ganges basin, the cumulative effects of reduced water supplies have received little attention. Meanwhile, new projects to divert dry-season flow, such as Kanpur Barrage in the upper Ganges, continue to be constructed (Smith et al. 2000)

Toxic Contaminants 

Organochlorine and butyltin concentrations in samples from the tissues of Ganges dolphins were high enough to cause concern about effects (Kannan et al. 1993, 1994, 1997; Senthilkumar et al. 1999). Pollutant loads can be expected to increase with industrialization and the spread of intensive agricultural practices facilitated by water diversion. River dolphins may be particularly vulnerable to industrial pollution because their habitat in counter-current pools downstream of confluences and sharp meanders often places them in proximity to point sources in major urban areas (e.g., Allahabad, Varanasi, Patna, Calcutta, and Dhaka). Furthermore, the capacity of rivers to dilute pollutants (e.g., arsenic, DDT) and salts has been drastically reduced in many areas because of upstream water abstraction, diversion, and impoundment. Again, this problem is destined to worsen as more development takes place.

Hunting

Deliberate killing of river dolphins is believed to have declined in most areas but still occurs at least occasionally in the middle Ganges near Patna, India (Smith and Reeves 2000, Sinha 2002), in the Kalni-Kushiyara River of Bangladesh (Smith et al. 1998), and in the upper reaches of the Brahmaputra River in Assam, India (Mohan et al. 1997). Dolphins are killed by tribal people in the upper Brahmaputra for their meat and by fishermen in the middle reaches of the Ganges for their oil, which is used as a fish attractant.

Mortality in Fishing Gear

Mortality in fishing gear, especially gillnets, is a severe problem for Ganges River dolphins throughout most of their range (Mohan 1995, Smith and Reeves 2000). They are particularly vulnerable because their preferred habitat is often in the same location as the fishing grounds. In the middle Ganges, although harpooning is now "rare", mortality in fishing nets remains "widespread" (Sinha 2002). A specific problem is that, because dolphin oil is highly valued as a fish attractant, fishermen have a strong incentive to kill any animals found alive in their nets and even to set their nets strategically in the hope of capturing dolphins (described by Sinha 2002 as "assisted incidental capture").

Meaningful quantitative data on the magnitude of catches, either deliberate or incidental, are unavailable and unlikely to become available in the absence of a well-organized, adequately funded, and incorruptible fishery/wildlife management system.

Ganges River dolphins are legally protected from hunting in all range states. The Vikramshila Gangetic Dolphin Sanctuary, Bihar, India, between Sultanganj and Kahalgaon in the mainstem of the Ganges River was designated as a protected area for dolphins in August 1991 but there is little government support to enforce protective measures. The legal protection in India has been described as "completely ineffective" (Sinha 2002), however, a small measure of progress was the convening of the Regional Seminar on Environmental Laws in the Vikramshila Gangetic Dolphin Sanctuary, Bhagalpur, India in November 2003 (Anon 2002). Also, recent proceedings of the Patna High Court (C.J.W.C. No. 5628 of 2001) directed the state and federals governments to allocate funds for supporting efforts to conserve and monitor dolphins in the Ganges. In a few smaller tributaries, dolphins receive nominal protection by virtue of the fact that small portions of their habitat are within or adjacent to national parks and sanctuaries (e.g., Kaziranga National Park in Assam, India, National Chambal Sanctuary in Madhya Pradesh, India, and Royal Bardia National Park and Katerniya Ghat Gharial Sanctuary, respectively north and south of the Nepal-India border. Although field trials have shown that shark or fish oils would be efficient substitutes for dolphin oil as a fish attractant and some fishermen in the middle Ganges are now using oil made from fish scraps as an alternative, most apparently continue to use dolphin oil by preference or because suitable alternatives are not widely available in either the Ganges or Brahmaputra systems (Mohan and Kunhi 1996, Smith et al. 1998, Bairagi 1999, Sinha 2002). 

Further Reading

  1. "Platanista gangetica (Roxburgh, 1801)". Encyclopedia of Life, available from "http://www.eol.org/pages/328477". Accessed 04 May 2011.
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  3. Smith, B.D. & Braulik, G.T. 2008. Platanista gangetica. In: IUCN 2010. IUCN Red List of Threatened Species. Version 2010.4. <www.iucnredlist.org>. Downloaded on 04 May 2011
  4. IUCN (2008) Cetacean update of the 2008 IUCN Red List of Threatened Species.
  5. Ahmed, B. 2000. Water Development and the Status of the Shushuk (Platanista gangetica) in Southeast Bangladesh. In: R. R. Reeves, B. D. Smith and T. Kasuya (eds), Biology and Conservation of Freshwater Cetaceans in Asia<, pp. 62-66. IUCN Species Survival Commission Occasional Paper 23, Gland, Switzerland and Cambridge, UK.
  6. Anderson, J. 1879. Anatomical and Zoological Researches: Comprising an Account of Zoological Results of the Two Expeditions to Western Yunnan in 1868 and 1875; and a Monograph of the Two Cetacean Genera, Platanista and Orcella. Bernard Quaritch, London, UK.
  7. Anonymous. 2002. Proceedings of Regional Seminar on Environmental Laws: Violations and Regulations in Vikramshila Gangetic Sanctuary (November 30, 2002). Unpublished report, the Vikramshila Biodiversity Research and Education Centre, University Department of Botany, T. M. Bhagalpur University, Bhagalpur, Bihar, India.
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Life, E. (2012). Ganges River dolphin. Retrieved from http://www.eoearth.org/view/article/165679

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