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Platanista gangetica (Roxburgh,
1801)
English: South Asian river dolphin, Ganges river dolphin, susu
(P. g. gangetica) and Indus river dolphin, bhulan (P.
g. minor)
German: Ganges-Delphin, Indus-Delphin
Spanish: Delfín del río Ganges, Delfín del
río Indus
French: Plataniste du Gange, Plataniste de l'Indus
Family Platanistidae
Platanista gangetica © Wurtz-Artescienza
(see links)
1. Description
The body of the South Asian river dolphin is subtle and robust,
attenuating behind the dorsal fin to a narrow tail stock. The colouration
is grey all over and becomes blotchy with age. The snout is long
and widens at the tip, resembling a forceps. In females, the snout
is generally longer and may curve upwards and to one side. The eyes
are extremely small, resembling pinhole openings slightly above
the mouth. The dorsal fin is a low triangular hump. The broad flippers
have a crenellated margin, with visible hand and arm bones. The
flukes are also broad. Males are smaller than females, with 210
and 250 cm, respectively (Smith, 2002). Body mass can reach at least
85 kg (Jefferson et al. 2008).
2. Distribution
The Indus and Ganges populations were long regarded as identical
until Pilleri and Gihr (1971) divided them into two species, but
Kasuya (1972) reduced the two taxa to subspecies of a single species.
This is supported by the results of Yang and Zhou (1999), who found
that the difference between cytochrome-b sequences of Ganges and
Indus river dolphins was very small. According to Rice (1998, and
refs. therin) 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.
Current main distribution of P. gangetica
in the Indus and Chenab Rivers and the Ganges-
Brahmaputra river system (Smith and Braulik, 2008; © IUCN;
enlarge
map)
Susus and bhulans live exclusively in freshwater. One conservative
current view is that there are two disjunct subspecies, but this
view is controversial (Jefferson et al. 2008) and further research
is needed to establish the true relationship between these two populations:
P. g. minor Owen, 1853: formerly ranged in the Indus River
and its tributaries, the Jhelum, Chenab, Ravi, and Sutlej rivers,
of Pakistan and India, from tidal limits to the foothills (Rice,
1998). The range is now limited to the mainstream in areas located
between barrages (Reeves and Chaudry, 1998). A small population
has recently been discovered in a 60 km stretch of the River Beas
in Punjab, northern India in 2008 (WWF, 2009)
P. g. gangetica: Formerly distributed throughout the Ganges-Brahmaputra
river system of India, Bangladesh, Nepal, and possibly Sikkim and
Bhutan, below an elevation of about 250 m. In the Ganges valley
it ranges into most of the major affluents, including some of their
tributaries: the Son, Yamuna, Sind, Chain-bal, Rainganga, Gumti,
Ghaghara, Rapti, Gandak, Baginati, Ghugri, Kosi, Kankai, and Atrai
rivers. In the Brahmaputra valley it also ranges into many of the
major tributaries: the Tista, Gadadhar, Champamat, Manas, Bhareli,
Ranga, Dihang, Dibang, Lohit, Disang, Dikho, and Kapili rivers.
Downstream it ranges through most of the larger tributaries between
the Hugh and Meghna rivers, as far as the tidal limits at the mouths
of the Ganges. Also reported from the Fenny, Karnafuli, and perhaps
the Sangu, rivers to the southeast of the mouths of the Ganges (Rice,
1998). Ganges River dolphins live not only in the main channels,
but also during the flood season, in seasonal tributaries, and the
flooded lowlands (Jefferson et al. 1993). The distribution is said
to be restricted only by the lack of water and by rocky barriers
(Reyes, 1991).
3. Population size
Indus: A survey conducted in 2001 (Choudhary et al. 2006)
covered 1.375 km of the Indus River main channel, 136 km of Indus
River secondary channels, and 24 km of the Panjnad River, a tributary
of the Indus. The resulting abundance estimate was 965 dolphins.
Dolphins occurred in five subpopulations separated by irrigation
barrages. The three largest subpopulations were estimated between
Chashma and Taunsa Barrages (84 dolphins; 0.28/km), Taunsa and Guddu
Barrages (259 dolphins; 0.74/km) and Guddu and Sukkur Barrages (602
dolphins; 3.60/km). Reasons suggested for the high encounter rate
between Guddu and Sukkur Barrages include high carrying capacity,
low levels of anthropogenic threat, effective conservation, and
augmentation of the subpopulation by downstream migration of dolphins
from upstream (Choudhary et al. 2006). A few scattered individuals
may still occur upstream of the Chasma barrage in the Indus (Smith,
2000 and refs. therein).
In 2008, a small population of 10-15 dolphins has been discovered
in India, in a 60 km stretch of the River Beas (WWF,
2009). This lies in the drainage of the Indus, but upstream
of a lake formed by Beas Dam, in the Indian state of Himachal Pradesh.
The population is separated from conspecifics of the main population
occurring downstream in Pakistan by the Beas Dam and further downstream
by yet another barrage (Bill Perrin, pers. comm. 2010).
Low numbers may also occur downstream of the Trimmu, Sidhnai, and
Pandjnad barrages in the Chenab, Ravi, and Sutlej rivers, respectively
(Smith, 2000 and refs. therein).
Extrapolation of encounter rates to unsurveyed channels and application
of a correction factor to account for missed dolphins indicates
that the metapopulation may number approximately 1,200 individuals
(Choudhary et al. 2006). This compares well to the latest survey
conducted in Pakistan by WWF-Pakistan and the Ministry of Environment,
estimating the minimum population of Indus River dolphins at 1,341
animals (WWF, 2009).
Current estimates are higher than those from 1996: in the Sindh
Dolphin Reserve between the Guddu and Sukkur barrages 458 individuals
were counted in 1996, between Taunsa and Guddu barrages 143 individuals;
between the Chasma and Taunsa barrages 39 individuals (Smith, 2000,
and refs. therein). Reeves (1998, in Smith, 2000) interpreted the
counts reported in 1996 to indicate a total of approximately 600-700
individuals for the subspecies as a whole, about half the current
estimate.
Ganges: A recent survey conducted by WWF-India and its partners
in the entire distribution range in the Ganges and Brahmaputra river
system - around 6,000 km - identified fewer than 2,000 individuals
in India (WWF, 2009). This count compares well with the estimate
of Mohan et al. (1997). In mangrove channels of the Sundarbans Delta
in Bangladesh, data analysis using two different models resulted
in 196 individuals (CV = 12.7%) and 225 individuals (CV = 12.6%),
respectively (Smith et al. 2006). In the past there were fewer than
100 dolphins in Nepal, with the group of about 20 in the Karnali
River above Chisapani being the largest single concentration (Jones,
1982; Reeves and Brownell, 1989).
Two detailed accounts have recently been published: In the Brahmaputra
River between the Assam-Arunachal Pradesh border to the India- Bangladesh
border (856 km) a population assessment conducted in 2005 yielded
an estimate of 197 animals. Encounter rate was low with 0.23 dolphins
per km (Wakid, 2009). This is less than 50% of a former estimate
of 400 between South Salmara and Sadiya (Mohan et al. 1997). In
the Vikramshila Gangetic Dolphin Sanctuary, a ca. 60 km long segment
of the middle Ganges River in Bihar, India, the mean number of dolphins
recorded between 2001 and 2003 during upstream surveys was 119.4
(SD 31.8; range 88-174), with an encounter rate of 1.8 dolphins
km² (range 1.4-2.8) (Choudhary et al. 2006).
4. Biology and Behaviour
Habitat: This species is exclusively riverine. Relatively
high densities of dolphins are found at sites where rivers join
or just downstream of shallow stretches, in areas where the current
is relatively weak, off the mouths of irrigation canals, and near
villages and ferry routes. In the Indus, about 40-45% of the dolphin
population is found at junctions of tributaries with the mainstream,
at least during the dry season, presumably being attracted to these
areas by concentrations of prey (Reeves and Brownell, 1989, and
refs. therein).
In the river basins in India, the Ganges river dolphin is present
mostly in plains where the rivers run slowly. This seems to be opposite
to the habitat observed in Nepal, where the dolphin can be found
in relatively clear waters and rapids. In both areas, however, there
is a preference for deep waters (Reyes, 1991, and refs. therein).
Primary habitats are characterised by an eddy counter-current system
in the main river flow caused by a fine sand/silt point bar formed
from sediment deposits of a convergent stream branch or tributary.
Marginal habitats are characterised by a smaller eddy counter-current
system caused by an upstream meander. Dolphins concentrate in locations
of high prey availability and reduced flow (Smith 1993).
South Asian river dolphins have been found in water as cold as
8°C and as warm as 33°C (Reeves and Brownell, 1989 and refs.
therein). In the Bramaputra River, the number of dolphins occurring
in different depths was found to be significantly different and
the highest number was found in depths of 4.1- 6 m (Wakid, 2009).
In the Sundarbans mangrove forest of Bangladesh, Ganges River dolphin
distribution was conditionally dependent on low salinity, high turbidity,
and moderate depth during both low and high freshwater flow. Animals
prefer wide sinuous channels with at least two small confluences
or one large confluence (Smith et al. 2009).
Schooling: South Asian river dolphins are not usually considered
gregarious. In one of the few quantitative studies of group size,
it was found that 90% of the groups and 80.4% of the total dolphins
observed during the dry season in the Meghna and Jamuna Rivers of
Bangladesh were solitary individuals. However, other investigators
reported groups of up to 25 individuals near ferryboats in the Indus
River, or as many as 25-30 dolphins in a 1-km stretch of river (Reeves
and Brownell, 1989, and refs. therein).
Reproduction: Calving apparently can occur at any time of
the year, but there may be peaks in December to January and March
to May. Newborn calves have been observed mainly in April and May.
Calves are weaned within one year of birth (Jefferson et al. 2008).
Gestation lasts 10.5 months (Reidenberg and Laitman, 2009).
Food: South Asian river dolphins feed on several species
of fish, invertebrates, and possibly turtles and birds. They do
much of their feeding at or near the bottom, echolocating and swimming
on one side (Reeves and Brownell, 1989; Jefferson et al. 1993).
The long beak is possibly an adaptation for extracting prey from
crevasses or buried in soft sediment.
5. Migration
The marked seasonal changes in susu distribution and density over
much of its range are due, at least in large part, to fluctuations
in water levels. During the dry season from October to April, many
dolphins leave the tributaries of the Ganges - Brahmaputra systems
and congregate in the main channels, only to return to the tributaries
the following rainy season. They may become isolated in pools and
river branches during the dry season (Reeves and Brownell, 1989).
Observations in Nepal show that susus' move in and out of tributaries
of the Gandaki, Koshi, and Karnali systems during high water seasons,
probably spending lower-water seasons in deep pools of the tributaries.
In the main rivers, a decrease in abundance during the summer would
confirm a seasonal pattern of migration (Shrestha, 1989, in Reyes,
1991).
6. Threats
Direct catch: Oil extracted from blubber of the Ganges River
dolphin is used as a fish attractant in India and Bangladesh. This
oil fishery is associated with the mortality of hundreds of dolphins
every year. Whereas the hunting of river dolphins is now banned
in Pakistan, poaching presumably still occurs occasionally (IWC,
2000). Although deliberate killing is believed to have declined
in most areas, it presumably still occurs in the middle Ganges near
Patna, India, in the Kaini-Kushiyara River of Bangladesh, and in
the upper reaches of the Brahmaputra River in Assam, India (Mohan
et al. 1997). In Assam, they are also killed for their meat (IWC,
2000). Fish oil was repeatedly suggested as a substitute for susu
oil (Mohan and Kunhi, 1996; Bairagi, 1999) and shown to have a better
attractant effect on target species (Sinha, 2002).
Incidental catch:
Indus: Incidents of accidental killing and observations of
dolphin carcasses and products are documented in Reeves et al. (1991)
and Reeves and Chaudhry (1998). Little detailed information is available,
but the level of take is not thought to be high, even though the
Indus river dolphin is vulnerable to gillnets. Permanent losses
from the population also occur when animals swim into irrigation
channels. Since 1992 there have been reports of one or two dolphins
becoming trapped in these channels annually, but then were recorded
in the winter of 1999/2000 (IWC, 2000).
Ganges: Accidental killing is a severe problem for Ganges
River dolphins throughout most of their range. The primary cause
is believed to be entanglement in fishing gear, most often nylon
gillnets. Ganges River dolphins may be particularly vulnerable to
entanglement in gillnets because their preferred habitat is often
in the same location as primary fishing grounds. No rigorous estimates
of dolphin mortality have been published but the problem of accidental
killing is expected to worsen as the demand for fish and for fishing
employment increases (IWC, 2000 and refs. therein; Mansur et al.
2008). Dolphins may also become entangled in long-line fishing gear
very similar to the rolling hooks used in the Yangtze River that
have been cited as among the primary factors contributing to the
probable extinction of the baiji (Lipotes vexillifer) (Mansur et
al. 2008).
Gill net encounter rate in the Ganges River is significantly different
in different stretches of the river with maximum encounter rate
recorded from Goalpara to Dhubri. Accidental killing through gill
net and poaching of dolphin for oil are the most dangerous threats
to the survival of these dolphins. Close monitoring of dolphins
and their habitats involving local communities are required for
long term conservation of the species in the Brahmaputra River (Choudhary
et al. 2006).
Deliberate killing: It has been suggested that some fishermen
see Ganges river dolphins as rivals that scare away the fish or
tear the fish from the nets. For this reason, the fishermen would
scare the dolphins into the nets to kill them. This, however, is
unlikely because the high cost in repairing the nets would not be
compensated by selling the entire dolphin or its products (Reyes,
1991 and refs. therein).
Pollution:
Indus: Pollution may be affecting the viability of the species,
especially considering the decline in the flushing effect of moving
water above barrages. Mercury and arsenic concentrations sampled
from fish above the Guddu Barrage were high. Massive fish kills
have apparently become common from industrial pollution in urban
areas and the use of pesticides in the irrigated crops grown along
the riverbank (IWC, 2000 and refs. therein).
Ganges: Pollution by fertilisers, pesticides, and industrial
and domestic effluents is dramatic in the Ganges River: about 1.15
million metric tons of chemical fertilisers and about 2,600 tons
of pesticides were dumped annually to the river system. Industrial
effluents are also a source of increasing pollution in Nepal. The
effects of pollutants may be considered deleterious to dolphin populations
(Reyes, 1991, and refs. therein; Subramaian et al. 1999). Senthilkumar
et al. (1999) determined concentrations of polychlorinated biphenyls
(PCBs), hexachlorocyclohexane (HCH), chlordane compounds, and hexachlorobenzene
(HCB) in river dolphin blubber and prey fishes collected during
1993 through 1996. Comparison of organochlorine concentrations with
values reported for samples analysed during 1988 through 1992 suggested
that the contamination by these compounds has increased in the river
and persists (Kumari et al. 2002).
Kannan et al. (1997) determined concentrations of butyltin compounds
in dolphin, fish, invertebrates and sediment collected from the
River Ganges. Total level in dolphin tissues was up to 2,000-ng/g
wet wt, which was about 5-10 times higher than in their diet. The
biomagnification factor for butyltins in river dolphin from its
food was in the range 0.2-7.5. Butyltin concentrations in Ganges
river organisms were higher than those reported for several persistent
organochlorine compounds. Discharge of untreated domestic sewage
was one of the major sources. 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 close proximity to point sources in major urban areas (e.g.
Allahabad, Varanasi, Patna, Calcutta, and Dhaka). Furthermore, the
capacity of rivers to dilute pollutants has been drastically reduced
in many areas because of upstream water abstraction (IWC, 2000).
Habitat degradation:
Indus: According to the Scientific Committee of the IWC (2000)
the dramatic decline in the range of the species, from the historical
distribution of approximately 3,500km of river length to a range
of less than 700km of river length (Reeves et al. 1991) occurred
presumably after the mainstem and major tributaries were segmented
by three main barrages completed at Sukkur in 1932, at Kotri in
1955, and at Guddu in 1969. The greatly reduced volume of water,
particularly downstream of Sukkur Barrage, caused the dolphins'
dry-season range to shrink. Subpopulations on either side of barrages
are now isolated and thus are more vulnerable to extirpation by
hunting or environmental change (Reeves and Brownell, 1989).
Due to water abstraction, the Indus river becomes virtually dry
in several places in the low-water season, especially downstream
of the Sukkur Barrage, thereby eliminating suitable habitat in the
lower reaches. The greatest threat to the survival of the Indus
bhulan is probably the continuing decline in water supply due to
the construction of new diversion structures (e.g. Ghazi-Gariala
(Barotha) Dam in the upper Indus) and from increasing extraction
from aquifers. Increasing human populations and both industrial
and agricultural development in the area immediately surrounding
this dolphin's range will inevitably lead to even greater habitat
loss or damage (IWC, 2000).
Ganges:
For Nepal, Smith et al. (1996) and Sinha et al (2000) warn that
in the Karnali and Narayani river basins aquatic species are threatened
with local extinction from the effects of habitat degradation, segregation
of breeding groups by downstream barrages, incidental catches during
fishing operations and declines in prey fish populations.
There has been a dramatic decline in the extent of occurrence of
Ganges susus, as well as in the quality of their habitat, especially
in the Ganges river basin (IWC, 2000). This decline has been related
to the construction, since the late 1950's, of an extensive network
of barrages. The species is severely fragmented and additional barrages
continue to be built (e.g. Kanpur barrage on the Ganges mainstem).
Construction of dams for hydroelectric development and irrigation
in the Ganges system has divided dolphin populations into small
isolated subpopulations, preventing migrations and reducing food
availability. The population above the Kaptai dam in the Karnaphuli
River disappeared over a period of 6 or 7 years after the completion
of the dam. The diversion of water for irrigation caused high fluctuations
in the water flow, reducing suitable habitats for the dolphins.
Similar effects are expected with dolphin populations in the Karnali
River in Nepal, in addition to erosion of banks and changes in river
beds, as a result of deforestation and mining. Heavy river traffic
is increasing drastically in both India and Nepal, and this may
result in habitat restriction and changes in feeding behaviour (Reyes,
1991 and refs. therein). The population of the Padma River system
was said to be "fast declining" due to the construction
of the Farakka Barrage (Reeves and Brownell, 1989). Mohan et al.
(1998) observed a land-locked susu population in the Kulsi river,
a southern tributary of the Brahmaputra. Its number has come down
from 24 animals in 1992 to 12 in 1995. Large scale sand extraction
and operation of fishing gear hazardous to the dolphins were the
main causes for the decline.
In addition to fragmenting dolphin populations, dams and barrages
degrade downstream habitat and create reservoirs with high sedimentation
and altered assemblages of fish and invertebrate species (IWC,2000).
Luxuriant growth of macrophytes and excessive siltation have eliminated
suitable habitat immediately above the Farakka-Barrage. The insufficiency
of water release downstream of the barrage has eliminated dry-season
habitat for more than 300 km, or until the Ganges (Padma)-Brahmaputra
confluence (Smith et al.,1998) and resulted in salt water intruding
an additional 160 km into the Sundarbans Delta, further decreasing
the amount of suitable habitat for this obligate freshwater species.
Consequently, in Bangladesh Susu's are also threatened from the
effects of dams, large embankment schemes, dredging, fisheries bycatch,
directed hunting, and water pollution (Smith et al. 1998).
Other sources of habitat degradation include dredging (Smith et
al. 1998) and the removal of stones, sand (Mohan et al. 1998), and
woody debris (Smith, 1993). These activities threaten the ecological
integrity of the riverine environments, especially in small tributaries
where suitable habitat is more confined and therefore more vulnerable
to local sources of degradation. Suitable 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 been dramatically
reduced in recent years. The long-term implications of the reduction
of dry-season flows in the Ganges are catastrophic for the survival
of susus. New projects that divert dry-season flow, such as the
Kanpur barrage in the upper Ganges, continue to be constructed (IWC,
2000, and refs. therein).
7. Remarks
Range states (Smith and Braulik, 2008):
Bangladesh; India; Nepal; Pakistan
P. gangetica is listed in Appendix I of CITES and the subspecies
P. gangetica gangetica is listed in Appendix II of CMS. The
IUCN considers the species as "Endangered" (Smith and
Braulik, 2008). This is based on the diversity of ongoing threats,
the fragmentation of the populations, the small size of the populations
and the ongoing decrease in numbers.
A small subpopulation of the subspecies P. g. minor was
recently also found in India, but exchange with the main populations
in Pakistan is hampered by a series of dams and barrages. The conservation
of this isolated population in India should be a matter of priority.
The World Wide Fund for Nature (WWF) considers P. gangetica as one
of the most endangered small cetaceans world wide (WWF, 2009).
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© Maps by IUCN.
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