The Riverdolphin has a long beak, a stocky body, and large flippers. Its eye lacks a
lens, and the dolphin is sometimes referred to as being blind, although its
eyes do seem to function as a direction-finding device. The River dolphin measures 1.5 - 2.5 m
(4.9 - 8.2 ft) in length and weighs up to 90 kg (200 lb). It occurs only in
fresh water, in Bangladeshand India,
where the rivers flow slowly through the plains, as well as in Nepal,
where the dolphin can be found in relatively clear water and rapids. The River dolphin lives not only in the
main channels, but also, during the flood season, in seasonal tributaries and
flooded lowlands.
The Riverdolphin feeds on several species of fish, invertebrates, and possibly turtles
and birds. It does much of its feeding at or near the bottom. The waters that
it inhabits are extremely murky.Probably for this reason, the dolphin's sight has degenerated.To find food, it probably uses echolocation
and also probes with its sensitive snout and flipper for prey in the bottom
mud. Reports from the 19th century speak of ‘large schools’ of River dolphins to be seen near most
large towns on the GangesRiver.
However, in more recent times, this dolphin has usually been found to occur in small
groups or alone.
The Riverdolphin was formerly distributed throughout the Ganges-Brahmaputra river system
of Bangladesh, Indiaand Nepal, and
possibly Bhutan.
Although it still has a fairly extensive range, its distribution has
contracted, and its abundance has declined dramatically in some areas.
Currently it is found in the Ganges-Brahmaputra-Meghna and Karnaphuli-Sangu
river systems of Bangladeshand India. A
few individuals survive in Nepalin the KarnaliRiverand possibly the SaptaKosiRiver.The linear extent of its distribution in the Gangesmainstem appears to have diminished by some 100 km (62 mi) since the 19th
century, and even larger amounts of tributary habitat have been lost due to
reduction in water flow below irrigation barrages.Its habitat is severely fragmented, and
additional barrages continue to be built. Further reductions in the extent of
occurrence and area of occupancy of this dolphin are expected. It is almost
certainly declining in numbers and will continue to do so as habitat
degradation shows no sign of abating.
Tidbits
*** The Riverdolphin lives in one of the world's most densely populated areas - almost a
tenth of the world's population lives within the GangesRiver drainage area (Cetacea).
*** It has been recently found that fish oils can be used in
the place of Riverdolphin oil for the preparation of bait. Popularization of fish oil as bait may
reduce the poaching of dolphins for its oil, which causes the mortality of
hundreds of dolphins every year. (Sinha 2002)
Countries Where the River Dolphin Is Currently Found:
2006: Occurs in Bangladesh,India and Nepal(Ganges-Brahmaputra-Megna and Karnaphuli-Sangu river systems). May occur in Bhutan.
(IUCN 2006)
Taxonomy:
The Indus and GangesRiver dolphin populations,
previously treated as separate species, have recently been reduced to
subspecies of a single species.The new
species is named the "GangesRiverdolphin" (Platanista gangetica), with two subspecies - the GangesRiver dolphin (Platanista gangetica
gangetica) and the IndusRiverdolphin (P. g. minor).Under this new
assignment of scientific names, the newly named species as well as the two
subspecies retain the listing of "Endangered" by the IUCN.(IUCN 2003a, Reeves 2004)
Population Estimates:
[Note: Figures
given are for wild populations only.]
* WORLD
o 1982: 4000
- 5000 (Educated guess) (IWC 2000)
o 1997:
Fewer than 2000 (Educated guess) (IWC 2000)
o 2003: At
least hundreds and probably a few thousand (Reeves et al. 2003)
o 2006: At
least 1,200 - 1,800, but could be several times as high (IUCN 2006)
* Nepal
o 1986:
About 40 (Oryx 1986j)
o 2003:
Probably fewer than 100 (Culik 2003c)
History of
Distribution:
The GangesRiverdolphin was formerly distributed throughout the Ganges-Brahmaputra river system
of Bangladesh, Indiaand Nepal, and
possibly Bhutan,
below an elevation of about 250 m (820'). Although it still has a fairly
extensive range, its distribution has contracted, and its abundance has
declined dramatically in some areas. Currently it is found in the
Ganges-Brahmaputra-Meghna and Karnaphuli-Sangu river systems of Bangladeshand India. A
few individuals survive in Nepalin the KarnaliRiverand possibly the SaptaKosiRiver.The linear extent of its distribution in the Gangesmainstem appears to have diminished by some 100 km (62 mi) since the 19th
century, and even larger amounts of tributary habitat have been lost due to
reduction in water flow below irrigation barrages.Its habitat is severely fragmented, and
additional barrages continue to be built. Further reductions in the extent of
occurrence and area of occupancy of this dolphin are expected. It is estimated
to be declining in numbers and will continue to do so as habitat degradation
shows no sign of abating. The current population size has been reduced by an
unknown amount compared to historical levels, but it is still thought to be
large enough to be viable in the long-term if adequate conservation measures
are taken soon. (IWC 2000, Culik 2003c, Reeves et al. 2003, IUCN 2006)
Distribution Map #1S (23 Kb JPEG) (WCMC/CMS) (smaller map)
Distribution Map #1L (69 Kb JPEG) (WCMC/CMS) (larger map)
Distribution Map #2 (55 Kb JPEG) (IUCN 2006)
Threats and Reasons for Decline:
Construction of 50 or more dams and barrages within the GangesRiver dolphin’s historic range has
drastically altered its habitat and fragmented the population. More such
structures are planned or are under consideration. Approximately 3500 km (2200
mi) of embankments have been constructed along the main channel of the Gangesand its tributaries. Embankments interrupt access to spawning habitat for
floodplain-dependent fishes and eliminate eddy counter-currents where the
dolphins spend much of their time. Dredging and the removal of stones, sand,
and woody debris also compromise the ecological integrity of the riverine
environments, especially in small tributaries. Concentrations of toxic
chemicals such as organochlorine and butyltinin the tissues of GangesRiverdolphins are high enough to cause concern about adverse effects. Incidental
mortality in fishing gear, especially gillnets, is a severe problem for the GangesRiver dolphin throughout most of
its range. (Reeves et al. 2003)
Deliberate killing of GangesRiver dolphins for meat and oil is
believed to have declined in most areas, but it still occurs in some locations.
The demand for these products means that there is little incentive for
fishermen to reduce the bycatch or to release dolphins that are still alive
when found in nets. (Reeves et al. 2003)
Data on Biology and Ecology
Size and Weight:
The GangesRiver dolphin measures 1.5 - 2.5 m
(4.9 - 8.2 ft) in length and weighs up to 90 kg (200 lb).
Habitat:
The GangesRiver dolphin occurs only in fresh
water inBangladeshand India,
where the rivers flow slowly through the plains, as well as in Nepal,
where the dolphin can be found in relatively clear water and rapids.In both areas, there is a preference for deep
waters, especially deep counter-current eddy pools below channel convergences
and sharp meanders and above and below mid-channel islands, bridge pilings, and
other engineering structures that cause scouring. The GangesRiver dolphin lives not only in the
main channels, but also, during the flood season, in seasonal tributaries and
flooded lowlands.It has been found in
water between 8 and 33 deg C (46 - 91 deg F). Brackish waters are a major
component of the total range, but the Ganges River dolphin is not generally
known to occur in salinities greater than 10 ppt, although it has been recorded
in waters as saline as 23 ppt. (Klinowska 1991, Sinha 2000, Sinha et al. 2000,
Smith et al. 2001, Culik 2003c, IUCN 2006)
The GangesRiver dolphin lives in the
Indo-Burma Biodiversity Hotspot (Cons. Intl.).
Age to Maturity:
Males become
sexually mature at 1.7 m (5.6') or less, at an age of about 10 years.The smallest sexually mature female so far
reported was 2.0 m (6.6'). (Klinowska 1991) Age at first reproduction for
females is probably 6 - 10 years (IUCN 2006).
Gestation Period:
Approximately 1
year.
Birth Season:
Calving apparently
can occur at any time of the year, but there may be peaks during December -
January and March - May. (Culik 2003c)
Birth Rate:
A single young is
born.
Early Development:
The young begin
eating solid food 1 - 2 months after birth and are weaned within 1 year.
Maximum Age:
The oldest male
animal aged so far was about 28 years old (Klinowska 1991).
Diet:
The GangesRiver dolphin feeds on several
species of fish (e.g. catfish, freshwater shark, mahseers, gobies and carp),
invertebrates (e.g. prawns and clams), and possibly turtles and birds. It does
much of its feeding at or near the bottom, using echolocation, swimming on one
side, and probing the river bottom with its snout and flipper. (Klinowska 1991,
Culik 2003c)
Behavior:
The waters that
the GangesRiverdolphin inhabits are extremely murky.Probably for this reason, the dolphin's sight has degenerated.Its eye lacks a lens, and the dolphin is
sometimes referred to as being blind, although its eyes do seem to be used for
direction-finding. To find food, it probably uses echolocation and also probes
with its sensitive snout and flipper for prey in the bottom mud. (Nowak
1999)
The marked
seasonal changes in the distribution and density of the GangesRiver dolphin over much of its
range are due, at least in large part, to fluctuations in water levels. During
the dry season from October - April, many dolphins leave the tributaries of theGanges - Brahmaputra system 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. (Culik 2003c)
Social Organization:
Reports from the
19th century speak of ‘large schools’ of GangesRiver dolphins to be seen near most
large towns on the GangesRiver.
However, in more recent times, this dolphin has usually been found to occur in
small groups or alone. One study found that 80 % of the dolphins observed
during the dry season in the Meghna and Jamuna Rivers of Bangladesh were
solitary individuals. The size of a number of groups surveyed in the GangesRiver system averaged 1 - 3, with a
range of 1 - 20. Relatively high densities of dolphins are found at sites where
rivers join, in areas where the current is relatively weak, off the mouths of
irrigation canals, and near villages and ferry routes. (Klinowska 1991, Sinha
et al. 2000, Culik 2003c)
Density and Range:
In a 52 sq km (20
sq mi) area of the Karnali River system it was estimated that the density of
Ganges River dolphins was 0.23 individuals/sq km (0.60 individuals/sq mi) in
September 1982 and 0.38 individuals/sq km (0.98 individuals/sq mi) in January
1983. (Klinowska 1991)
Surveys have
resulted in the following estimates of linear densities of GangesRiver dolphins (Smith et al. 2001):
* 1.4
individuals/km (2.2 individuals/mi) (59 dolphins total; Sangu below the DohazariBridge) (Smith et al. 2001)
* 0.81
individuals/km (1.3 individuals/mi) (60.3 km (37.4 mi) covered; middle reaches
of the Ganges mainstem) (Smith et al. 2001)
* 0.24
individuals/km (0.39 individuals/mi) (89.6 km (55.6 mi) covered; middle reaches
of the Yamuna (Brahmaputra) River) (Smith et al. 2001)
* 0.30
individuals/km (0.48 individuals/mi) (113.0 km (70.2 mi) covered; Kalni-KushiyaraRiver) (Smith et al. 2001)
* 1.5
individuals/km (2.4 individuals/mi) (GangesRiver mainstem between Maniharighat
and Buxar (within this segment there are two subsegments with densities of
2.8/km (4.5 individuals/mi) and 3.4/km (5.5 individuals/mi))) (IUCN 2006)
* 0.09
individuals/km (0.15 individuals/mi) (Sundarbans of Bangladesh) (IUCN 2006)
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