The Ganges River Dolphin (Platanista gangetica gangetica) and Indus River Dolphin (Platanista gangetica minor) are two sub-species of freshwater or river dolphins found in the Indian sub-continent. From the 1970s they had commonly been regarded as separate species. However in 1998, a seminal work on cetacean taxonomy listed them as a single species once again (see taxonomy below). The single species is likely to have either the Ganges and Indus River Dolphin or Indian River Dolphin as its common name, although neither has yet spread into wide usage and the latter does not reflect that the Indus subspecies is resident primarily in Pakistan.
Other names:
Both subspecies: Blind River Dolphin, Side-swimming Dolphin, Susu, Bhulan
Ganges subspecies: Gangetic Dolphin, Ganges Susu
Indus subspecies: Bhulan, Indus Dolphin, Indus Susu
Taxonomy:
Until the 1970s the Indus and Ganges River Dolphins were regarded as a single
species, first named by Roxburgh in 1801. However at that time it became clear
that two populations, one centered on the Ganges River in India and the other
in the Indus River in Pakistan had not interbred for many hundreds of years.
Based on differences in skull structure, scientists declared the two populations
as separate species. However Rice (1998)[1], in his taxonomic classification
of cetaceans that has become standard in the field, found that there were
insufficient morphological differences to warrant distinction at the species
level. Thus just one species is recognised in the genus Platanista and four
true river dolphin species, each in its own family as well as genus.
Physical description:
The Ganges and Indus River Dolphins are essentially identical in appearance.
They have the long, pointed snout characteristic of all river dolphins. Both
the upper and lower jaw sets of teeth are visible even when the mouth is closed.
The snout thickens towards its end. The species does not have crystalline
eye lens, rendering it effectively blind, although it may still be able to
detect the intensity and direction of light. Navigation and the hunt for food
is carried out using echo-location. The body is a brownish colour and stocky
at the middle. The species has only a small triangular lump in the place of
a dorsal fin. The flippers and tail are thin and large in relation to the
body size, which is about 2-2.2m in males and 2.4-2.6m in females. The life
span and other life history data is unknown.
The species feeds on shrimp and small fish that prefer the waters close to the river bed. Dolphins are usually encountered on their own but have travelled in groups more often historically, when populations were larger.
Population and distribution:
The Ganges subspecies can be found in the Ganges River as well as the Brahmaputra,
Meghna, Karnaphuli and Sangu river systems of India, Bangladesh, Nepal and
Bhutan. Relatively high population densities have been observed near the Vikramshila
Gangetic Dolphin Sanctuary in Indian and in the Sangu River in southern Bangladesh.
Very few individuals (perhaps 20) are present in Nepal in the Karnali River.
The total population is unknown, but certainly numbers in the hundreds and
there are perhaps as many as a few thousand (but see "Human interaction"
below)
The Indus subspecies resides primarily in the Indus River in Pakistan. In the nineteenth century its range was approximately five times greater than it is today, and included the Sutlej, Ravi, Chenab and Jhelum tributaries of the Indus. The highest population density occurs in Sind Province.
A comprehensive population census of the Indus Dolphin that was completed in 2001 by WWF-Pakistan concluded that there are approximately 1,100 individuals alive today.
Human interaction:
Both species have been very adversely affected by the use by humans of the
river systems in the sub-continent. Entanglement in fishing nets can cause
significant damage to local population numbers. Some individuals are still
taken each year and their oil and meat used as a liniment, as an aphrodisiac
and as bait for catfish. Irrigation has lowered water levels throughout both
subspecies' ranges. Poisoning of the water supply from industrial and agricultural
chemicals may have also contributed to population decline. Perhaps the most
significant issue is the building of more than 50 dams along many rivers,
causing the segregation of populations and a narrowed gene pool in which dolphins
can breed. There are currently three sub-populations of Indus Dolphins considered
capable of long-term survival if protected.
Both subspecies are listed by the IUCN as endangered on their Red List of Endangered Species.
References:
Randall R. Reeves, Brent S. Stewart, Phillip J. Clapham and James A. Powell (2002). National Audubon Society Guide to Marine Mammals of the World. Alfred A. Knopf, Inc.
Rice, Dale W. (1998). Marine mammals of the world: systematics and distribution. Society of Marine Mammalogy Special Publication Number 4. 231 pp.