Biology Reference
In-Depth Information
I
NTRODUCTION
The Ganges River dolphin, commonly known as ‗susu', is discontinuously distributed in
the Ganges-Brahmaputra-Meghna and Karnaphuli-Sangu river systems of India, Nepal and
Bangladesh between tidal zones and as far up as the rivers are navigable from the foothill of
the Himalayas to the Bay of Bengal (Smith et al., 1994, 1998; Sinha, 1997; Sinha et al.,
2000). It belongs to the Order Cetacea, suborder Odontoceti (toothed whales), family
Platanistidae (South Asian river dolphin), genus
Platanista
, species
P. gangetica
and sub-
species
P. g. gangetica
. Rivers and associated freshwater ecosystems in the Indian
subcontinent are under threat due to a wide range of intensive human use and developmental
activities. The Ganga-Brahmaputra-Meghna river basins cover only 0.12% of the world's
land mass where about 10% of the world's population live. Increased population and
development pressures have led to depletion of fish stocks, severe pollution from point and
non-point sources, degradation of habitats, sediment load changes and hydrological
alterations (Mohan, 1989; Ansari et al., 1999; Dudgeon, 2000; Sinha 2006). These in turn
have had detrimental effects on the flora and fauna of the river ecosystems, including the
Ganges River dolphin
Platanista gangetica gangetica
, an endemic species of the Ganges-
Brahmaputra-Meghna river systems in India, Nepal and Bangladesh (Sinha et al., 2000;
Sinha, 2006). The total estimated population of the dolphin in its entire distribution range is
about 2000-2500. River dolphin conservation has become a very critical issue owing to the
recently reported extinction of the Baiji or Chinese River dolphin
Lipotes vexillifer
Miller
1918 (Turvey et al., 2007). The Ganges River dolphin
Platanista gangetica gangetica
Roxburgh 1801 has been declared endangered by the IUCN (IUCN Red List 2007) and the
species has been listed in Schedule-I in the Indian Wildlife (Protection) Act, 1972.
Different studies have proposed that water depth, channel width, direction and velocity of
flow, geomorphologic complexities, and substrate type affect dolphin habitat use (Smith et
al., 1998; Sinha et al., 2000, 2006; Choudhary et al., 2006). Along with these, prey
availability is another factor that can affect population size and habitat selection. River
dolphins in the Ganges have been recorded to feed on small fish, and occasionally on
crustaceans and snails (Sinha, 2006). The species is known to be mostly solitary, except
mother-calf pairs. They are also known to congregate sometimes in shallow water zones for
feeding on small fish groups in such areas (Sinha, 2006). For the Ganges River dolphins in
India, Nepal, and Bangladesh, population and threat assessment surveys have been carried out
throughout their distribution range in order to assess the conservation status and obtain
information on threats (Smith et al., 1994, 1998; Sinha et al., 2000; Wakid; 2005, Biswas &
Boruah, 2006; Choudhary et al., 2006; WWF, 2006).
The main threat to the Ganges dolphins, especially in Bihar and Assam states, were
reported to be direct killing by fishermen for extraction of oil from the blubber, which was
used as a fish-bait (Mohan, 1989; Sinha, 2002). One novel approach to prevent further
hunting has been the use of fish scrap oil instead of oil from hunted dolphins (Sinha, 2002).
Such approaches have contributed to an increase of awareness and conservation efforts
leading to the reduction of directly killing dolphins (Sinha, 2006; Choudhary et al., 2006).
Threats to dolphins and their habitat such as construction of dams and barrages have been
suspected to cause genetic isolation of dolphin populations (Reeves et al., 2000; Smith et al.,
2000; Sinha, 2006). However, accidental by-catch through the entanglement in gillnets
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