Journal of
Threatened Taxa | www.threatenedtaxa.org | 26 November 2018 | 10(13):
12738–12748
Possible range decline of Ganges River Dolphin Platanista
gangetica (Mammalia: Cetartiodactyla: Platanistidae) in Indian Sundarban
Sangita Mitra 1 & Mahua Roy Chowdhury 2
1 National Biodiversity Authority, TICEL Bio
park, 5th Floor, CSIR Road, Taramani, Chennai, Tamil
Nadu 600113, India
2 Research Scholar, University of Calcutta,
35 Ballygunj Circular Road, Kolkata, West Bengal
700019, India
1 mitras095@gmail.com (corresponding
author), 2 mahua.rishra@gmail.com
doi: https://doi.org/10.11609/jott.3746.10.13.12738-12748
Editor: Gill Braulik,
University of St Andrews, Scotland. Date
of publication: 26 November 2018 (online & print)
Manuscript details: Ms
# 3746 | Received 21 August 2017 | Final received 12 September 2018 | Finally
accepted 28 October 2018
Citation: Mitra, S. & M.R. Chowdhury
(2018). Possible range decline of Ganges River
Dolphin Platanista gangetica
(Mammalia: Cetartiodactyla: Platanistidae)
in Indian Sundarban. Journal
of Threatened Taxa 10(13): 12738–12748; https://doi.org/10.11609/jott.2493.10.13.12738-12748
Copyright: © Mitra & Chowdhury 2018. Creative Commons
Attribution 4.0 International License. JoTT
allows unrestricted use of this article in any medium, reproduction and
distribution by providing adequate credit to the authors and the source of
publication.
Funding: The study was funded by HSBC Bank.
Competing interests: The authors declare no competing interests.
Author Details: Sangita Mitra is presently engaged as senior consultant with National Biodiversity
Authority under Ministry of Environment, Forest & Climate Change, Govt of India; zoologist; specialization in wildlife
ecology; former coordinator, WWF-India, West Bengal State Office and Fellow,
Zoological Survey of India.
Mahua Roy Chowdhury, a marine biologist, presently a research scholar, University of
Calcutta and former Research Associate with WWF-India, West Bengal State
Office.
Author Contribution: Both the authors contributed equally in field work.
SM designed and wrote the paper.
Acknowledgements: The present study would not have been
possible without the generous support from Hong Kong Sanghai
Banking Corporation and WWF-India, West Bengal State Office. The authors remain
immensely thankful to all the staff in the WWF office who
extended full cooperation at every stage.
Keywords: Anthropogenic, freshwater, Hooghly,
Irrawaddy, occurrence, river, Salinity.
INTRODUCTION
The Ganges River Dolphin Platanista gangetica gangetica Lebeck, 1801
inhabits the river systems of the Ganga-Brahmaputra-Meghna
and Karnaphuli-Sangu in Nepal, India and Bangladesh
(Mohan 1995; Smith & Reeves 2000; Smith et al. 2001; Choudhary
et al. 2006; Braulik & Smith 2017). Many river dolphin species are among the
least known and endangered of all cetaceans (Hamilton et al. 2001).
Previous data on the status of
Ganges River Dolphins in this unique estuarine water of the Indian Sundarbans is limited to historical reports (Anderson 1879;
Jones 1982). In the past few decades,
there has been no systematic and continuous survey of GRD in the Indian Sundarbans. The
presence of GRD is doubtful in the Indian Sundarbans,
as reported in the initial phase of a survey conducted by IUCN in 2014 (Wakid, A, 2017 pers comm). Mandal & Nandi (1989) reported the common occurrence of
Platanista in estuarine waters of the Indian Sundarbans due to their entry from freshwater rivers. There was, however, no site specific data
or any supporting image recorded. Surveys
and observations carried out in the present study from West Bengal, India
indicate that the current distribution of GRDs is confined to small pockets of
habitat (Chowdhury et al. 2016). There has been no authentic record or
observation of GRD in recent times especially in the
central and eastern part of the Indian Sundarbans. Observations prior to this study were not
clear about the range decline of Platanista in
Indian Sundarbans, however, the present study confirms that
GRD populations do not inhabit the waterways of central and
eastern Sundarbans in
India.
Nearly 9,630km2 of area in the two
districts of North and South 24 Parganas of West
Bengal (Fig. 1) covering 1,692km2 of core area, 2,233km2
of buffer and 5,705km2 of transition zone is presently known as the Sundarbans Biosphere Reserve (SBR). The extent of mangroves in the Indian Sundarbans has decreased over the past centuries, due to
clearing of large tracts of forests and land reclamation for agricultural
purposes. Currently the extent of the Sundarbans mangrove forest is limited to 4,264km2
of SBR (Fig. 1) and the remaining landscape comprises of inhabited islands and
human settlements in the adjacent mainland.
Hydrological modifications like water diversion and commissioning of
large barrages upstream has had a great impact on the salinity profile of the
rivers downstream in
central Sundarbans, which lose their
freshwater supply for much of the year.
The channels of the western segment (Hooghly and Muriganga)
are hyposaline than in the 80s and 90s, primarily due
to the increased amount of freshwater.
Intrusion of marine phytoplankton species indicates the increased
salinity profile in the central Sundarbans (Banerjee
2013). Surface water salinity has
increased in central Sundarbans at a rate of 1.09 psu per year during 1984–2013 (Trivedi
et al. 2016). According to Manna et al.
(2010) salinity in the Sundarbans estuary ranges from
11–25 psu, being highest in the dry season and lowest
in the wet season.
Recent work from the Bangladesh Sundarbans shows that the Ganges River Dolphins share their habitat with
Irrawaddy Dolphins Orcaella brevirostris but that Irrawaddy Dolphin distribution extends further southwest and
offshore into the saline coastal waters of the Bay of Bengal (Smith et al. 2006). The range of Platanista
has declined since the 19th century when it was
mapped by Anderson (1879) especially in the upstream reaches (Sinha et al. 2000; Smith et al. 2001). Sighting of Irrawaddy Dolphins is common in
the Indian Sundarbans (Chakraborty
& De 2007), however, there is no estimated count
available so far.
In August 2010, a floating carcass
of a Ganges River Dolphin was reported from the river Bidya
flowing through the eastern part of the Indian Sundarbans
and a similar incident was recorded earlier in June 2010 from Jhingakhali (Fig. 3).
Bahuguna & Mallick
(2010) reported the occurrence of GRD in river Ichchamati
based on a previous survey undertaken in 2002.
Sighting records of three Indo-Pacific Hump-backed Dolphins Sousa chinensis in the Gomor River
close to Sajneakhali Wildlife Sanctuary (Saha & Palchowdhury 2008) and
by B.D. Smith in 2002 on the Raimangal River on the
Indo-Bangladesh border indicate the likely continued occurrence of these marine
cetaceans in the eastern Indian Sundarbans. There is no consistent sighting record or
evidence of the GRD population from the central and eastern segment of the Indian
Sundarbans in the recent past. Occurrence has been recorded only in the
water channels of the transition zones of SBR during 2015–2016.
Study Area
The present study was carried out in
the river Hooghly, as the lower stretch is termed from the point where the
Ganges meets with the river Jalangi at Nabadwip until it reaches the Bay of Bengal, eight
tributaries of river Ganges upstream and also in the comparatively saline,
estuarine creeks and channels of the Sundarbans in
West Bengal, India (Fig. 2).
Selection of the river courses for
survey was based on the secondary data collected from the literature review,
interactions with communities presently residing along the river banks and
primary data generated from direct observations and occurrence records of the
Ganges River Dolphin in the last decade from the waterways flowing in this
region. The coastline of southern West
Bengal was avoided on the basis of less likelihood of sighting GRD and greater
possibility of Irrawaddy Dolphins Orcaella brevirostris occurring in some channels and along the
coast.
MATERIALS AND METHODS
Interviews
Secondary data,
both on the occurrence and absence of GRD, was collected by conducting
interviews with local boatmen and fishers who spend a considerable amount of
time on the rivers during their day-to-day activities. A total of
251 respondents of all age groups including boatmen, fishermen, fish vendors
from the local community, officials of the forest department, and tourists were
interviewed about the frequency and seasonal pattern of dolphin sightings in
last 10 years, recent and past records of entanglement in fishing nets,
mortality records, availability of fish species, habitat preference, and human
induced pressures, etc.
Boat-based surveys
A survey effort of a total stretch
of 96.3km in rivers (Table 1) was utilized in selected channels of the Indian Sundarbans including visits to all segments in summer
(March–June), monsoon (July–October) and winter (November–February) seasons
during 2013–2016. The waterways were
scanned in all the seasons to confirm the presence or absence of dolphins and
to avoid any information gap due to seasonal differences. The observations following the method of
Smith & Reeves (2000) were carried out by a team of 3–4 observers by sailing
with the help of traditional country boats and mechanized boats as well, at a
speed of a maximum of 5-8km/hr. Country
boats of low height and comparatively higher mechanized boats with double decks
were used. All
encounters with dolphins and the respective geographical coordinates were
recorded by a hand held GPS (GARMIN e-trex) and river
depths measured by a hand held echo sounder. River width at specific
locations was measured with the help of Google Earth imagery.
Land-based surveys
Point observations were made during 2012–2016 at
different stretches of river Ganges, its tributaries and selected rivers and
channels in the Sundarbans based on previous records
of GRD. The observations were carried
out by a team of 3–4 observers along the river banks or
channels in summers, monsoons and winters.
The observers watched the river courses from specific observation points
on river banks for 7–9 hours, especially at
confluences of rivers and stretches where there are regular sighting records of
GRD. All land-based survey locations are
shown in Fig. 1. All
the sightings were confirmed by more than one observer. Photographs were captured in the areas with
higher visibility of dolphins. River dolphins are extremely difficult to
photograph, as the unpredictable surfacing of a solitary animal or two for a
moment is difficult to capture. Wherever
possible, images of surfacing GRD were captured and documented during the field
observations to authenticate location, timing and records of the
encounters. Digital photographs were
taken using a DSLR camera Nikon D700, D300s with lenses of Nikkor
70-200 mm f/2.8 G ED VR II and Nikkor 24-70 mm f/2.8
G ED.
Monocular spotter (Bushnell) was
used for better visibility in wider channels.
Surface water salinity was obtained by using an optical refractometer and monitored every season in the study area
at all focal points to understand the salinity preference of the GRD population
in this region. Online Google
Earth imagery was used for geo-referencing and plotting of the ground
observation points recorded from the field.
Analysis
Platanista and Orcaella both inhabiting the Sundarbans waterways
are often considered as a single species by the locals. Therefore the secondary information was
verified and the taxonomic identity of both the species was confirmed by direct
sighting, photographic records and official records obtained from the forest
department. Sighting of dolphins in any
channel for two consecutive years or more in different seasons during this
study was considered to confirm their year round occurrence. Absence of records or direct observation of
the live species in any season at any channel or river for a period of 10 years
or more based on primary and secondary data has been considered as local
extirpation. Salinity was recorded in
low water and high water seasons to understand the minimum-maximum range and
preference to specific salinity range was calculated based on the frequency of
dolphins’ occurrence in consecutive seasons.
RESULTS
Out of a total survey effort of
373.5km in different waterways of southern West Bengal, nearly 96.3km estuarine
section in five channels of the western, central and eastern Sundarbans in India was studied every season during
2012–2016. This could not confirm the
presence of GRD from the central and eastern segment.
The major congregations of GRD or sighting points in the lower stretch
of the Hooghly and some of the confluences show a preference to a hyposaline (Table 2) environment.
Sightings of GRD by boat and land-based surveys
reflect that distribution of the species is affected by salinity.
Among the
hydrological parameters recorded so far, surface water salinity was found to be
a key factor influencing the distribution of GRD in the estuary. Encounter rate (sightings/hr)
of GRD has been recorded at different salinity levels in a boat-based survey
effort at the lower reaches of Hooghly.
There has been no sighting record in the waterways wherever the salinity
level crosses 10 ppt (Fig. 4).
The present study clearly indicates that the encounter
rate of dolphins was consistent in all seasons mostly in hyposaline
waters (<1 ppt) and in moderate salinity (1-10ppt) which occurs close to the estuarine mouth of Ganges. The absence of GRD at salinity >10 ppt, however, indicates that the dolphins do prefer lower
salinities. Findings and sighting records
in the present study shows a correlation between dolphin sightings and salinity which suggests that this is an important
environmental factor influencing the distribution.
Significant increase in salinity
levels were documented in the river Ganges in India after the
commissioning of the Farakka Barrage. Five rivers namely the Saptamukhi,
Thakuran, Matla, Gosaba, and Harinbhanga in
central section (Fig: 2) of Indian Sundarban are also
tidally fed and lost their upstream freshwater connectivity. Based on the current study, and historical
records, there is no evidence that GRD occurs in the central section of the Sundarbans; however a group of Irrawaddy Dolphins (Fig: 4)
was sighted in 2014 and also in 2016 in the central section (present study).
Salinity range varies on account of seasonal changes,
fresh water flow and tidal influence (Fig. 5).
The recorded salinity range from three zones in the Indian Sundarbans reflects a comparatively hypersaline
central part, which is not preferred by the GRD. The eastern section is comparatively less
saline upstream, however there is no sighting record in the present study.
The present study recorded the use
of moderate to shallow depth (3.9–37 m) by GRD in the river Hooghly and its
tributaries of southern West Bengal depending on seasonal flow in different
channels having variable width from 88.39m - 4.3km.
Table 1. Boat-based surveys in selected
water channels in Sundarban
Section |
River/ Channel |
Survey length (km) |
Sighting |
Salinity range |
Western |
Hooghly |
38 |
Y |
<1.0–19 ppt |
Muriganga |
14 |
Y |
||
Central |
Matla |
15 |
N |
9.0–24.6 ppt |
Eastern |
Gomor |
17 |
N |
5.0–24 ppt |
Bidya |
12.3 |
N |
Y - sighted, N - not sighted
Table 2. Sightings of Platanista gangetica
in lower reaches of river Hooghly and its tributaries
Land reference |
District |
River |
Salinity (ppt) |
Boat-based (B)/ land-based (L) Survey |
Geographic Coordinates |
Year |
Budge budge
river side |
South 24 Paraganas |
Hooghly |
0 |
B, L |
22.4830N & 88.1830E |
2015 |
Bata nagar |
South 24 Paraganas |
Hooghly |
0 |
L |
22.5000N & 88.2000E |
2015–16 |
Pujali |
South 24 Paraganas |
Hooghly |
0 |
L |
22.4660N & 88.1500E |
2015 |
Millenium park |
Kolkata |
Hooghly |
0 |
B |
22.5500N & 88.0310E |
2014–15 |
Diamond Harbour |
South 24 Paraganas |
Hooghly |
0 |
B, L |
22.1600N & 88.0170E |
2015 |
Kachuberia |
South 24 Paraganas |
Muriganga |
0 |
B, L |
21.8500N & 88.1330E |
2015–16 |
Namkhana |
South 24 Paraganas |
Confluence of River Muriganga
and Hatania-Doania |
1.01–9.0 |
B |
21.7500N & 88.2100E |
2015–16 |
Gadiara |
Howrah |
Confluence of Rupnarayan
and Hooghly |
0 |
B, L |
22.2160N & 88.0330E |
2012–15 |
Haldia |
South 24 Paraganas |
Confluence of Haldi
and Hooghly |
0 |
L |
22.0000N & 88.0500E |
2012 |
Garchumuk |
Howrah |
Confluence of Damodar
and Hooghly |
0 |
L |
22.3330N & 88.0800E |
2012–2015 |
Bakshi |
Howrah |
Confluence of Rupnarayan,
Damodar and Mundeshwari |
0 |
L |
22.5100N & 88.0160E |
2014–15 |
Kolaghat |
East Midnapur |
Rupnarayan |
0 |
B, L |
22.4160N & 87.8830E |
2011–15 |
Belur |
Howrah |
Hooghly |
0 |
L |
22.6160N & 88.3500E |
2015 |
Table 3. Recorded Salinity data and
occurrence of Platanista gangetica
|
Zone |
Salinity Level |
Occurrence of Platanista |
1 |
River Ganga, Hooghly & tributaries |
<1–1.014 ppt
(present study) |
Recorded in all the stretches
(observation from nearly a total of 300km except the estuarine part in the
present study) |
2 |
Tidal lower reaches of Hooghly and Muriganga River |
1–19 ppt
(present study) |
Occurrence in the salinity level up to 10 ppt
(present study) |
3 |
Rivers in Central and Eastern Sundarban |
9–26.59 ppt (Mitra et al.
2010) 5–24.6ppt (present study) |
Not recorded (present study) |
Table 4. Recent sightings of Orcaella brevirostris in
West Bengal (India)
Land reference |
District |
River/ coast |
Geographic Coordinates |
Year |
Observation from |
Source |
Kakdwip (Steamer Ghat) |
South 24 Paraganas |
Hooghly |
21.8500N & 88.1660E |
2012–13 |
River bank |
Present study |
Namkhana-Narayanpur |
South 24 Paraganas |
Hatania-duania |
21.7500N & 88.2160E |
2012–13 |
Boat |
Present study |
Sundarban |
South 24 Paraganas |
Bidya |
21.9330N & 88.7000E |
2016 |
Boat |
(pers. comm) |
Digha coast |
West Medinipur |
Bay of Bengal |
21.6160N & 87.5000E |
2014 |
Land |
Present study |
DISCUSSION
The stretch selected in the present
study so far is very limited compared to the span of this estuary. The likelihood of GRD occurring in the selected
channels, however, was comparatively better than the other stretches downstream
with higher salinity levels and no sighting records in the last decade. The GRD subpopulation occurring downstream of Farakka Barrage in the river
Ganges and its tributaries in West Bengal was studied by Chowdhury
et al. (2016). The present study
confirmed the sightings of GRD in the western part of the Indian Sundarbans.
The study conducted so far indicates
a possible decline in the range of Platanista
gangetica in the Indian Sundarbans.
Occurrence
of the GRD in the river Hooghly, its tributaries and estuarine rivers depends
on a combination of various factors. The geo-climatic factors gained prominence in
all recent studies, more so because of climate change and the subsiding delta
complex in the Ganga River Basin, which has a geotectonic
evolutionary history. The decline of Platanista in the Indian Sundarbans,
however, may be attributed to the combined effect of increased sedimentation,
reduced freshwater discharge and increased salinity. It has been observed that the salinity level
has a marked influence on the distribution of GRD in the Sundarbans. Salinity in this deltaic system is influenced
by the combined action of the following factors:
Natural salinity level of the rivers is regulated by
evaporation and recharge by rainwater as well as tidal flow downstream.
Both the glacial melting and sea level rise due to climate induced
changes affect the salinity level.
Anthropogenic factors like reduced discharge from barrages, runoff from adjacent lands, and water abstraction for
irrigation etc.
The estuarine channels in the Indian Sundarbans at present are mostly fed by tidal flow and have lost their freshwater
connectivity.
Primary and secondary data on surface water salinity obtained from Sundarbans in West Bengal, India show three distinct salinity zones in the Indian
Sundarbans (Trivedi et al. 2016).
In the present study, GRD has been recorded only in the hyposaline zone in western Sundarbans, which includes the flow of the river Ganges, Muriganga and Saptamukhi rivers (Figs. 2 & 3). The
lower stretch of the Ganga in Southern West Bengal receives snow melt and
monsoon fed river water from upstream but that is only the discharge allowed through the Farakka Barrage. Occurrence of Platanista was recorded in this section up to a maximum salinity of 10 ppt. Salinity recorded from this zone in the
present study varied between <1.0–19 ppt depending on seasonal fluctuation.
Hypersaline zone in the central part lost the connectivity with upstream
freshwater flow. Siltation and clogging
of some channels in the late 15th century and thereafter completely
deprived this central sector from freshwater flow (Choudhuri & Chaudhury 1994). Takhuran, Matla and Gosaba are the major waterways in this segment (Fig. 2). There has been no recent sighting record of
GRD from this segment.
The eastern most part of the Indian Sundarbans having freshwater connectivity with river Padma of Bangladesh is
moderately saline. The trans-boundary
waters adjacent to Bangladesh Sundarbans are low in salinity (5–15 ppt), but it increases downstream and the southwest part of
Bangladesh Sundarbans is hypersaline. There has been no sighting of Platanista reported in recent times or recorded in this
study from this zone.
Biodiversity loss in the Sundarban
waterways due to the cumulative effects of changing weather patterns,
subsidence and reduced freshwater input due to dams and diversions have not
been discussed much, however, these factors changed the salinity dynamics of
the region (Raha et al. 2012). The reduced freshwater
supply not only led to higher river water salinity, but the aquatic subsystem
was significantly altered, resulting in a sharp decline in the fish varieties
from the central tracts of the Sundarbans (Mitra et al. 2010).
All of these may have certainly influenced an obligate freshwater
species like Platanista and the present study
recorded a similar finding. Smith & Braulik (2017) postulated that GRD are not commonly found
in salinities greater than 10 ppt. The
findings from the present study conforms with the fact opined by Jensen et al. (2013) that distribution limits of GRD is related to its
preference of low salinity in the water channel. The present study
clearly indicates that the encounter rate of dolphins was consistent in all
seasons mostly in hyposaline waters (<1 ppt) and sometimes in moderate salinity (1–10ppt) close to
the estuarine mouth of the Ganges.
However, the disappearance of GRD from channels due to increased
salinity >10 ppt strongly confirms its preference
to low salinity. Findings and sighting records in the present study reveal that
distribution of GRD is directly influenced by the salinity
level of the waterways.
At the northeastern edge of
the mangrove forest in Bangladesh, there has been increase of salinity and
decline in river flow of the Ganges. Increased
sedimentation due to reduced discharges also contributed to the gradual drying
up of tributaries (Smith et al. 2009).
Smith et al. (2007) opined that the upstream range of Irrawaddy Dolphins
in Bangladesh Sundarbans is more affected by
interspecific competition with GRD than by any dependence on a particular
salinity. Irrawaddy Dolphins also co-occur with Platanista gangetica in a relatively small
portion of their range in the Sundarbans mangrove
forest (Smith et al. 2006). In India, it needs further study to confirm
if there is any interspecific competition or range overlap of both the species
in this mangrove region. Present
findings indicate the suitability of hyposaline
stretches for GRD and areas with salinity level >5 ppt
for Irrawaddy dolphins.
In a study by Choudhary
et al. (2012) the minimum mid-channel depth requirements were estimated at 5.2m
for dolphin adults and between 2.2m and 2.4m for mother-calf pairs. The channel depth and width recorded under
the present study find that the channels and creeks of the Indian Sundarbans are suitable for Ganges River Dolphins.
Dolphins depend on freshwater
fishes. River Hooghly and estuarine
waters in the Sundarbans are intensive fishing zones.
Around 94–95 % of the Hilsa, an iconic fish Tenualosa ilisha in
this sub region are captured by drift gill nets in the lower stretch of the
Hooghly estuarine system (De 2014).
These nets have a direct or indirect impact on the availability of fish
in the river. Fishermen have reported that dolphins target fisherman’s nets
under water in search of prey. It was
reported that dolphins directly collect fishes from fishing nets and also
damage the nets. However, entanglement
in fishing nets was also recorded by local fishermen in Diamond Harbour, Garchumuk and Raghunathpur near Farakka. Fishing nets increase vulnerability of
dolphins to entanglement, however the present study didn’t have the scope to
establish the relationship of overfishing and use of unsustainable fishing gear
with the non-occurrence of GRD in comparatively low saline water stretches of
eastern Sundarbans.
Choudhury & Mitra (2014) reported
the unsustainable use of fishing gear in the river Hooghly and its tributaries
contributing to the loss of many fish varieties, especially their breeding grounds. Fishing is a common livelihood for the
communities living in the Sundarbans or elsewhere in
southern West Bengal. Exploitative
fishing practices in many channels and use of destructive fishing gear
contribute to the decline of fishes and incidental mortality of dolphins due to
entanglement of dolphins in fishing nets.
The most abundant gear observed in a survey at the lower reaches of
Hooghly was the bottom set bag net (behundi jal) which accounted for nearly
98% of all gears recorded. Other gears
were drag net, cast net along with a small percentage of hook and line
fishing. The bag net is made of mosquito
net and is mainly used to collect shrimps and their larvae. These nets are set in series across the river
in many places obstructing the movement of dolphins. Mansur et al. (2008) has
also cited incidents of entanglement of Platanista
in the fishing nets in Bangladesh Sundarban.
Decline in fish variety and reduced
wild catch prompted the fisher folks to look for other alternatives to compensate
the economic loss. Establishment of
brick kilns (10–11 / km) along river banks and sand or
soil collection from riverbeds (8–9 boats/ 2–3 km stretch of river) in prolific
quantity are recent practices that damage the riverfront and directly interfere
with the fluvial habitat of river dolphins.
These are more prominent in the main course of the Hooghly and its
tributaries. All of these potential anthropogenic threats
have been discussed by Chowdhury et al.
(2016). Almost 70% of the respondents
were unable to differentiate between different cetacean species, which often
generates a generalized idea about the existence of GRD in the Sundarbans. Only
confirmed sightings, however, have been considered in this study.
Relationship of dolphins with the
fisher folk and the effect of underwater noise from the motorized boats are not
favourable for dolphins. An encounter
rate of 0.35 dolphins/km was recorded from the western part of the Indian Sundarbans. This
rate was proportionately higher by around 55% in stretches of the rivers with
limited use of motorized boats, less river traffic and more use of country
boats; however, Kelkar et al. (2010) in his study on
the habitat use and distribution of the Ganges River Dolphin, opined that the
number of motorised boats and boat noise were not significantly correlated with
dolphin encounter rates. Motorised boats
in good number (10–25/hr) travel at an average speed
of 6–10km/hr across the dolphin movement route in
different segments of the major rivers in South Bengal and near the inhabited
islands of the Sundarbans. It is likely that underwater noise does
affect the behavior of dolphins, which rely on sound
for sensing of the environment. In
Mahakam River of Indonesia, Irrawaddy dolphins dive for longer periods and
avoid river traffic (Kreb & Rahadi
2004). Their study also highlighted the
risk of vessel collision and impact of underwater noise pollution.
It was beyond the scope of this
study to conclude that unsustainable fishery practices and underwater noise have
a direct impact on the GRD population of the Sundarbans;
however, salinity profile in different parts of the Indian Sundarbans,
continued disappearance of Ganges Dolphin there along with its occurrence in hyposaline western segment indicate that the present
habitat in the Indian Sundarbans may not be
preferable to Ganges River Dolphins and none of the recent evidences indicate
the existence of a population of GRD in most of the waterways of this
estuary. Chowdhury
et al. (2016) reported the confinement of the species in isolated pockets. Braulik & Smith
(2017) reported non availability of information on the
status of Ganges River Dolphins in the Indian Sundarbans. All secondary information or records of
historical evidence indicate the existence of dolphins in estuarine channels of
the Indian Sundarbans similar to the presence of GRD
in the Bangladesh Sundarbans. Unlike Bangladesh, however, there has been no
systematic and continuous survey on GRD in the Indian Sundarbans. In this context, the distribution of GRD in
the Indian Sundarbans has been reviewed and visual
surveys in this study and contemporary survey/ observations by other workers
indicate a restricted distribution of GRD in the westernmost part of the
estuary at present. The decline of GRD population
in the Indian Sundarban needs to be substantiated
with further study, however the reduced freshwater flow in the channels and
gradual rise of salinity clearly suggest a habitat
unsuitability for GRD in this estuary.
The extent of the former range of the species in light of previous
records has definitely changed, but that has happened over a considerable time
period while the waterways/ channels have been losing their freshwater
connectivity and some others have been drying up. Intensive study and long term survey data on this species in the Indian Sundarbans could further substantiate the viewpoints
presented here.
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