Possible Range Decline of Ganges River Dolphin Platanista Gangetica (Mammalia: Cetartiodactyla: Platanistidae) in Indian Sundarban

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Journal of Threatened Taxa Building evidence for conservaton globally www.threatenedtaxa.org ISSN 0974-7907 (Online) | ISSN 0974-7893 (Print) Communication Possible range decline of Ganges River Dolphin Platanista gangetica (Mammalia: Cetartiodactyla: Platanistidae) in Indian Sundarban

Sangita Mitra & Mahua Roy Chowdhury

26 November 2018 | Vol. 10 | No. 13 | Pages: 12738–12748 10.11609/jot.3746.10.13.12738-12748

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Threatened Taxa Journal of Threatened Taxa | www.threatenedtaxa.org | 26 November 2018 | 10(13): 12738–12748

Possible range decline of Ganges River Dolphin

Communication Platanista gangetica (Mammalia: Cetartiodactyla:

ISSN 0974-7907 (Online) Platanistidae) in Indian Sundarban ISSN 0974-7893 (Print) Sangita Mitra 1 & Mahua Roy Chowdhury 2 OPEN ACCESS 1 Natonal Biodiversity Authority, TICEL Bio park, 5th Floor, CSIR Road, Taramani, Chennai, Tamil Nadu 600113, India 2 Research Scholar, University of Calcuta, 35 Ballygunj Circular Road, Kolkata, West Bengal 700019, India 1 [email protected] (corresponding author), 2 [email protected]

Abstract: Recent survey reports and observatons from rivers in southern West Bengal (India) indicate the extrpaton of Ganges River Dolphin (GRD) from the Indian Sundarbans. The present study undertaken during 2011–16 reviews the possible factors accountable for the disappearance of this obligatory freshwater cetacean from the major waterways of the Sundarbans, India and conclude that it is due to reasons of anthropogenic and geo-climatc origin. Sundarbans, the largest contguous mangrove forest on earth encompassing almost 10,000km2 of India and Bangladesh is located at the head of the Bay of Bengal within 21.533–22.666 0N and 88.083–89.850 0E, of which 62% lies within Bangladesh and 38% in India (Spalding et al. 2010). The landscape is a network of mudfats and islands at the deltaic mouth of the rivers Ganga, Brahmaputra and Meghna created by accumulated sediments carried by the snow-fed Himalayan rivers and their tributaries along with anastomosing tdal water channels. Historic reports reveal the occurrence of GRD in the Sundarbans waters of both India and Bangladesh (Anderson 1879). Current data, however, confrms the disappearance of Platanista gangetca but there is contnued occurrence of Orcaella brevirostris in the Indian part of the estuary. Analysis of causatve factors in light of existng evidence validates the potental extrpaton of Platanista from the majority of the Sundarbans in India, except for its persistence in only the westernmost segment in the lower reaches of river Hooghly as confrmed by this study. The present study also records the habitat preferences and limitng factors afectng GRD distributon, and maps the decline of its range.

Keywords: Anthropogenic, freshwater, Hooghly, Irrawaddy, occurrence, river, Salinity.

DOI: htps://doi.org/10.11609/jot.3746.10.13.12738-12748

Editor: Gill Braulik, University of St Andrews, Scotland. Date of publicaton: 26 November 2018 (online & print)

Manuscript details: Ms # 3746 | Received 21 August 2017 | Final received 12 September 2018 | Finally accepted 28 October 2018

Citaton: Mitra, S. & M.R. Chowdhury (2018). Possible range decline of Ganges River Dolphin Platanista gangetca (Mammalia: Cetartodactyla: Platanistdae) in Indian Sundarban. Journal of Threatened Taxa 10(13): 12738–12748; htps://doi.org/10.11609/jot.2493.10.13.12738-12748

Copyright: © Mitra & Chowdhury 2018. Creatve Commons Atributon 4.0 Internatonal License. JoTT allows unrestricted use of this artcle in any medium, reproducton and distributon by providing adequate credit to the authors and the source of publicaton.

Funding: The study was funded by HSBC Bank.

Competng interests: The authors declare no competng interests.

Author Details: Sangita Mitra is presently engaged as senior consultant with Natonal Biodiversity Authority under Ministry of Environment, Forest & Climate Change, Govt of India; zoologist; specializaton in wildlife ecology; former coordinator, WWF-India, West Bengal State Ofce and Fellow, Zoological Survey of India. Mahua Roy Chowdhury, a marine biologist, presently a research scholar, University of Calcuta and former Research Associate with WWF-India, West Bengal State Ofce.

Author Contributon: Both the authors contributed equally in feld 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 Corporaton and WWF- India, West Bengal State Ofce. The authors remain immensely thankful to all the staf in the WWF ofce who extended full cooperaton at every stage.

12738 Range decline of Ganges River Dolphin in Indian Sundarban Mitra & Chowdhury

INTRODUCTION primarily due to the increased amount of freshwater. Intrusion of marine phytoplankton species indicates The Ganges River Dolphin Platanista gangetca the increased salinity profle in the central Sundarbans gangetca Lebeck, 1801 inhabits the river systems of (Banerjee 2013). Surface water salinity has increased in the Ganga-Brahmaputra-Meghna and Karnaphuli-Sangu central Sundarbans at a rate of 1.09 psu per year during in Nepal, India and Bangladesh (Mohan 1995; Smith & 1984–2013 (Trivedi et al. 2016). According to Manna et Reeves 2000; Smith et al. 2001; Choudhary et al. 2006; al. (2010) salinity in the Sundarbans estuary ranges from Braulik & Smith 2017). Many river dolphin species are 11–25 psu, being highest in the dry season and lowest in among the least known and endangered of all cetaceans the wet season. (Hamilton et al. 2001). Recent work from the Bangladesh Sundarbans Previous data on the status of Ganges River Dolphins shows that the Ganges River Dolphins share their in this unique estuarine water of the Indian Sundarbans habitat with Irrawaddy Dolphins Orcaella brevirostris is limited to historical reports (Anderson 1879; Jones but that Irrawaddy Dolphin distributon extends further 1982). In the past few decades, there has been no southwest and ofshore into the saline coastal waters systematc and contnuous survey of GRD in the Indian of the Bay of Bengal (Smith et al. 2006). The range of Sundarbans. The presence of GRD is doubtul in the Platanista has declined since the 19th century when Indian Sundarbans, as reported in the inital phase of a it was mapped by Anderson (1879) especially in the survey conducted by IUCN in 2014 (Wakid, A, 2017 pers upstream reaches (Sinha et al. 2000; Smith et al. 2001). comm). Mandal & Nandi (1989) reported the common Sightng of Irrawaddy Dolphins is common in the Indian occurrence of Platanista in estuarine waters of the Sundarbans (Chakraborty & De 2007), however, there is Indian Sundarbans due to their entry from freshwater no estmated count available so far. rivers. There was, however, no site specifc data or any The trans-boundary waters adjacent to Bangladesh supportng image recorded. Surveys and observatons Sundarbans is low in salinity (5–15 ppt) compared to carried out in the present study from West Bengal, India other segments eastward in Bangladesh which are indicate that the current distributon of GRDs is confned moderately saline (15–25 ppt) and hypersaline (25– to small pockets of habitat (Chowdhury et al. 2016). 30 ppt) as reported by the Centre for Environmental There has been no authentc record or observaton of Geographic Informaton Services, Dhaka (CEGIS 2006; GRD in recent tmes especially in the central and eastern Hussain et al. 2013). Smith et al. (2009) recorded part of the Indian Sundarbans. Observatons prior to distributon of GRD is dependent on low salinity in the this study were not clear about the range decline of Sundarbans mangrove forests of Bangladesh. They Platanista in Indian Sundarbans, however, the present reported the presence of GRD in waterways with a mean study confrms that GRD populatons do not inhabit the salinity of 0.00 ppt in the low water seasons (March) and waterways of central and eastern Sundarbans in India. around 6.00 ppt during the high water seasons (Sept/ Nearly 9,630km2 of area in the two districts of North Oct). and South 24 Parganas of West Bengal (Fig. 1) covering In August 2010, a foatng carcass of a Ganges River 1,692km2 of core area, 2,233km2 of bufer and 5,705km2 Dolphin was reported from the river Bidya fowing of transiton zone is presently known as the Sundarbans through the eastern part of the Indian Sundarbans and a Biosphere Reserve (SBR). The extent of mangroves in similar incident was recorded earlier in June 2010 from the Indian Sundarbans has decreased over the past Jhingakhali (Fig. 3). Bahuguna & Mallick (2010) reported centuries, due to clearing of large tracts of forests and the occurrence of GRD in river Ichchamat based on a land reclamaton for agricultural purposes. Currently previous survey undertaken in 2002. Sightng records the extent of the Sundarbans mangrove forest is limited of three Indo-Pacifc Hump-backed Dolphins Sousa to 4,264km2 of SBR (Fig. 1) and the remaining landscape chinensis in the Gomor River close to Sajneakhali Wildlife comprises of inhabited islands and human setlements Sanctuary (Saha & Palchowdhury 2008) and by B.D. Smith in the adjacent mainland. Hydrological modifcatons in 2002 on the Raimangal River on the Indo-Bangladesh like water diversion and commissioning of large barrages border indicate the likely contnued occurrence of these upstream has had a great impact on the salinity profle marine cetaceans in the eastern Indian Sundarbans. of the rivers downstream in central Sundarbans, which There is no consistent sightng record or evidence of the lose their freshwater supply for much of the year. GRD populaton from the central and eastern segment The channels of the western segment (Hooghly and of the Indian Sundarbans in the recent past. Occurrence Muriganga) are hyposaline than in the 80s and 90s, has been recorded only in the water channels of the

Journal of Threatened Taxa | www.threatenedtaxa.org | 26 November 2018 | 10(13): 12738–12748 12739 Range decline of Ganges River Dolphin in Indian Sundarban Mitra & Chowdhury

Figure 1. Map of Sundarban Biosphere Reserve transiton zones of SBR during 2015–2016. coast.

Study Area The present study was carried out in the river Hooghly, MATERIALS AND METHODS as the lower stretch is termed from the point where the Ganges meets with the river Jalangi at Nabadwip untl Interviews it reaches the Bay of Bengal, eight tributaries of river Secondary data, both on the occurrence and absence Ganges upstream and also in the comparatvely saline, of GRD, was collected by conductng interviews with estuarine creeks and channels of the Sundarbans in local boatmen and fshers who spend a considerable West Bengal, India (Fig. 2). amount of tme on the rivers during their day-to-day Selecton of the river courses for survey was based on actvites. A total of 251 respondents of all age groups the secondary data collected from the literature review, including boatmen, fshermen, fsh vendors from the interactons with communites presently residing along local community, ofcials of the forest department, the river banks and primary data generated from direct and tourists were interviewed about the frequency and observatons and occurrence records of the Ganges River seasonal patern of dolphin sightngs in last 10 years, Dolphin in the last decade from the waterways fowing in recent and past records of entanglement in fshing nets, this region. The coastline of southern West Bengal was mortality records, availability of fsh species, habitat avoided on the basis of less likelihood of sightng GRD preference, and human induced pressures, etc. and greater possibility of Irrawaddy Dolphins Orcaella brevirostris occurring in some channels and along the

12740 Journal of Threatened Taxa | www.threatenedtaxa.org | 26 November 2018 | 10(13): 12738–12748 Range decline of Ganges River Dolphin in Indian Sundarban Mitra & Chowdhury

Figure 2. Locaton of Sundarban in the study area of West Bengal, India

Figure 3. Occurrence records of Platanista (P) and Orcaell (O) from southern West Bengal

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Boat-based surveys was used for geo-referencing and plotng of the ground A survey efort of a total stretch of 96.3km in rivers observaton points recorded from the feld. (Table 1) was utlized in selected channels of the Indian Sundarbans including visits to all segments in summer Analysis (March–June), monsoon (July–October) and winter Platanista and Orcaella both inhabitng the (November–February) seasons during 2013–2016. The Sundarbans waterways are ofen considered as a waterways were scanned in all the seasons to confrm single species by the locals. Therefore the secondary the presence or absence of dolphins and to avoid any informaton was verifed and the taxonomic identty informaton gap due to seasonal diferences. The of both the species was confrmed by direct sightng, observatons following the method of Smith & Reeves photographic records and ofcial records obtained (2000) were carried out by a team of 3–4 observers by from the forest department. Sightng of dolphins in any sailing with the help of traditonal country boats and channel for two consecutve years or more in diferent mechanized boats as well, at a speed of a maximum of seasons during this study was considered to confrm 5-8km/hr. Country boats of low height and comparatvely their year round occurrence. Absence of records or higher mechanized boats with double decks were direct observaton of the live species in any season at any used. All encounters with dolphins and the respectve channel or river for a period of 10 years or more based geographical coordinates were recorded by a hand held on primary and secondary data has been considered GPS (GARMIN e-trex) and river depths measured by a as local extrpaton. Salinity was recorded in low water hand held echo sounder. River width at specifc locatons and high water seasons to understand the minimum- was measured with the help of Google Earth imagery. maximum range and preference to specifc salinity range was calculated based on the frequency of dolphins’ Land-based surveys occurrence in consecutve seasons. Point observatons were made during 2012–2016 at diferent stretches of river Ganges, its tributaries and selected rivers and channels in the Sundarbans RESULTS based on previous records of GRD. The observatons were carried out by a team of 3–4 observers along Out of a total survey efort of 373.5km in diferent the river banks or channels in summers, monsoons waterways of southern West Bengal, nearly 96.3km and winters. The observers watched the river courses estuarine secton in fve channels of the western, central from specifc observaton points on river banks for 7–9 and eastern Sundarbans in India was studied every hours, especially at confuences of rivers and stretches season during 2012–2016. This could not confrm the where there are regular sightng records of GRD. All presence of GRD from the central and eastern segment. land-based survey locatons are shown in Fig. 1. All the The major congregatons of GRD or sightng points sightngs were confrmed by more than one observer. in the lower stretch of the Hooghly and some of the Photographs were captured in the areas with higher confuences show a preference to a hyposaline (Table 2) visibility of dolphins. River dolphins are extremely environment. Sightngs of GRD by boat and land-based difcult to photograph, as the unpredictable surfacing surveys refect that distributon of the species is afected of a solitary animal or two for a moment is difcult by salinity. to capture. Wherever possible, images of surfacing Among the hydrological parameters recorded so GRD were captured and documented during the feld far, surface water salinity was found to be a key factor observatons to authentcate locaton, tming and infuencing the distributon of GRD in the estuary. records of the encounters. Digital photographs were Encounter rate (sightngs/hr) of GRD has been recorded taken using a DSLR camera Nikon D700, D300s with at diferent salinity levels in a boat-based survey efort lenses of Nikkor 70-200 mm f/2.8 G ED VR II and Nikkor at the lower reaches of Hooghly. There has been no 24-70 mm f/2.8 G ED. sightng record in the waterways wherever the salinity Monocular spoter (Bushnell) was used for beter level crosses 10 ppt (Fig. 4). visibility in wider channels. Surface water salinity The present study clearly indicates that the was obtained by using an optcal refractometer and encounter rate of dolphins was consistent in all seasons monitored every season in the study area at all focal mostly in hyposaline waters (<1 ppt) and in moderate points to understand the salinity preference of the GRD salinity (1-10ppt) which occurs close to the estuarine populaton in this region. Online Google Earth imagery mouth of Ganges. The absence of GRD at salinity >10

12742 Journal of Threatened Taxa | www.threatenedtaxa.org | 26 November 2018 | 10(13): 12738–12748 Range decline of Ganges River Dolphin in Indian Sundarban Mitra & Chowdhury

Table 1. Boat-based surveys in selected water channels in Sundarban Salinity level (ppt)

Survey River/ Salinity Secton length Sightng Channel range (km) Hooghly 38 Y Western <1.0–19 ppt Muriganga 14 Y

Central Matla 15 N 9.0–24.6 ppt Frequency of sightng Frequency Gomor 17 N Eastern 5.0–24 ppt Bidya 12.3 N

Y - sighted, N - not sighted Figure 4. Encounter rate of GRD at diferent salinity levels from western Sundarban

© Mrudul Godbole Figure 5. Average salinity range in diferent zones of Indian Image 1. A pair of Orcaella brevirostris near eastern Sundarban, Sundarban India ppt, however, indicates that the dolphins do prefer lower shallow depth (3.9–37 m) by GRD in the river Hooghly salinites. Findings and sightng records in the present and its tributaries of southern West Bengal depending study shows a correlaton between dolphin sightngs on seasonal fow in diferent channels having variable and salinity which suggests that this is an important width from 88.39m - 4.3km. environmental factor infuencing the distributon. Signifcant increase in salinity levels were documented in the river Ganges in India afer the commissioning of DISCUSSION the Farakka Barrage. Five rivers namely the Saptamukhi, Thakuran, Matla, Gosaba, and Harinbhanga in central The stretch selected in the present study so far is secton (Fig: 2) of Indian Sundarban are also tdally fed very limited compared to the span of this estuary. The and lost their upstream freshwater connectvity. Based likelihood of GRD occurring in the selected channels, on the current study, and historical records, there is no however, was comparatvely beter than the other evidence that GRD occurs in the central secton of the stretches downstream with higher salinity levels Sundarbans; however a group of Irrawaddy Dolphins and no sightng records in the last decade. The GRD (Fig: 4) was sighted in 2014 and also in 2016 in the subpopulaton occurring downstream of Farakka Barrage central secton (present study). in the river Ganges and its tributaries in West Bengal was Salinity range varies on account of seasonal changes, studied by Chowdhury et al. (2016). The present study fresh water fow and tdal infuence (Fig. 5). The confrmed the sightngs of GRD in the western part of recorded salinity range from three zones in the Indian the Indian Sundarbans. Sundarbans refects a comparatvely hypersaline central The study conducted so far indicates a possible part, which is not preferred by the GRD. The eastern decline in the range of Platanista gangetca in the secton is comparatvely less saline upstream, however Indian Sundarbans. Occurrence of the GRD in the river there is no sightng record in the present study. Hooghly, its tributaries and estuarine rivers depends The present study recorded the use of moderate to on a combinaton of various factors. The geo-climatc

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Table 2. Sightngs of Platanista gangetca in lower reaches of river Hooghly and its tributaries Salinity Boat-based (B)/ Geographic Land reference District River Year (ppt) land-based (L) Survey Coordinates Budge budge river 22.4830N & South 24 Paraganas Hooghly 0 B, L 2015 side 88.1830E 22.5000N & Bata nagar South 24 Paraganas Hooghly 0 L 2015–16 88.2000E 22.4660N & Pujali South 24 Paraganas Hooghly 0 L 2015 88.1500E 22.5500N & Millenium park Kolkata Hooghly 0 B 2014–15 88.0310E 22.1600N & Diamond Harbour South 24 Paraganas Hooghly 0 B, L 2015 88.0170E Muriganga 21.8500N & Kachuberia South 24 Paraganas 0 B, L 2015–16 88.1330E Confuence of River Muriganga 21.7500N & Namkhana South 24 Paraganas 1.01–9.0 B 2015–16 and Hatania- 88.2100E Doania Confuence of 22.2160N & Gadiara Howrah Rupnarayan and 0 B, L 2012–15 88.0330E Hooghly Confuence 22.0000N & Haldia South 24 Paraganas of Haldi and 0 L 2012 88.0500E Hooghly Confuence of 22.3330N & Garchumuk Howrah Damodar and 0 L 2012–2015 88.0800E Hooghly Confuence of Rupnarayan, 22.5100N & Bakshi Howrah 0 L 2014–15 Damodar and 88.0160E Mundeshwari 22.4160N & Kolaghat East Midnapur Rupnarayan 0 B, L 2011–15 87.8830E 22.6160N & Belur Howrah Hooghly 0 L 2015 88.3500E

Table 3. Recorded Salinity data and occurrence of Platanista gangetca

Zone Salinity Level Occurrence of Platanista Recorded in all the stretches (observaton from nearly a total 1 River Ganga, Hooghly & tributaries <1–1.014 ppt (present study) of 300km except the estuarine part in the present study) Tidal lower reaches of Hooghly and Occurrence in the salinity 2 1–19 ppt (present study) Muriganga River level up to 10 ppt (present study) 9–26.59 ppt Rivers in Central and Eastern 3 (Mitra et al. 2010) Not recorded (present study) Sundarban 5–24.6ppt (present study) factors gained prominence in all recent studies, more fow downstream. so because of climate change and the subsiding delta 2) Both the glacial meltng and sea level rise due complex in the Ganga River Basin, which has a geotectonic to climate induced changes afect the salinity level. evolutonary history. The decline of Platanista in the 3) Anthropogenic factors like reduced discharge Indian Sundarbans, however, may be atributed to the from barrages, runof from adjacent lands, and water combined efect of increased sedimentaton, reduced abstracton for irrigaton etc. freshwater discharge and increased salinity. It has been 4) The estuarine channels in the Indian Sundarbans observed that the salinity level has a marked infuence at present are mostly fed by tdal fow and have lost their on the distributon of GRD in the Sundarbans. Salinity in freshwater connectvity. this deltaic system is infuenced by the combined acton Primary and secondary data on surface water salinity of the following factors: obtained from Sundarbans in West Bengal, India show 1) Natural salinity level of the rivers is regulated three distnct salinity zones in the Indian Sundarbans by evaporaton and recharge by rainwater as well as tdal (Trivedi et al. 2016).

12744 Journal of Threatened Taxa | www.threatenedtaxa.org | 26 November 2018 | 10(13): 12738–12748 Range decline of Ganges River Dolphin in Indian Sundarban Mitra & Chowdhury

Table 4. Recent sightngs of Orcaella brevirostris in West Bengal (India)

Geographic Observaton Land reference District River/ coast Year Source Coordinates from Kakdwip South 24 Paraganas Hooghly 21.8500N & 88.1660E 2012–13 River bank Present study (Steamer Ghat) Namkhana- South 24 Paraganas Hatania-duania 21.7500N & 88.2160E 2012–13 Boat Present study Narayanpur 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

Image 2. Incidental mortality due to river trafc in Kolkata © Koushik Bhatacharjee Image 3. Watercrafs in the movement route of dolphins on River In the present study, GRD has been recorded only Hooghly in the hyposaline zone in western Sundarbans, which includes the fow of the river Ganges, Muriganga and Saptamukhi rivers (Figs. 2 & 3). The lower stretch of waters adjacent to Bangladesh Sundarbans are low in the Ganga in Southern West Bengal receives snow melt salinity (5–15 ppt), but it increases downstream and the and monsoon fed river water from upstream but that is southwest part of Bangladesh Sundarbans is hypersaline. only the discharge allowed through the Farakka Barrage. There has been no sightng of Platanista reported in Occurrence of Platanista was recorded in this secton up recent tmes or recorded in this study from this zone. to a maximum salinity of 10 ppt. Salinity recorded from Biodiversity loss in the Sundarban waterways due this zone in the present study varied between <1.0–19 to the cumulatve efects of changing weather paterns, ppt depending on seasonal fuctuaton. subsidence and reduced freshwater input due to Hypersaline zone in the central part lost the dams and diversions have not been discussed much, connectvity with upstream freshwater fow. Siltaton however, these factors changed the salinity dynamics of and clogging of some channels in the late 15th century the region (Raha et al. 2012). The reduced freshwater and thereafer completely deprived this central sector supply not only led to higher river water salinity, but the from freshwater fow (Choudhuri & Chaudhury 1994). aquatc subsystem was signifcantly altered, resultng Takhuran, Matla and Gosaba are the major waterways in in a sharp decline in the fsh varietes from the central this segment (Fig. 2). There has been no recent sightng tracts of the Sundarbans (Mitra et al. 2010). All of these record of GRD from this segment. may have certainly infuenced an obligate freshwater The eastern most part of the Indian Sundarbans species like Platanista and the present study recorded having freshwater connectvity with river Padma of a similar fnding. Smith & Braulik (2017) postulated Bangladesh is moderately saline. The trans-boundary that GRD are not commonly found in salinites greater

Journal of Threatened Taxa | www.threatenedtaxa.org | 26 November 2018 | 10(13): 12738–12748 12745 Range decline of Ganges River Dolphin in Indian Sundarban Mitra & Chowdhury than 10 ppt. The fndings from the present study Farakka. Fishing nets increase vulnerability of dolphins conforms with the fact opined by Jensen et al. (2013) to entanglement, however the present study didn’t have that distributon limits of GRD is related to its preference the scope to establish the relatonship of overfshing of low salinity in the water channel. The present study and use of unsustainable fshing gear with the non- clearly indicates that the encounter rate of dolphins was occurrence of GRD in comparatvely low saline water consistent in all seasons mostly in hyposaline waters (<1 stretches of eastern Sundarbans. ppt) and sometmes in moderate salinity (1–10ppt) close Choudhury & Mitra (2014) reported the unsustainable to the estuarine mouth of the Ganges. However, the use of fshing gear in the river Hooghly and its tributaries disappearance of GRD from channels due to increased contributng to the loss of many fsh varietes, especially salinity >10 ppt strongly confrms its preference to low their breeding grounds. Fishing is a common livelihood salinity. Findings and sightng records in the present study for the communites living in the Sundarbans or reveal that distributon of GRD is directly infuenced by elsewhere in southern West Bengal. Exploitatve fshing the salinity level of the waterways. practces in many channels and use of destructve fshing At the northeastern edge of the mangrove forest gear contribute to the decline of fshes and incidental in Bangladesh, there has been increase of salinity mortality of dolphins due to entanglement of dolphins and decline in river fow of the Ganges. Increased in fshing nets. The most abundant gear observed in a sedimentaton due to reduced discharges also survey at the lower reaches of Hooghly was the botom contributed to the gradual drying up of tributaries set bag net (behundi jal) which accounted for nearly 98% (Smith et al. 2009). Smith et al. (2007) opined of all gears recorded. Other gears were drag net, cast net that the upstream range of Irrawaddy Dolphins in along with a small percentage of hook and line fshing. Bangladesh Sundarbans is more afected by interspecifc The bag net is made of mosquito net and is mainly used competton with GRD than by any dependence on a to collect shrimps and their larvae. These nets are set partcular salinity. Irrawaddy Dolphins also co-occur in series across the river in many places obstructng the with Platanista gangetca in a relatvely small porton of movement of dolphins. Mansur et al. (2008) has also their range in the Sundarbans mangrove forest (Smith et cited incidents of entanglement of Platanista in the al. 2006). In India, it needs further study to confrm if fshing nets in Bangladesh Sundarban. there is any interspecifc competton or range overlap Decline in fsh variety and reduced wild catch of both the species in this mangrove region. Present prompted the fsher folks to look for other alternatves fndings indicate the suitability of hyposaline stretches to compensate the economic loss. Establishment of for GRD and areas with salinity level >5 ppt for Irrawaddy brick kilns (10–11 / km) along river banks and sand dolphins. or soil collecton from riverbeds (8–9 boats/ 2–3 km In a study by Choudhary et al. (2012) the minimum stretch of river) in prolifc quantty are recent practces mid-channel depth requirements were estmated at that damage the riverfront and directly interfere with 5.2m for dolphin adults and between 2.2m and 2.4m the fuvial habitat of river dolphins. These are more for mother-calf pairs. The channel depth and width prominent in the main course of the Hooghly and its recorded under the present study fnd that the channels tributaries. All of these potental anthropogenic threats and creeks of the Indian Sundarbans are suitable for have been discussed by Chowdhury et al. (2016). Almost Ganges River Dolphins. 70% of the respondents were unable to diferentate Dolphins depend on freshwater fshes. River between diferent cetacean species, which ofen Hooghly and estuarine waters in the Sundarbans are generates a generalized idea about the existence of GRD intensive fshing zones. Around 94–95 % of the Hilsa, in the Sundarbans. Only confrmed sightngs, however, an iconic fsh Tenualosa ilisha in this sub region are have been considered in this study. captured by drif gill nets in the lower stretch of the Relatonship of dolphins with the fsher folk and the Hooghly estuarine system (De 2014). These nets have efect of underwater noise from the motorized boats are a direct or indirect impact on the availability of fsh in not favourable for dolphins. An encounter rate of 0.35 the river. Fishermen have reported that dolphins target dolphins/km was recorded from the western part of the fsherman’s nets under water in search of prey. It was Indian Sundarbans. This rate was proportonately higher reported that dolphins directly collect fshes from fshing by around 55% in stretches of the rivers with limited use nets and also damage the nets. However, entanglement of motorized boats, less river trafc and more use of in fshing nets was also recorded by local fshermen in country boats; however, Kelkar et al. (2010) in his study Diamond Harbour, Garchumuk and Raghunathpur near on the habitat use and distributon of the Ganges River

12746 Journal of Threatened Taxa | www.threatenedtaxa.org | 26 November 2018 | 10(13): 12738–12748 Range decline of Ganges River Dolphin in Indian Sundarban Mitra & Chowdhury

Dolphin, opined that the number of motorised boats and REFERENCES boat noise were not signifcantly correlated with dolphin encounter rates. Motorised boats in good number (10– Anderson, J. (1879). Anatomical and Zoological Researches: Comprising 25/hr) travel at an average speed of 6–10km/hr across an Account of the Zoological Results of the two Expeditons to Western Yunnan in 1868 and 1875 and a Monograph of the two the dolphin movement route in diferent segments of cetacean genera Platanista and Orcaella. Bernard Quaritch, the major rivers in South Bengal and near the inhabited Piccadilly, London. islands of the Sundarbans. It is likely that underwater Bahuguna, N.C. & J.K. Mallick (2010). Handbook of the Mammals of South Asia. Natraj Publishers, Dehra Dun, 541pp noise does afect the behavior of dolphins, which rely Banerjee, K. (2013). Decadal change in the surface water salinity on sound for sensing of the environment. 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12748 Journal of Threatened Taxa | www.threatenedtaxa.org | 26 November 2018 | 10(13): 12738–12748

ISSN 0974-7907 (Online); ISSN 0974-7893 (Print)

November 2018 | Vol. 10 | No. 13 | Pages: 12715–12858 Date of Publicaton: 26 November 2018 (Online & Print) www.threatenedtaxa.org DOI: 10.11609/jot.2018.10.13.12715-12858

Artcles Odonata of eastern Bangladesh with three new records for the country -- M. Kawsar Khan, Pp. 12821–12827 The patern of bird distributon along the elevaton gradient of the Sutlej River basin, western Himalaya, India Two new species of phytoseid mites Euseius (Acari: Phytoseiidae) from -- Balraj Santhakumar, P. Ramachandran Arun, Ramapurath Kozhummal Sony, Kerala, India Maruthakut Murugesan & Chinnasamy Ramesh, Pp. 12715–12725 -- P.P. Santhosh, Mary Anithalatha Sadanandan & M.P. Rahul, Pp. 12828– 12832 Morphological variatons in marine puferfsh and porcupinefsh (Teleostei: Tetraodontformes) from Tamil Nadu, southeastern coast of India --K. Kaleshkumar, R. Rajaram, P. Purushothaman & G. Arun, Pp. 12726–12737 Notes

First photographic record of tger presence at higher elevatons of the Communicatons Mishmi Hills in the Eastern Himalayan Biodiversity Hotspot, Arunachal Pradesh, India Possible range decline of Ganges River Dolphin Platanista gangetca -- Aisho Sharma Adhikarimayum & G.V. Gopi, Pp. 12833–12836 (Mammalia: Cetartodactyla: Platanistdae) in Indian Sundarban -- Sangita Mitra & Mahua Roy Chowdhury, Pp. 12738–12748 An old collecton reveals an additonal distributon record of the Greater Long-tongued Fruit Bat Macroglossus sobrinus K. Anderson, 1911 Retrospectve study on epidemiology of snakebites in Sarpang District, (Chiroptera: Pteropodidae) from southern West Bengal, India southern Bhutan -- Tauseef Hamid Dar, M. Kamalakannan, C. Venkatraman & Kailash Chandra, -- Bal Krishna Koirala, Jaganath Koirala & Sunil Sapkota, Pp. 12749–12754 Pp. 12837–12839

Individual identfcaton of Dutaphrynus melanostctus (Schneider, 1799) Breeding reports and conservaton implicatons of the Endangered (Amphibia: Anura: Bufonidae) based on dorsal wart paterns Black-bellied Tern Sterna acutcauda J.E. Gray, 1831 (Aves: Charadriiformes: -- Uddalak Tathagato Bindhani & Abhijit Das, Pp. 12755–12768 Laridae) in Odisha, eastern India -- Tuhinansu Kar, Himanshu Shekhar Palei & Subrat Debata, Pp. 12840–12843 A preliminary checklist of buterfies from the northern Eastern Ghats with notes on new and signifcant species records including three new reports for A frst record of the Redbelly Yellowtail Fusilier Caesio cuning (Bloch, 1791) peninsular India (Teleostei: Caesionidae) from Visakhapatnam coastal waters, India -- Rajkamal Goswami, Ovee Thorat, Vikram Aditya & Seena Narayanan -- Muddula Krishna Naranji, Govinda Rao Velamala & Kondamudi Ramesh Karimbumkara, Pp. 12769–12791 Babu, Pp. 12844–12846

Aquatc and semi aquatc Hemiptera community of Sonebeel, the largest A record afer 92 years, and a frst report of the moth Mecodina metagrapta wetland of Assam, northeastern India Hampson, 1926 (Lepidoptera: Erebidae: Aganainae) from the Western -- Anupama Saha & Susmita Gupta, Pp. 12792–12799 Ghats’ part of Maharashtra, India -- Aparna Sureshchandra Kalawate, Pp. 12847–12849

Short Communicatons A new record of the Malay Cardamom Amomum aculeatum Roxb. (Zingiberaceae) for mainland India First record of colour aberraton in Basra Reed Warbler Acrocephalus griseldis -- Sameer Chandrakant Patl & P. Lakshminarasimhan, Pp. 12850–12853 (Hartlaub, 1891) (Passeriformes: Acrocephalidae) from Central Marshes of southern Iraq, with notes on its intraspecifc/interspecifc behavior New distributon records of the leopard plants Ligularia amplexicaulis DC. -- Omar F. Al-Sheikhly, Mukhtar K. Haba, Nadheer A. Faza’a & Ra’ad H. Al-Asady, and Ligularia sibirica (L.) Cass. (Asteraceae) in the Indian Himalaya Pp. 12800–12804 -- Bikarma Singh, Sumit Singh & Bishander Singh, Pp. 12854–12858

Avian fauna of Amboli Ghat, Sindhudurg District, Maharashtra State, India -- Varun Satose, Vikrant Choursiya, Rakesh Deulkar & Sasikumar Menon, Miscellaneous Pp. 12805–12816 Natonal Biodiversity Authority DNA barcoding and morphological characterizaton of moth Antoculeora Member ornatssima (Walker, 1858) (Lepidoptera: Noctuidae), a new range record from western Himalayan region of India -- Twinkle Sinha, P.R. Shashank & Pratma Chaudhuri Chatopadhyay, Pp. 12817– 12820

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