Identification and channel characteristics of cetacean hotspots in waterways of the eastern Sundarbans mangrove forest, Bangladesh B rian D. Smith,M.Abdullah A bu D iyan,Rubaiyat M owgli M ansur E lisabeth F ahrni M ansur and B enazir A hmed Abstract Sightings data of two freshwater-dependent ce- Introduction taceans, the Ganges River dolphin Platanista gangetica gangetica and Irrawaddy dolphin Orcaella brevirostris, wo threatened freshwater-dependent cetaceans, the were recorded by captains of three nature tourism vessels TGanges River dolphin or shushuk Platanista gangetica operating in waterways of the eastern Sundarbans man- gangetica and Irrawaddy dolphin or Iraboti Orcaella bre- grove forest, Bangladesh. These data were used to in- virostris, occur in waterways of the Sundarbans mangrove vestigate channel-type preferences and identify cetacean forest, Bangladesh. The Ganges River dolphin is an obligate hotspots according to a scoring system based on group, freshwater species that ranges far upstream in the Ganges– individual and calf encounter rates, the co-occurrence of Brahmaputra–Meghna rivers of Nepal, India and Bangladesh both species, and encounter rates in neighbouring seg- and in the comparatively much smaller Karnaphuli–Sangu ments recorded during monsoon, post-monsoon and dry rivers of southern Bangladesh. The Irrawaddy dolphin occurs seasons. Six 5-km segments were identified for priority in some of Asia’s largest rivers (Mekong, Ayeyarwady and conservation attention from a total of 38 that were Mahakam) and in coastal waters of the Indo-Pacific affected 2002 surveyed on at least three occasions during each season. by freshwater inputs (Smith & Jefferson, ; Smith et al., 2007 An investigation of habitat preferences evaluated 5-km ). The Ganges River and Irrawaddy dolphins are cate- segments that had been surveyed on five or more occasions gorized as Endangered and Vulnerable, respectively, on the 2009 (n 5 69) and assigned them to one of 12 categories defined IUCN Red List (IUCN, ). Both species are threatened in by channel width, sinuosity and the number of large and the Sundarbans by incidental killing in gill nets, destruction of small confluences. Significant differences were found be- fish-spawning habitat through mangrove deforestation, pol- tween observed and expected frequencies of occurrence in lution from large human population centres, the enormous the different segment categories for Ganges River dolphin bycatch of fish fingerlings and crustacean larvae in mosquito groups and individuals and for Irrawaddy dolphin individ- nets, and saline encroachment and excessive sedimentation uals. Both Ganges River and Irrawaddy dolphins selected from declining freshwater supplies and sea-level rise (Reeves 2003 2006 2009 channels with more than two small confluences or at least et al., ;Smithetal., , ). one large confluence. Understanding the preferred habitat The only comprehensive cetacean survey of the Sundar- and identifying hotspots of freshwater-dependent cetaceans bans mangrove forest in Bangladesh was conducted in 2002 1 510 in the Sundarbans is the first step of a planning process for March along , km of waterways. The survey used the potential establishment of a network of protected water- double concurrent counts from independent observer teams 225 ways for these threatened species. to generate abundance estimates of Ganges River dol- phins (CV 5 12.6%) and 451 Irrawaddy dolphins (CV 5 9.6%; Keywords Bangladesh, encounter rate, Ganges River dol- Smith et al., 2006). This survey inspired us and the captains phin, habitat preference, Irrawaddy dolphin, Orcaella of three nature tourism vessels operated by The Guide brevirostris, Platanista gangetica, Sundarbans Tours Ltd in the eastern Sundarbans of Bangladesh to initiate a cetacean monitoring programme. Here we use sightings data recorded during nature tourism cruises from April 2002 to March 2005 to identify hotspots of abundance and investigate channel-type preferences of freshwater- BRIAN D. SMITH* (Corresponding author) Wildlife Conservation Society, 2300 Southern Boulevard, Bronx, NY 10460, USA. E-mail [email protected] dependent cetaceans. M. ABDULLAH ABU DIYAN, RUBAIYAT MOWGLI MANSUR and ELISABETH FAHRNI MANSUR Bangladesh Cetacean Diversity Project, Khulna, Bangladesh Study area BENAZIR AHMED Department of Zoology, University of Chittagong, Chittagong, Bangladesh The eastern Sundarbans is defined by the Bay of Bengal in the south, Mongla Port in the north, Baleswar River in the *Current address: 27/16 Soi Naya, Moo1, Muang, Rawai, Phuket 83130, Thailand east and Passur River in the west. It includes the Sundar- 31 000 Received 8 April 2008. Revision requested 25 June 2008. bans East Sanctuary covering c. , ha of which part Accepted 17 September 2008. was designated as a UNESCO World Heritage Site in 1997 ª 2010 Fauna & Flora International, Oryx, 44(2), 241–247 doi:10.1017/S0030605309990159 Downloaded from https://www.cambridge.org/core. IP address: 170.106.33.14, on 29 Sep 2021 at 03:52:25, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0030605309990159 242 B. D. Smith et al. because of its unique and diverse biological character. Major The scoring system used a suite of weighted variables to freshwater inputs to the eastern Sundarbans come from the characterize the relative value of each segment as particu- Baleswar and Passur rivers that fan out into a complex larly important habitat for each species. The system gave (1) network of estuarine waterways ranging from a few metres one point if the encounter rate of groups, individuals or to a few kilometres wide and increase in salinity from north- calves was in the top 20% of all segments (calculated east to south-west (Hussain & Karim, 1994). separately for each species and season and for groups, individuals and calves for a maximum of 18 points), (2) one Methods point if the encounter of groups, individuals or calves was in the top 40% of all segments during all seasons (calculated Sighting network and channel mapping separately for each species and for groups, individuals and calves for a maximum of six points), (3) two points if both 25 During nature tourism cruises conducted between Ganges and Irrawaddy dolphins were recorded in the 2002 31 2005 March and March the captains and crew of segment during all seasons, and (4) two points if both 35 8 the vessels M. V. Aboshar (length m, beam m, height of Ganges and Irrawaddy dolphins were recorded during one 5 265 2 bridge above waterline m and engines hp 3 ), M. V. season (maximum of six points). The top 40% of these 25 6 5 Chhuti (length m, beam . m, height of bridge above scored segments were then shortlisted and each one was 4 2 265 waterline . m and engine hp) and M. L. Bonbibi assigned 0.5 points for each adjacent segment that was also 21 6 (length m, beam m, height of bridge above waterline shortlisted in the top 40% (calculated separately for each 4 5 265 . m and engine hp) recorded observations of season and for all three seasons together for a maximum of cetaceans on a standardized data sheet, in Bengali. four points) and 0.25 points if the segment 5 km away was Sightings were opportunistic and no assumption was made shortlisted in the top 40% (also calculated separately for that all dolphins present were recorded. However, sighting each season and for all three seasons together for a total of 2000 biases (Smith & Reeves, ) were assumed to be consistent. two points), resulting in a grand total of 38 possible points. The data sheet included entry fields for the date, time, species The aim of adjacent segment scores was to reflect the identity, group size, numberofcalvesandgeographical positive value of having a continuous stretch or closely location taken with a global positioning system (GPS). Calves spaced segments of preferred habitat vs isolated patches 1 were defined as animals , m long. Group sizes were with energetically demanding travel distances in between. estimated according to the best estimate of dolphins visible within the channel reach (Smith et al., 2006). The captains and crew were familiar with both cetacean species and Channel-type preferences received training in the use of the GPS, defining groups and To investigate the frequency of dolphin occurrence accord- calves and in completing the data sheets. ing to channel characteristics, values for the mean width, Data were periodically compiled, quality-checked and sinuosity and the number of large and small confluences entered into a spreadsheet. Routes from the vessel logs were were calculated and assigned to each segment. Channel 1 50 000 overlaid on a digitized map derived from : , SPOT width was measured at the beginning, end and middle satellite imagery and divided into channel segments cover- of each segment and then averaged. Segments with a width 5 ing linear km using the geographical information system # 400 m were classified as narrow and segments with 3 3 5 ArcView v. (ESRI, Redlands, USA). Each -km segment a width . 400 m as wide. Sinuosity was calculated by was then assigned a unique identification number and all dividing the length of each segment (i.e. 5 km) by the sightings were matched to the segment number according straight-line distance between the beginning and the end to the location where they occurred. points. Segments with sinuosity values of 1.0–1.49 were 1 5 Scoring system for identifying hotspots classified as straight and segments with a value $ . were classified as sinuous. The number of large (. 400 m) and Hotspots were identified according to a scoring system small (# 400 m) confluences was recorded for each applied to segments surveyed on at least three occasions segment. Each segment was then coded according to the during three seasons: dry (December–March), monsoon composite criteria used by Smith et al. (2009; Table 1). (June–September) and post-monsoon (October–November). Insufficient data were available to include the pre-monsoon Results season (April–May).