| Citation |
Minton, G., Smith, B.D., Braulik, G.T., Kreb, D., Sutaria, D. & Reeves, R. 2017. Orcaella brevirostris (errata version published in 2018). The IUCN Red List of Threatened Species 2017: e.T15419A123790805. https://dx.doi.org/10.2305/IUCN.UK.2017-3.RLTS.T15419A50367860.en. Downloaded on 07 February 2020. |
Description |
RANGE DESCRIPTION
Irrawaddy Dolphins have a discontinuous distribution in coastal waters of the tropical and subtropical Indo-Pacific, and are predominantly associated with freshwater inputs (Stacey and Arnold 1999, Smith 2017). Coastal and estuarine populations occur from Borneo and the central islands of the Indonesian Archipelago north to Palawan, Philippines, and west to the Bay of Bengal, including the Gulf of Thailand. There are also freshwater subpopulations in three large rivers: Ayeyarwady in Myanmar (up to 1,400 km upstream from the river mouth), Mahakam in Indonesia (with a suspected current range between 90 km to 500 km upstream from the river mouth and the area of highest density between 180 and 280 km from the mouth), and Mekong in Cambodia and Lao Peoples Democratic Republic (a range of less than 200 km between Kampi pool, near Kratie, Cambodia, and Khone Falls, Laos, some 690 km from the river mouth). Within the extent of Irrawaddy Dolphin occurrence in these rivers, their area of occupancy is concentrated in deep pools associated with confluences and above and below rapids. Three other subpopulations inhabit marine-appended brackish water bodies: Chilika Lagoon in India, Songkhla Lagoon in Thailand, and Malampaya Sound in the Philippines.
Although the map accompanying this assessment implies a continuous distribution throughout the coastal areas of the species’ range, subpopulations are known to be fragmented and patchily distributed, with long stretches of coastline where the species is now absent, either due to lack of freshwater inputs or likely local extirpation. For example, while Irrawaddy Dolphins are present in the coastal waters of the Trat Province, Thailand, they appear to be absent in the neighbouring provinces of Chanthaburi and Rayong in the upper Gulf of Thailand (Hines et al. 2015). Similarly, while populations were documented in Kuching Bay and Similajau in Sarawak, Malaysia, the species was not observed during repeated cetacean surveys off the coast of Miri and the Baram River, where the species is known to have been present historically (Minton et al. 2011, Pilleri and Gihr 1974).
DESCRIPTION
No range-wide survey has been conducted for this species, nor is there a synoptic estimate of the total number of Irrawaddy Dolphins from local or regional surveys. Statistically rigorous abundance estimates are available for only a few portions of the range. As summarized in Smith (2017) and Thomas and Gulland (2017), listed from largest to smallest, these are: 5,383 (CV = 40%) in open estuarine waters of Bangladesh (Smith et al. 2008); 451 (CV = 10%) in waterways of the Sundarbans mangrove forest in Bangladesh (continuous with the estuarine waters in Bangladesh mentioned above; Smith et al. 2006); 423 averaged over five years of line-transect surveys along the Trat coast in the Gulf of Thailand (Hines et al. 2015); 233 (CV = 22.5%, 95% confidence limits (CL) = 151–360) in Kuching Bay, Sarawak, Malaysia (Minton et al. 2013); 189 (CV = 23%, 95% CL = 122-292) for the Bintulu-Similajau coastline in Sarawak (C. Peter and G. Minton, unpub. data through 2015); 111 (CV = 8%) in Chilika Lagoon, India (Sutaria and Marsh 2011); 77 (CV = 8%; 95% CI = 71-84, D. Kreb, unpub. data) in the Mahakam River in 2016; and 58-72 in the Ayeyarwady River (Smith and Tun 2007). In the Mekong River estimates dropped from 93 individuals in 2007 (95% CL = 86–101; Beasley et al. 2013) to 85 in 2010 (95% CL = 78-91; Ryan et al. 2011), to 80 in 2015 (95% CL 64-100; Phan et al. 2015). The most recent estimate for Malampaya Sound in the Philippines decreased from 77 individuals in 2001 (CV = 27%; Smith et al. 2004a) to 35 in 2012 (CV = 22.9%; 95% CI = 22-55; Whitty 2016). Additionally, an estimate of minimum abundance of Irrawaddy Dolphins in Koh Kong, Cambodia (contiguous with the population on the Trat coast in Thailand), was 69 individuals (95% CI = 53-85) based on the number of photo-identified individuals (47) corrected by the proportion of unmarked individuals (46.9%; Smith et al. 2016). A population of roughly 50 individuals has been documented in Balikpapan Bay, Kalimantan, Indonesia (D. Kreb, unpub. data).
For each of the five of the subpopulations classified as Critically Endangered (CR), and a sixth subpopulation in Chilika Lake, India, the number of reproductively mature individuals is estimated to be less than 50. Limited research and data from additional locations in Borneo, where other presumably isolated subpopulations have been documented, indicate that in contrast to the larger subpopulation in Bangladesh, most parts of the species’ range where suitable habitat occurs likely host very small local subpopulations, such as those in the Sesayap River and estuary in Kalimantan (estimated at 40-96 individuals with seasonal fluctuation; Kreb unpubl. data) and Cowie Bay in Sabah (estimated at 31 individuals; Woan et al. 2013).
HABITAT AND ECOLOGY
Irrawaddy Dolphins occur in varied habitats. In rivers, they occur almost exclusively in relatively deep (10-50 m) pools located at confluences or above and below rapids. In coastal waters, Irrawaddy Dolphins most commonly occur in areas affected by freshwater inputs and they may enter the lower reaches of rivers (Smith 2017). In the Rajang River in Sarawak, Malaysia, Irrawaddy Dolphins were observed both in the lower reaches of the river and as far as 86 km upriver (Bali et al. 2017). In the coastal areas of Sarawak, Irrawaddy Dolphins were statistically more likely to be observed within 6 km of shore, with a mean water depth of 4.2 m (standard deviation (SD) = 2.4) and salinity of 31.2 ± 2.3 ppt with their distribution shifting inshore during high tide and offshore during low tide (Peter et al. 2016a). In Malampaya Sound, Philippines, 76% of sightings occurred in waters shallower than 6 m (Dolar et al. 2002) with a mean salinity of 28.3 ppt (Smith et al. 2004a). In Chilika Lagoon, 75% of 517 dolphin groups occurred in waters 0.6-2.5 m deep, with a maximum depth of 5.2 m for all groups, while salinity ranged from 1-22 ppt (Sutaria 2009). In coastal waters of Bangladesh, sighting locations had a mean depth of 7.5 m (range 2.7-16.0 m) and a mean salinity of 16.1 ppt (Smith et al. 2008). In Balikpapan Bay, Indonesia, Irrawaddy Dolphins occur in slightly deeper (mean 14.6 m; n = 94; SD = 9.3; range = 2-46 m) and higher salinity (mean 26 ppt; n = 35; SD = 3.1; range = 21-33 ppt) waters (D. Kreb pers. comm. 2017). Irrawaddy Dolphins have been observed in the same areas as Indo-Pacific Finless Porpoises (Neophocaena phocaenoides) in coastal waters of Bangladesh and Myanmar (Smith et al. 2008, Smith and Tun 2008), and their range overlaps those of both Finless Porpoises and Indo-Pacific Humpback Dolphins (Sousa chinensis) in coastal areas of Borneo (Kamaruzzan et al. 2011, Minton et al. 2011). Irrawaddy Dolphins also co-occur with Ganges River Dolphins (Platanista gangetica) in a relatively small portion of their range in the Sundarbans mangrove forest (Smith et al. 2006). In waterways of the Sundarbans mangrove forest, Irrawaddy Dolphin distribution shifts downstream and upstream according to high and low freshwater flow, respectively, and is conditionally dependent (P < 0.05) on low salinity, moderate to high water depths, high temperature and increasing numbers of channel con?uences (Smith et al. 2009).
THREATS
As mentioned above, the patchy and fragmented distribution of Irrawaddy Dolphins in both coastal waters and rivers renders them particularly vulnerable to threats from human activities concentrated in the same areas. The most severe threat to most subpopulations is incidental mortality from entanglement in fishing gear, particularly gillnets. Habitat loss, particularly from dams in riverine populations, and degradation from declining or altered freshwater flows affecting estuarine populations, is a looming conservation threat with the potential for extirpating subpopulations and further fragmenting the already patchy distribution of the species. Irrawaddy Dolphin habitat in both freshwater and estuarine systems is also being degraded by pollutants (oil, pesticides, industrial wastes, coal dust, etc.), siltation from poor land practices, and in some areas noise (e.g., Kreb et al. 2010, Peter et al. 2016a, Thomas and Gulland 2017). Live captures for aquarium display as well as hunting may have affected some populations in the past. In the Mekong River Basin, Irrawaddy Dolphins were also killed by wartime bombing and by explosives used to catch fish (Baird and Mounsouphom 1994).
Irrawaddy Dolphins are caught accidentally in fishing nets in all areas where they have been studied (Smith et al. 2007). The most detailed information on such bycatch comes from the Mekong River where of 15 confirmed human-caused deaths documented between 2001 and 2005, 13 (87%) were due to gillnet entanglement (Beasley et al. 2007). Fewer carcasses have been recovered since 2008, but six (four adults and two neonates) were recovered in 2016 (Hang in Thomas and Gulland 2017).
In the Mahakam River between 2005 and 2016, an average of four carcasses were recovered every year, of which 16% were calves, 9% juveniles and 75% adults. About two thirds of these deaths were attributed to entanglement in large-mesh (10-17.5 cm) gillnets (Kreb in Thomas and Gulland 2017).
Of 49 Irrawaddy Dolphin deaths recorded in Bangladesh from February 2007 to December 2016, two thirds were judged to be due to entanglement, mostly in gillnets, based on net and rope marks, entangled gear attached to the carcasses, and reports from fishermen (Alom in Thomas and Gulland 2017). Mortality has also been recorded in drift gillnets targeting elasmobranchs in coastal waters of Bangladesh (Smith et al. 2008) and bottom-set gillnets targeting crabs in Malampaya Sound (Smith et al. 2004a). Fishermen in some areas report that dolphins are released if found still alive (Smith and Hobbs 2002, Peter et al. 2016b) but in the case of drowned animals, the oil may be used for medicinal purposes or the flesh eaten (Smith et al. 2004a).
In Chilika Lagoon, India, 72 dolphin carcasses were found between 2003 and 2009 (Sutaria et al. 2017) but the causes of death were not specified. Between 2002 and 2016, 42 dolphin deaths were recorded in the Ayeyarwady in Myanmar, 29 of these inside the Ayeyarwady Dolphin Protected Area (Thomas and Gulland 2017).
Interview surveys of 210 fishermen in Bangladesh and 27 fishermen in Sarawak, Malaysia confirmed that gillnets, and occasionally set-bag nets and longlines, are responsible for the majority of dolphin deaths (Peter et al. 2016b, Alom in Thomas and Gulland, 2017). Fishing with electricity has been reported as a threat to the Ayeyarwady subpopulation (Smith and Tun 2007) but details of the dolphin mortality attributed to this destructive fishing practice are unclear.
Dams have degraded some channels inhabited by Irrawaddy Dolphins in the Mekong River Basin and more dams have been proposed in the river’s mainstem (Stone 2016, Brownell et al. 2017). Four major hydropower projects are of extreme concern to dolphins in the Mekong River: Don Sahong dam, currently under-construction in Lao Peoples Democratic Republic, and the proposed Sambor, Stung Treng, and Lower Sekong dams in Cambodia. Since construction began on the Don Sahong dam in 2014, the tiny group of dolphins in the transboundary pool between Laos and Cambodia had declined from five to three animals in 2017 and all are now restricted to the Anlung Cheuteal deep pool upstream from the construction site. If built, the other three proposed dams are almost certain to cause further loss of core habitat and fragmentation of the Irrawaddy Dolphin population in the Mekong (Ryan in Thomas and Gulland 2017). In particular, experts believe that construction of the Sambor and Stung Treng dams, to be located in the middle of their core habitat in the Mekong mainstem, would lead to the extinction of Irrawaddy dolphins in the Mekong. These dams threaten not only dolphins but also fisheries and therefore human livelihoods (Smith et al. 2007). A high dam proposed for the headwaters of the Ayeyarwady River, Myanmar, in Myitsone just below the confluence of the Mali Hka and N’Mai Hka tributaries, provides reason for concern about effects on the population of Irrawaddy Dolphins downstream (https://www.internationalrivers.org/resources/the-myitsone-dam-on-the-irrawaddy-river-a-briefing-3931).
Deforestation and gold, sand, and gravel mining are causing major changes to the geomorphic and hydraulic features of rivers and marine-appended lakes where Irrawaddy Dolphins occur (Smith et al. 2007). Increased sedimentation resulting from deforestation in surrounding watersheds has resulted in declining water depths in Songkhla, Chilika, and Jempang Lakes. The last of these water bodies adjoins the Mahakam River and previously supported dolphins throughout most of its breadth. Now it is no longer suitable habitat for the species (D. Kreb pers. comm). Between 1992 and 1997, the maximum depth of Chilika Lagoon declined from 3.4 to 1.4 meters and the accumulation of sediments led to shrinkage of the opening channel and a dramatic decline in salinity. A new channel dredged in the northern portion of the lake in 2000 is thought to have mitigated at least some of the problems caused by sedimentation (Pattnaik et al., 2007). Maintenance dredging now maintains waterflow in and out of the lake, although the opening of the lake to the sea is continually shifting, followed by shifts in dolphin distribution (D. Sutaria pers comm). The lake is also affected by high levels of heavy metal contamination (Banerjee et al. 2017).
Habitat loss and population fragmentation in several areas have resulted from the proliferation of fixed fishing gear. In the early 2000s, about 27,000 Sai nong or sitting traps and 13,000 Sang sai or barrier traps created more than 8,000 linear kilometres of barrier in multiple rows in the middle and southern portions of Songkhla Lake. These fishing structures were left in place year-round and restricted dolphin movements such that their habitat was substantially reduced and the potential for demographic interaction with dolphins from the Gulf of Thailand was eliminated (Smith et al. 2004b). No new information on the situation in Songkhla Lake has become available but there is no reason to believe it has improved in any way, and the Songkhla Lake subpopulation of Irrawaddy Dolphins is in imminent danger of extirpation.
A 2014 study of six Irrawaddy Dolphin populations in Malaysia, Bangladesh, and India found varying degrees of presence of cutaneous skin nodules, thought to be caused by a fibropapilloma, with the highest prevalence (13.6%) in Chilika Lagoon, where the two most serious cases were observed, and in Kuching, Malaysia. The progression of this disease may be associated with environmental pollutants (Van Bressem et al. 2014), and as such may be an indicator of habitat deterioration in the areas where it is detected.
USE AND TRADE
Hunting or trade is not known to be a major factor explaining population declines of Irrawaddy Dolphins with the exception of Ton Le Sap in Cambodia where intensive hunting by the Khmer Rouge was reported during the 1970s and the species is now extirpated from the lake. Live captures for aquarium display, including export (e.g., to Japan), have taken place in some areas (e.g., Thailand, Myanmar and Indonesia) but are not believed to be frequent at present.
CONSERVATION ACTIONS
The Irrawaddy Dolphin is listed in Appendix I of the Convention on International Trade in Endangered Species (CITES) as well as the Convention on Migratory Species (CMS).
The Action Plan for the Conservation of Freshwater Populations of Irrawaddy Dolphins (Smith et al. 2007) noted that multiple-use protected areas will play a key role for conserving freshwater subpopulations of Irrawaddy Dolphins. Protected areas could be a particularly effective conservation tool due to the fidelity of the species in freshwater systems to relatively circumscribed areas, as this can facilitate management. The Action Plan also provided details on strategies for mitigating bycatch that included (1) establishing core conservation areas where gillnetting is banned or severely restricted; (2) promoting net attendance rules and providing training on the safe release of entangled dolphins; (3) initiating a program to compensate fishermen for damage caused to their nets by entangled dolphins that are safely released; (4) providing alternative or diversified employment options for gillnet fishermen; (5) encouraging the use of fishing gears that do not harm dolphins by altering or establishing fee structures for fishing permits to make gillnetting more expensive while decreasing the fees for non-destructive gears; and (6) experimenting with acoustical deterrents and reflective nets.
In some countries conservation progress is underway. In Myanmar, the Department of Fisheries is collaborating with the Wildlife Conservation Society (WCS) to implement the Management Plan for the Ayeyarwady Dolphin Protected Area (ADPA) (Thomas and Gulland 2017). In the Mekong River, a River Guard Program administered by the local governments in relevant provinces and supported by WWF-Cambodia has stationed 68 River Guards at 16 different outposts along the Mekong from Kratie to the Cambodian-Lao border to help ensure compliance with laws and regulations designed to protect fisheries and dolphins (Samnang in Thomas and Gulland 2017).
In the Mahakam River, East Kalimantan, Indonesia, conservation efforts have focused on training fishermen to rescue dolphins when they are entangled and raising awareness among communities and government authorities. A local non-governmental organization also encourages alternative livelihoods, for example aquaculture and nature-oriented tourism. Since 2002, there have been eight successful releases of dolphins entangled in nets and three dolphins have been rescued from swamp areas when they were unable to return to the main river. The designation of a protected area (24,000 ha core, 51,000 ha total) is currently in progress and will include core dolphin habitat as well as freshwater swamps (Kreb et al. 2010, Kreb in Thomas and Gulland 2017).
In Bangladesh three wildlife sanctuaries for freshwater dolphins have been in place since 2012 in the eastern Sundarbans. While primarily designed to protect Ganges River Dolphins rather than Irrawaddy Dolphins, the sanctuaries include areas where both species regularly co-occur. In 2014, the Government of Bangladesh declared a 1,738 km² marine protected area in the Swatch-of-No-Ground submarine canyon and adjacent estuarine waters. This includes 233 km² of priority habitat for Irrawaddy Dolphins. |
