Orcaella brevirostris (Mekong River subpopulation)
|Scientific Name:||Orcaella brevirostris (Mekong River subpopulation)|
|Red List Category & Criteria:||Critically Endangered C2a(i,ii); D ver 3.1|
|Assessor(s):||Smith, B.D. & Beasley, I.|
|Reviewer(s):||Reeves, R. & Taylor, B.L. (Cetacean Red List Authority)|
The best estimate of abundance for the Kratie to Khone Falls river segment is 69 individuals, based on the pool-count survey in May 2003. This number is probably close to the actual size of the Mekong subpopulation because of:
1) the low probability that dolphins occur below Kratie and in the Sekong River during the low-water season, and in Tonle Sap (Great Lake) and its connecting channel at any time, as indicated by interview surveys and the observations by researchers conducting water-bird surveys and hydrological investigations;
2) the nearly comprehensive search coverage of navigable channels during vessel-based surveys; and
3) the 100% match in concurrent detections of dolphin groups and close agreement in group size estimates by land-based and boat-based observers.
Guidelines for considering measurement error (Annex 1: Uncertainty, in IUCN 2001) suggest using plausible lower bounds, rather than best estimates, to determine population size. In the case of Mekong dolphins, this implies that it would be appropriate to use 57, the sum of minimum estimates of group size from the May 2003 pool-count survey, as the estimate of abundance for this subpopulation. The threshold of 50 mature individuals for listing a species or population as CR according to Criterion D (and C2a(i)) refers to the number of individuals known, estimated or inferred to be capable of reproduction. Although the proportion of mature individuals typical for this species is unknown, it is reasonable (and certainly precautionary) to infer that the number of mature individuals in the Mekong River is less than 50. Therefore, the subpopulation qualifies for listing as CR based on Criterion D. The subpopulation also qualifies as CR on the basis of Criterion C2a (i,ii) as it certainly has fewer than 250 mature individuals, a continuing decline in the number of mature individuals is projected or inferred based on the continuing known threat of gillnet entanglement and potential threats from water development and navigation improvement projects, and both subcriteria of subcriterion 2a apply (no subpopulation contains more than 50 mature individuals and more than 90% of the total mature individuals are in one subpopulation).
|Range Description:||The Irrawaddy dolphin is patchily distributed in shallow, near-shore tropical and subtropical marine waters of the Indo-Pacific, from northeastern Australia in the south, north to the Philippines (Dolar et al. 2002) and west to northeastern India (Stacey and Leatherwood 1997; Stacey and Arnold 1999). Its marine distribution is concentrated in estuaries and semi-enclosed water bodies (i.e., bays and sounds), generally adjacent to mangrove forests. Freshwater populations occur in three river systems - the Mahakam of Indonesia, the Ayeyarwady (formerly Irrawaddy) of Myanmar (formerly Burma) and the Mekong of southern Laos, Cambodia and Viet Nam. Irrawaddy dolphins also occur in partially isolated brackish or fresh-water bodies, including Chilka Lake in India and Songkhla Lake in Thailand. The first published record of the species in the Mekong was from the diary of a 19th century explorer (Mouhout 1966). |
The effective range of Irrawaddy dolphins in the Mekong River is a 190 km segment from Kratie (about 500 km upstream of the river mouth in Viet Nam) to slightly upstream of the Laos/Cambodia border at Khone Falls (or Lee Pee), which physically obstructs further upstream movement. During the high water season (June-October), anecdotal reports suggested that the dolphins ascended the Sekong River and its tributaries, the Houay Khaliang, Xepian (to Xepha Falls about 50 km above the Sekong confluence), Xenamnoi (to Tatkhek Falls about 8 km above the Sekong confluence), and Xekaman (to about 50 km above the Sekong confluence and including the Houay Twai tributary and possibly the Xepian). In the Sekong, the dolphins were reported to range as far upstream as Kalaum Town, Laos, about 280 km above the Mekong confluence near Stung Treng, Cambodia (Baird and Mounsouphom, 1997). Based on interviews conducted by Baird (1997) and Beasley et al. (2003), dolphins probably now only rarely, if ever, ascend these rivers. During interview surveys downstream from Kratie to Phnom Penh, children were unaware of the existence of dolphins, whereas adults reported that before 1975, dolphins were observed every day during both low and high water seasons (Isabel Beasley, pers. comm.).
Based on visual surveys conducted by Beasley et al. (2003), dolphins are frequently found during the low-water season in nine deep areas in the Kratie to Khone Falls segment. Approximately 2 km below the falls, dolphins regularly occur in a small (ca. 600 m diameter), deep (> 50 m during the high-water season) pool, known locally in Laos as Boong Pa Gooang and in Cambodia as Anlong Chiteal. Dolphins were observed daily in the pool during the dry seasons of 1992-93, generally in groups of 2-10; 17 were seen at least once (Baird et al. 1994). Using visual and acoustic methods, Borsani (1999) estimated that there were 8-10 dolphins present in Boong Pa Gooang in late March/early April 1998. Other pools occupied by dolphins in the Kratie to Khone Falls segment are at Koh Suntuk, Kang Kohn Sat and Tbong Klar in the Stung Treng Province, and Sampan, Khasak Makak, Gopidau, Chroy Bantey and Kampi in the Kratie Province. Kampi pool, located 15 km north of Kratie, is currently considered the most important dolphin habitat in the Mekong, due to the 100% reliability of sightings, as recorded during 165 visits to the pool over three years, and the relatively large number of animals observed during each visit (mean group size = 7, range = 1-19) (Beasley et al. 2003, Beasley, unpublished).
Dolphins previously inhabited Tonle Sap (Great Lake) (Lloze 1973) but apparently have been extirpated there. Fishing is extremely intensive within the lake and in the channel connecting it to the Mekong. Researchers conducting extensive water-bird surveys in the lake from 1999-2003 have not observed any dolphins (Federic Goes, pers. comm.). Moreover, researchers from the Water Utilization Program - Finnish International Development Aid (WUP-FIN) Tonle Sap Modeling Project visited sampling stations throughout the lake every month from 2001-2003 and never observed dolphins (Juha Sarkkula, pers. comm.).
The only documentation of Irrawaddy dolphins in the Mekong of Viet Nam consists of a few records reported by Lloze (1973), skulls housed in whale temples near the delta and in the mouth of the nearby Dong River (Smith et al. 1997, Beasley et al. 2003) and a single carcass found in a fishing net in the Tien distributary near the Cambodian border in March 2002 (Chung and Ho 2002). During a survey of almost the entire length (224 km) of the two main distributaries of the Mekong, Tien and Hau Giang, in April 1996, Smith et al. (1997) were unable to find a single dolphin.
During March and May 1997, Baird (1997) observed 40 dolphins in the segment of the Mekong from Kratie to the Laos/Cambodia border. He estimated, on the basis of surveys and interviews, that the total population in the Mekong was roughly 100 individuals. Beasley et al. (2003) conducted 11 boat-based direct-count surveys, traveling upstream from Jum Neight (about 30 km downstream of Kratie) to the Laos/Cambodia border during January-May from 2001 to 2003. All navigable channels were surveyed (zigzagging when widths were greater than one km and transiting through the center when less than 1 km). Unsurveyed channels were either too shallow or unsafe to survey due to high-velocity currents, which also meant a low probability of dolphins occurring there; interview surveys of local people living along these channels supported this assumption. The largest number of dolphins observed during an upstream survey conducted at the height of the low-water season in April 2003 was 64. This number was based on the sum of best estimates of group size, with a range of 55-82, according to the sum of low and high estimates of group size, respectively. A slightly different method was used during 2002-2003, traveling downstream in the same river segment but stopping for 10-30 minutes in the nine deep pool areas where dolphins had been observed during previous surveys. The maximum number of animals recorded using the pool-count method was 69 individuals based on the sum of best estimates of group size, with a range of 57-84 individuals based on low and high estimates, during a survey in May 2003. Paired observation experiments, using land-based survey teams and concurrent boat-based observations, were conducted during each of the pool-count surveys in an attempt to assess the proportion of dolphins missed by the boat-based team. This resulted in a 100% match between the two methods in terms of the number of dolphin groups detected in each of the nine pools and very similar group size estimates when experienced observers were present on both teams (Beasley et al. 2003).
Native:Cambodia; Lao People's Democratic Republic; Viet Nam
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||No quantitative estimates of population trends are available, but significant range declines in Tonle Sap (Great Lake) and the Mekong mainstem below Kratie imply that the number of dolphins in the Mekong River has declined substantially over the past several decades. Also, for small cetaceans generally, it is recommended that yearly removals should not exceed 1-2% of the population size (Wade 1998) - the lower bound being more applicable to very small populations that are already vulnerable because of demographic and genetic factors. Four deaths per year (the mean number of carcasses recovered and determined to have died from gillnet entanglement in 2001-2003; Beasley et al.  and Beasley [unpublished]) would represent 5.8% of the population, assuming a best estimate of abundance of 69, based on pool count surveys. Considering that the small size of the Mekong population already makes it vulnerable from demographic stochasticity, inbreeding depression and catastrophic environmental and epizootic events, the current rate of incidental mortality in gillnets will almost certainly lead to extirpation.|
|Current Population Trend:||Decreasing|
|Habitat and Ecology:||Irrawaddy dolphins inhabit deep pools of large rivers, sheltered inshore marine environments with substantial freshwater inputs, and partially isolated brackish or freshwater bodies (Stacey and Leatherwood 1997, Stacey and Arnold 1999, Smith and Jefferson 2002).|
Anecdotal reports suggest high dolphin mortality from deliberate killing for oil (reportedly for use in the motors of fishing boats and lamps) during the rule of the Khmer Rouge in 1970-1975 and then from target practice and the effects of explosives used for blast fishing during the Viet Namese occupation in 1975-1980. Dedicated studies on the dolphin population in 1990-1996 (Baird and Mounsouphom 1997) and 2000-2003 (Beasley et al. 2002, Beasley, unpublished) also recorded high mortality with a large proportion of the deaths caused by gillnet entanglement.
Smith et al. (1997) noted the presence of several dozen 200-400 m long stow nets in the Mekong River mouth, followed upstream by more than 10 rows of nylon gillnets stretched across the entire channel, with only small openings to permit vessel traffic. Those authors suggested that dolphin bycatch and displacement caused by the nets could explain the lack of cetacean sightings during their survey of the lower Mekong in Viet Nam during April 1996.
Potential additional threats
Numerous dams have been proposed for the Mekong River system. If built, these would degrade essential habitat features and interrupt the movements of dolphins and their prey. Of greatest concern are the large run-of-the-river dams proposed for the Mekong mainstem at Stung Treng and Sambor (Perrin et al. 1996). In the Sekong River system, at least two dams have been proposed tens of kilometers below the reported upstream limit of the dolphins. Dolphins are also threatened in the Sekong system by the proposed Xakaman and Xepian/Xenamnoi dam projects. This last project would divert almost all of the flow from the Xepian River to a reservoir behind another dam in the Xenamnoi River (Baird and Mounsouphom 1997).
Proposed navigation improvement schemes, which entail blasting the pool-riffle sequences that compose dolphin habitat, would probably lead to a dramatic decline, if not extinction, of the Mekong dolphin population due to the direct effects of the explosions and the indirect effects from eliminating or severely degrading their deep pool habitat. Prey declines from overfishing (particularly from the use of explosives and electricity) and unregulated dolphin-watching tourism may also be affecting the population.
Dolphins in the Mekong River receive some degree of protection from the traditional respect afforded by local fishermen (Baird et al. 1994, Beasley et al. 2003). Fishermen in Viet Nam worship whales and dolphins because they believe that the animals will aid them if they are in distress (Smith et al. 1997). Most Cambodians and Laotians say that they do not hunt dolphins and believe that bad luck will result from killing them (Baird et al. 1994). The Lao Community Fisheries and Dolphin Protection Project was working with local fishermen at Chiteal Pool to reduce incidental catches of dolphins in gillnets, stop explosive fishing and manage aquatic resources in a sustainable manner (Perrin et al. 1996). One practical measure was the establishment of a fund so that fishermen who found dolphins entangled in their nets and cut them free would be compensated for damages (Baird et al. 1994). However, this project has now stopped. Small-scale dolphin watching operations were established at Chiteal Pool (Laos/Cambodia border) in 1997, and this provided substantial income to a few local boat owners. However, due to the decline in dolphin numbers at this pool, the tour operations are now on the verge of collapse. Dolphin-watching also occurs at Kampi Pool and a project partially funded by the Whale and Dolphin Conservation Society, UK, is planned for 2004 to manage the operations so that they provide maximum benefits to the local community (and thereby increase the value of the dolphins as living resources) while not adversely affecting the animals.
In Laos, dolphins are legally protected from hunting, capture and trade, with fines of US$ 65-650 and imprisonment for three months to one year. In Viet Nam, all cetaceans are protected by a decree of the national assembly but this is not generally enforced. During the last three years, the Viet Namese government has been drafting a new law that will give authorities greater power to enforce fishery regulations (Perrin et al., in press). Approval by the national assembly is expected in the near future. No legal protection for cetaceans currently exists in Cambodia. However, a fisheries law is being drafted that includes specific regulations pertaining to marine mammals and the Cambodian Department of Fisheries has proposed to formulate a Royal Decree for protecting the Mekong River dolphin population.
The Mekong Dolphin Conservation Project (currently supported by the Mekong River Commission and Ocean Park Conservation Foundation) was initiated in January 2001. The aims of the project are to assess the Mekong Irrawaddy dolphin population, initiate conservation and management efforts, and build capacity among local government officials. The project works in cooperation with the Cambodian Department of Fisheries, the Wildlife Conservation Society - Cambodia Program and Community Aid Abroad.
|Citation:||Smith, B.D. & Beasley, I. 2004. Orcaella brevirostris (Mekong River subpopulation). The IUCN Red List of Threatened Species 2004: e.T44555A10919444.Downloaded on 24 September 2018.|
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