Mekong River Irrawaddy dolphins

Updated August 2021

Background and Status

Irrawaddy dolphins (Orcaella brevirostris) are coastal dolphins that are patchily distributed in the nearshore waters of south and southeast Asia. Three discrete riverine populations occur in the Mekong, Mahakam and Ayeyarwady Rivers of Cambodia, Indonesia and Myanmar, respectively. The Irrawaddy dolphin species is red-listed as Endangered, while the three riverine subpopulations are all Critically Endangered (Mekong, Mahakam, Ayeyarwady).

The effective range of Irrawaddy dolphins in the Mekong River is a 180 km segment from Kratie Township, Kratie Province, Cambodia (about 500 km upstream of the Mekong river mouth in Viet Nam) to slightly upstream of the Laos/Cambodia border just below the Khone Falls (or Lee Pee) in Champassak province, Lao PDR. In the distant past, in addition to the Mekong River, dolphins also occurred in the Sekong, Sesan and Srepok Rivers, however, these rivers are no longer inhabited by dolphins (Beasley 2007). In the last 10-15 years, the primary habitat of the dolphins during the low-water season has been in nine deep pools (Beasley 2007). However, during surveys conducted in 2020 and 2021, no dolphins were observed in some of the pools, likely due to changing and reduced dry-season water levels.

Mekong River Irrawaddy dolphin, Orcaella brevirostris, photographed at the Cambodia-Lao Border, Feb-2010. Photo: NIIMURA Yasuo

Map of the portion of the Mekong River inhabited by dolphins, illustrating the main areas of occurrence and key management locations.

Starting in 1997, a series of surveys have provided estimates for the Mekong dolphin population size (see Table 1).

Table 1 – Recent estimates of Mekong dolphin abundance (from Phan et al. 2015, updated with data from 2017 and 2020 surveys, WWF-Cambodia unpublished).

Year	1997	2005	2007	2010	2013	2015	2017	2020
Estimate	≤200	127	93	85	70	80	92	89
Confidence Interval	–	108-146	86-101	78-91	62-80	64-100	80-106	78-102
Method	Direct Observation and guess	Photo-ID Mark-Recapture	Photo-ID Mark-Recapture	Photo-ID Mark-Resight	Photo-ID Mark-Resight	Photo-ID Mark-Resight	Photo-ID Mark Resight	Photo-ID Mark-Resight
Reference	Baird & Beasley 2005	Beasley et al. 2009	Beasley et al. 2012	Ryan et al. 2011	Ryan 2015	Phan et al. 2015	Phan et al. 2018	Eam et al. 2020

All estimates were below 150 individuals and since 2007 they have been below 100 individuals. In 2015 abundance was estimated as 80 individuals (95% CI: 64-100) (Phan et al., 2015; Limsong et al., 2017) and in 2020 as 89 individuals (95% confidence interval = 78-102 individuals) (Khan and Willems 2021). The average annual population growth rate was estimated at 0.98; indicating an average annual decline of 1.6% per year between 2007 and 2015. Mortality rates have been high, for example, 48 dead dolphins (24 adults and 24 calves) were recorded between January 2001 and June 2005, with an additional four reports remaining unconfirmed (Beasley et al. 2007) and there were 88 confirmed dolphin deaths between 2003 and 2008, of which 56 were described as involving calves (Dove 2009, Reeves et al. 2009). A recent study estimated the ages of ten stranded individuals based on Growth Layer Groups (GLGs) in the teeth. The oldest animal aged was 13 years old (Brownell and Chivers 2021). The maximum age of Mekong dolphins is unknown but Australian snub-fin dolphin (Orcaella heinsohni, a closely related marine species) can live for at least 27-30 years (Stacey and Leatherwood 1997, Marsh et al. 1989) and the natural life span of Mekong dolphins should be at least 25 years.

Mekong Irrawaddy dolphin stranded in a gill net near Sambor, 12/16/2005. Photo: WWF-FiA

An investigation into the genetic variation, health status and exposure to contaminants of Mekong dolphins was recently published (Schnitzler et al. 2021). Residue levels of organochlorines and polybrominated diphenyl ethers in blubber samples from 10 Mekong dolphins were lower than the concentrations reported for other cetaceans in the coastal and riverine waters of Asia, (1 to 2 orders of magnitude lower than the threshold PCB concentration for toxicity of 8700 ng⋅g) except for Dichlorodiphenyltrichloroethane (DDT). Amongst DDT metabolites, p,p’-DDE was the predominant metabolic compound, suggesting exposure to aged residues from the environment. A high ratio of organic mercury compared to the immuno-toxic methylmercury was observed. Schnitzler et al. concluded that chemical contaminants could be having an adverse impact on the health of Mekong dolphins, although there was no evidence from histology of any morphological effects and measured levels were lower than in most other cetacean populations.

Samples from 7 dead calves collected between 2015 and 2018 were tested at the University of Illinois, USA for the presence of Brucella spp. DNA using polymerase chain reaction. All samples were negative. Brucellosis can cause reproductive failure, abortion and death in dolphins, and further testing of fresh tissues will continue to rule out this infection as a cause of the high calf mortality of Mekong dolphins.

A genetic study brought together samples collected between 2000 and 2009 to look at the genetic diversity, phylogeny, and demographic history of the Mekong dolphin population, and consider what this means for conservation. The genetic diversity of the population is low, though the results weren’t clear whether this was due to (i) recent genetic collapse from the currently small population size or (ii) the low diversity inherited from the Irrawaddy dolphins that first moved into the Mekong River long ago (with such a long life span, dolphins evolve slowly). However, the results were clear that Mekong dolphins are very distinct from other Irrawaddy dolphins, even those in nearby coastal areas in Cambodia. It’s possible that the population represents a sub-species, though more evidence is needed to clarify this (Krützen et al., 2018).

Phylogenetic relationships of the genus Orcaella based on 384bp of the hyper-variable region I of mitochondrial DNA. Numbers indicate Bayesian posterior probability values for each clade (replicated from Krützen et al. 2018).

Threats

The two primary threats facing Mekong dolphins are:

direct mortality from fisheries, especially entanglement in gillnets, and

habitat loss and degradation, including declining or altered freshwater flows due to the construction of dams and embankments.

Gillnetting is formally banned under a sub-decree of the Cambodian government. However, gillnetting continues and the high rate of adult dolphin mortality has been caused primarily by entanglement in gillnets. The cause(s) of the recorded high calf mortality have proven difficult to diagnose and additional, pathological investigation and behavioural investigations are needed before the causes can be known definitively (Reeves et al. 2009, Khan and Willems 2021). River guards regularly conduct patrols to confiscate all types of nets as well as eliminate illegal fishing practices along the river (see below for details).

Proposals for hydropower dams cover almost the entire current range of Mekong dolphins. The construction of the Don Sahong dam near the Laos/Cambodia border began in 2014, and the nearby subpopulation of dolphins has declined from five to only one individual dolphin and there is now virtually no hope for this group’s persistence.

Dead Irrawaddy dolphin calf with neck lesions found near Kratie Photo: Gordon Congdon

If they go ahead, construction of the proposed Sambor and Stung Treng hydropower dams on the Mekong mainstem as well as the Xayaburi and Luang Prabang dams, both located farther upstream in major tributaries of the Mekong in Laos, would eliminate or transform most of the dolphins’ remaining riverine habitat and would likely have serious detrimental impacts on the remaining Mekong dolphins.

River Guards Programme

As of 2020 there were 72 river guards comprising fisheries officers, police officers, and local community members. The river guards operate out of 16 posts along the Mekong. The highest priority of the River Guard program is to confiscate illegal gillnets. The river guards removed an average of more than 99,125 meters of gillnet annually from 2015 to 2020. In addition to gillnets, the river guards confiscated long-lines with multiple hooks, and the number of long-lines removed has significantly increased since 2016 (4,485 m in 2016, 14,775 m in 2017, 38,650 m in 2018, 48,682 m in 2019 and 41,579 m in 2020). The river guards have also arrested illegal fishers who used electric fishing gears; a total of 48 fishers were arrested for electrofishing from 2015 to 2020, of which 43 were sent to the provincial court. Essential equipment for the river guards such as boats, boat engines, and walkie-talkies have been provided with support from multiple donors including the Cambodian government, WWF and Tiergarten Nürnberg/Yaqu Pacha.

Chronology of Conservation Actions and Meetings

2006: The Government of Cambodia created the “Commission for Conservation and Development of Mekong River Dolphin Eco-Tourism Zone (DC)” to help the Fisheries Administration (FiA) stop the ongoing decline of Mekong dolphins.

2009: On 27-28 October 2009, the World Wide Fund for Nature (WWF) and the Cambodian Government convened the Workshop to Develop a Recovery Plan for the Irrawaddy Dolphin in the Mekong River. WWF-Cambodia requested help from the CSG in examining the evidence and helping identify research and management priorities and a small team of CSG members as well as Frances Gulland on behalf of the IUCN/SSC Veterinary Specialist Group, met with government officials and NGO representatives in Phnom Penh. The report produced by the expert group can be found here and concluded that high calf mortality was likely to be due to entanglement in fishing gear (Reeves et al. 2009).

2012: Following on from the 2009 meeting, an international team of scientists, including several from the CSG, spent the week of 9-13 January 2012 in Cambodia working with Cambodian counterparts on efforts to refine understanding of the status of dolphins in the Mekong River, determine cause(s) of the exceptionally high calf mortality documented in recent years, and improve protection measures (especially pertaining to bycatch in gillnets). A key finding by the visiting scientific team (which included veterinarians Frances Gulland, Thijs Kuiken, Antonio Fernández, and Paul Jepson) was that there is no evidence to support the idea that a contaminant exposure is involved in the high incidence of calf deaths. Nor was there any support for the view that this population is suffering significantly from immunosuppression. Entanglement in fishing gear, mainly gillnets, was unquestionably the primary cause of death for non-calves, but the primary cause(s) for the very high mortality of calves remained unknown.

The culmination of this workshop was the signing of the “Kratie Declaration on the Conservation of the Mekong River Irrawaddy Dolphins” (which can be found in full here). The Declaration was a landmark for dolphin conservation in the Mekong, with the Commission for Conservation and Development of Mekong River Dolphin Eco-Tourism Zone, the Fisheries Administration, and WWF-Cambodia, all agreeing to work together on a joint strategy for Mekong dolphin conservation. The Declaration specifies management and research recommendations that have been updated and used as the basis for conservation efforts.

On-the-ground protection also made big steps forward with the declaration of protected areas for Mekong dolphins covering their entire permanent range in Cambodia, from the Kratie Town to the Laos border. A grant to WWF-Cambodia from the Save Our Species funded support for implementation and enforcement of this new law by equipping and training the Fisheries Officers and the Dolphin Commission’s local River Guard network.

2014: A workshop was organised in Phnom Penh to assess progress on implementation of the recommendations from the Kratie Declaration and to update Cambodia’s river dolphin conservation strategy. The workshop affirmed that significant progress had been made since the Kratie Declaration, notably in the reinforcement of patrolling activities by river guards and the refinement of protocols to investigate mortality. Nonetheless, the group concluded that entanglement in gillnets remains the most critical and immediate threat to the survival of dolphins in the Mekong and that concerted efforts are needed to address this issue even more vigorously. Also, other threats had emerged, most importantly the notification by Laos of its intention to proceed with construction of a hydropower dam at Don Sahong near the Cambodia border. The workshop made a series of recommendations related to strengthened research on demography and mortality of the dolphin population as well as more rigorous analyses of the booming dolphin-watching tourism industry. The full report of the workshop is available here.

2014-2016: The WWF-Cambodia team based in Kratie, working in collaboration with the Fisheries Department in Phnom Penh, continue their valiant efforts to conserve and study Mekong dolphins including implementing an effective river guards programme. Some of the outcomes of the river guards work are described here. A small number of calves were reported to have survived their first year of life – the first such reports in several years. In late March 2016, Frances Gulland (SSC Wildlife Health Specialist Group) visited Kratie to carry out necropsies on five dolphins that died in 2015. She concluded that one of the two adults had died from entanglement in fishing gear; the other was too decomposed to diagnose the cause of death but it was fat and heavily scarred (typical of adults in this population). Of the three calves, two had been stillborn and one died from trauma.

2017: In January 2017 an international workshop on the Mekong dolphins was held in Kratie, Cambodia (Limsong et al. 217) [click here to read report]. This was the fourth such workshop convened by the World Wide Fund for Nature (WWF) Cambodia and the Cambodian Fisheries Administration, all of them organized and conducted in collaboration with the CSG and the US Marine Mammal Commission.

At this workshop the external advisory group concluded that significant progress had been made on implementing most of the recommendations from the 2014 workshop, thanks to the commitment of the WWF-Cambodia team, the Fisheries Administration, the River Guards and the local community. The River Guards have worked hard to confiscate gillnets in the core dolphin zones and the consequent reduction in entanglement risk may have been a significant factor contributing to the recent increase in calf survival. The River Guards nevertheless continue to face a number of obstacles and gillnetting remains a serious threat.

Somany Phay, Frances Gulland, and H.E. Srun Limsong, Deputy Director General of the Fisheries Administration, Royal Government of Cambodia. Photo Credit: Peter Thomas

The threat of hydropower development, addressed in detail at the 2014 workshop, is now a reality for this population as construction of the Don Sahong dam near the Laos/Cambodia border began in 2014 (Brownell et al. 2017). A letter co-signed by the IUCN Director General and the Chair of the Species Survival Commission was sent to the Prime Minister of Cambodia, emphasizing the concern of the international conservation community about the impacts of dam construction on the Mekong dolphins and other biodiversity. The dam issue was also discussed by the IWC Scientific Committee at its annual meeting in May 2017. The committee concluded that “if the proposed construction of large hydropower projects on the Mekong mainstem in Cambodia proceeds, almost all of the dolphins’ habitat in the Mekong will be modified or eliminated and the risk of extinction will be greatly increased.”

River guards burning confiscated gillnets. Photo credit: Peter Thomas

2020: In December 2020 a Trinational Workshop on the Irrawaddy dolphin was held virtually and brought together those working with the three freshwater riverine populations of Irrawaddy dolphin, Orcaella brevirostris. One goal of the workshop was to assess implementation of recommendations of the 2017 Mekong workshop, update recommendations and the national species action plan, and prioritize actions for the future. Overall, 24 of the 40 recommendations had been fully implemented, 13 were partially implemented, and three were not implemented. The failure to fully implement some of the recommendations was due to limitations in funding and capacity. Recommendations were made with regards to the river guards programme, and regular monitoring to estimate abundance and movements of Mekong dolphins.

2021: The number of dolphins in the Anlung Chuteal pool in Cambodia downstream of the Lao PDR border and just below the Don Sahong dam construction site has dropped from 8 individuals in 2007 to a single animal in 2021.