Orcaella brevirostris 

Scope: Global
Language: English

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Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Chordata Mammalia Cetartiodactyla Delphinidae

Scientific Name: Orcaella brevirostris (Owen in Gray, 1866)
Infra-specific Taxa Assessed:
Common Name(s):
English Irrawaddy Dolphin
French Orcelle
Spanish Delfín Del Irrawaddy
Taxonomic Notes: Dolphins of the genus Orcaella were recently split into two species, the Irrawaddy Dolphin Orcaella brevirostris and the Snub-fin Dolphin O. heinsohni (Beasley et al. 2002, 2005).

Assessment Information [top]

Red List Category & Criteria: Vulnerable A4cd ver 3.1
Year Published: 2008
Date Assessed: 2008-06-30
Assessor(s): Reeves, R.R., Jefferson, T.A., Karczmarski, L., Laidre, K., O’Corry-Crowe, G., Rojas-Bracho, L., Secchi, E.R., Slooten, E., Smith, B.D., Wang, J.Y. & Zhou, K.
Reviewer(s): Brownell Jr., R.L. & Cooke, J. (Cetacean Red List Authority)
The species was listed as Data Deficient in 1996, but a great deal of new information has become available and five subpopulations have been listed as Critically Endangered since that time. Where the species has been studied: (1) subpopulation sizes are generally low (10s to low 100s) with the single exception of Bangladesh (approximately 5800), (2) there have been significant range declines, and (3) threats, especially bycatch and habitat degradation, have been well documented and remain severe and pervasive. Even within the largest known population of the species in Bangladesh (see above), opportunistic observations of deaths in drifting gillnets and reports from local fishermen suggest that bycatch rates are not sustainable (Smith et al. 2005).

At least a 30% reduction in the range-wide population size is suspected over a period of three generations (45-48 years), including the past and future, based on increasing levels of bycatch and habitat degradation in recent years. Generation length (15-16 years) was assumed to be similar to that of Sotalia fluviatilis – a species that lives in similar habitat and has similar, but better known, life history characteristics (Taylor et al. 2007 estimated generation length for S. fluviatilis as 15.6 years). The species therefore qualifies as Vulnerable A4cd. Given the vast area and complexity of coastline inhabited by this species, it is unlikely that a more quantitative assessment of the global population will be feasible in the near future.
Previously published Red List assessments:

Geographic Range [top]

Range Description:Irrawaddy dolphins have a discontinuous distribution in the tropical and subtropical Indo-Pacific, almost exclusively in estuarine and fresh waters (Stacey and Arnold 1999; Arnold 2002). They 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 freshwater subpopulations in three large rivers: Ayeyarwady (up to 1,400 km upstream) in Myanmar, Mahakam (up to 560 km upstream) in Indonesia, and Mekong (up to 690 km upstream) in Viet Nam, Cambodia and Lao PDR, and two marine-appended brackish water bodies or lakes: Chilika in India and Songkhla in Thailand. The fine-scale range of the species is poorly documented throughout much of its range in estuarine waters.
Countries occurrence:
Bangladesh; Brunei Darussalam; Cambodia; India; Indonesia; Lao People's Democratic Republic; Malaysia; Myanmar; Philippines; Singapore; Thailand; Viet Nam
FAO Marine Fishing Areas:
Indian Ocean – eastern; Pacific – western central
Additional data:
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:No range-wide survey has been conducted for this species; nor is there a synoptic estimate of total numbers from local or regional surveys. Statistically rigorous abundance estimates are available for only a few portions of the range: 77 (CV = 27.4%) in Malampaya Sound, Philippines (Smith et al. 2004); at least 125 (95% CI = 114-152) in the Mekong River (Beasley et al. 2007); 70 (CV = 10%; 95% CL = 58-79) in the Mahakam River, Indonesia (Kreb et al. 2007); 58-72 in the Ayeyarwady River, Myanmar (Smith et al. 2007-a); 5,383 (CV=40%) in coastal waters of Bangladesh (Smith et al. 2005); and 451 (CV=9.6%) in the Sundarbans mangrove forest of Bangladesh (Smith et al,/i>. 2006).

Recent surveys indicate dramatic range declines in the Mekong, Mahakam and Ayeyarwady freshwater subpopulations (IWC 2001, Smith et al. 2007-b). All three of these subpopulations were classified as Critically Endangered in the 2004 Red List because the numbers of reproductively mature individuals were estimated to be < 50 and continuing population declines were projected based on known and potential threats. Two other geographically isolated subpopulations – one living in inner Malampaya Sound, Palawan, Philippines, and the other in Songkhla Lake, Thailand, were classified as Critically Endangered, also in 2004, due to their low numbers and limited ranges.
Current Population Trend:Decreasing
Additional data:
Population severely fragmented:Yes

Habitat and Ecology [top]

Habitat and Ecology:Irrawaddy dolphins prefer coastal areas associated with the muddy, brackish waters at river mouths, ranging offshore as far as the extent of the freshwater plume – often only a few km but more than 60 km at the Meghna River mouth in Bangladesh (Smith et al. 2005). In rivers and mangrove channels, the species is most often observed at channel confluences and divergences and downstream of sharp meanders. They have been seen in the same area as finless porpoises in coastal waters of Bangladesh and Myanmar (Smith et al. 2005), and Ganges River dolphins in the waterways of the Sundarbans mangrove forest (Smith et al. 2006).
Systems:Freshwater; Marine

Use and Trade [top]

Use and Trade: In the past, this species was hunted for food in the Mekong and Mahakam Rivers. However, it is revered by local people in many parts of Asia. Live captures for aquarium display continue in some areas.

Threats [top]

Major Threat(s): The estuarine and freshwater occurrence of this species makes it particularly vulnerable to threats from the human activities that occur in these environments. Threats include direct mortality from fisheries interactions (particularly gillnet entanglement), vessel strikes, and habitat loss and degradation (e.g. declining or altered freshwater flows due to dam and embankment construction, environmental contamination). Live captures for aquarium display also have been a conservation issue in some local areas. Irrawaddy dolphins have been hunted directly in the past, at least in the Mekong and Mahakam Rivers, but are revered by local people in many areas of Asia.

Irrawaddy dolphins are caught accidentally in fishing nets in almost all areas where they have been studied (Smith et al. 2007-b). The most detailed information on bycatch comes from the Mekong River where, of 15 confirmed human-caused deaths 2001-2005, 13 (87%) were due to gillnet entanglement (Beasley et al. 2007). Based on reports from local fishermen and the retrieval of eight carcasses along the Mahakam River between 1995 and 2005, Kreb et al. (2007) documented 48 deaths, 66% of them from entanglement in large-mesh (10 –17.5 cm) gillnets. Mortality also has been recorded in drift gillnets targeting elasmobranchs in coastal waters of Bangladesh (Smith et al. 2005) and bottom-set gillnets targeting crabs in Malampaya Sound (Smith et al. 2004). Fishermen in some areas report the dolphins are released if found still alive (Smith and Hobbs 2002, Kreb et al. 2007), but in the case of drowned animals, the oil may be used for medicinal purposes or the flesh eaten (Smith et al. 2004).

There have been no systematic observer schemes in freshwater or coastal regions, but evidence of bycatch and increased use of gillnets is cause for concern (IWC 2000). Fishing with electricity is considered a dire threat to the Ayeyarwady subpopulation (Smith et al. 2007-a).

Many dams have been proposed that are likely to degrade the channels inhabited by Irrawaddy dolphins in the Mekong River Basin. Of greatest concern are the large run-of-the-river dams (dams without a reservoir that generally preserve a relatively natural flow regime) proposed for the Mekong mainstem near Stung Treng and Sambor (Perrin et al. 1996; Mekong Secretariat 1995). Dam projects in Lao PDR, Cambodia and Viet Nam threaten not only dolphins but also fisheries and therefore human livelihoods (Smith et al. 2007-b). A recent report of a high dam planned 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 its effects on the population of Irrawaddy dolphins downstream (Anon. 2007).

Deforestation and gold, sand and gravel mining are causing major changes to the geomorphologic and hydraulic features of rivers and marine-appended lakes where Irrawaddy dolphins occur (Smith et al. 2007-b). Increased sedimentation resulting from deforestation in surrounding watersheds has resulted in declining water depths in Songkhla, Chilika and Semayang Lakes. The last of these water bodies is appended to the Mahakam River and previously supported dolphins throughout most of its breadth. Now it contains suitable habitat only in a small area near the channel connecting it with the mainstem (Kreb et al. 2007). Between 1992 and 1997 the maximum depth of Chilika Lake 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 apparently has mitigated at least some of the problems caused by sedimentation (Pattnaik et al. 2007).

Habitat loss and population fragmentation in several areas have resulted from the proliferation of fixed fishing gears. In the middle and southern portions of Songkhla Lake about 27,000 Sai nong or sitting traps and 13,000 Sang sai or barrier traps create more than 8000 linear km of barrier in multiple rows. These fishing structures are left in place year-round and restrict dolphin movements such that their habitat is substantially reduced and the potential for demographic interaction with individuals in the Gulf of Thailand is eliminated (Smith et al. 2004).

Conservation Actions [top]

Conservation Actions: The species is listed in Appendix I of CITES.

The Action Plan for the Conservation of Freshwater Populations of Irrawaddy Dolphins (Smith et al. 2007-c) notes that multiple-use protected areas will play a key role for conserving freshwater populations 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.

Citation: Reeves, R.R., Jefferson, T.A., Karczmarski, L., Laidre, K., O’Corry-Crowe, G., Rojas-Bracho, L., Secchi, E.R., Slooten, E., Smith, B.D., Wang, J.Y. & Zhou, K. 2008. Orcaella brevirostris. The IUCN Red List of Threatened Species 2008: e.T15419A4579987. . Downloaded on 24 November 2017.
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