Orcaella brevirostris (Owen in Gray, 1866)

Russell A. Mittermeier & Don E. Wilson, 2014, Delphinidae, Handbook of the Mammals of the World – Volume 4 Sea Mammals, Barcelona: Lynx Edicions, pp. 410-526 : 492-493

7.

Irrawaddy Dolphin

Orcaella brevirostris

French: Orcelle de |'lrrawaddy / German: Irawadi-Delfin / Spanish: Delfin del Irawadi

Other common names: Larger Indian Porpoise, Snubfin Dolphin

Taxonomy. Orca brevirostris Owen in Gray, 1866,

“Inhab. East coast of India, the harbor of Vizagapatam,” Bay of Bengal.

Orcaella brevirostris was split in 2005 by I. Beasley and colleagues into two species, O. brevirostris and O. heinsohni , based on morphological, genetic, and geographic differences; O. heinsohn: was limited to the coastal waters of northern Australia and southern Papua New Guinea. Genetic evidence may also support further differentiation within O. brevirostris , which may warrant new subspecies designations in the future. Nevertheless, this taxonomy has not yet been widely accepted. Monotypic.

Distribution. SE Asian coastal waters and river mouths including Bay of Bengal, Ganges Delta, Strait of Malacca, Gulf of Thailand, Sunda Shelf, and Malampaya Sound (Palawan), also in Irrawady (= Ayeyarwady), Mekong, and Mahakam river systems, Chilika and Songkhla lakes. A population that once inhabited Tonle Sap Great Lake in Cambodia has been extirpated.

Descriptive notes. Total length up to 280 cm (males) and 210-220 cm (females); weight 115-130 kg. Neonates are c.100 cm in length and weigh 10-12 kg. The Irrawaddy Dolphin has rounded head set on flexible neck, no visible beak, and small dorsalfin (3% of total length) with rounded tip positioned at ¢.60% of the total length along back. Flippers are large and wide, 7-3% of total body length on average, with rounded tips. Shallow groove also runs from neck to dorsal fin along back, a feature that distinguishes the Irrawaddy Dolphin from the closely related Australian Snubfin Dolphin ( O. heinsohni ). Skin pigmentation ranges from uniform dark gray along most of dorsal surface to paler gray on ventral areas. There are 8-19 pairs of teeth in upper jaw and 11-18 pairs in lower jaw, although teeth may not erupt in some populations.

Habitat. Five distinct populations in freshwater: the Irrawaddy (up to 1500 km upstream), Mekong (up to 690 km upstream), and Mahakam (up to 560 km upstream) river systems; Malampaya Sound in the Philippines; and the brackish Songkhla Lake in Thailand. These populations of Irrawaddy Dolphins are demographically isolated from marine populations and prefer shallow coastal, estuarine, and fresh turbid waters. All sightings of non-freshwater individuals have been within a few kilometers of coastlines. In the Mekong River system, the Irrawaddy Dolphin occurs most often near sand banks and stream tributaries and is confined to pool river sections that are at least 8 m deep during the dry season. Populations that inhabit nearshore waters off Bangladesh appear to prefer shallow depths and low salinity, which likely reflects distributions of preferred prey.

Food and Feeding. The Irrawaddy Dolphin is a generalist and primarily diurnal feeder, preying on a wide variety small fish, cephalopods, and crustaceans that are abundant in near-shore marine and freshwater environments. Echolocation clicks are emitted in short pulses of 25-30 psec at a frequency of ¢.60 kHz. Individuals from freshwater populations can spit water while foraging as a kind of herding strategy. In upper reaches of the Irrawaddy River system, the Irrawaddy Dolphin has established a cooperative fishery with local cast-net fishermen. One or two dolphins will herd a school offish toward shore and wave their flukes to surge the school toward waiting fishermen, who then cast their nets over the mass. Fish that are confused by the net’s movement or trapped under the lead line become easy prey for the Irrawaddy Dolphins. Fishermen that cooperate with the dolphins have a much lower zero-catch frequency.

Breeding. Breeding of Irrawaddy Dolphins may occur throughout the year, but it peaks during the pre-monsoon season in April-June. Gestation is c.14 months, and longevity is ¢.30 years, although thereis likely great variation among populations.

Activity patterns. In the Mekong River, the Irrawaddy Dolphin is most active in the morning while foraging. Dives are generally less than three minutes long but can last up to twelve minutes. Surfacing behavior of Irrawaddy Dolphins is mostly cryptic unless they are disturbed, socializing, or swimming against strong currents. Dives are slow and rolling, often with only a small part of the dorsal area becoming visible above the water's surface. Irrawaddy Dolphins spy-hop and breach occasionally, but they are not known to bow-ride.

Movements, Home range and Social organization. Group sizes of Irrawaddy Dolphins are generally 2-6 individuals, but groups of up to 25 individuals have been documented aggregating in deep pool sections of the Mekong River during the dry season. Daily linear ranges in the Mahakam River average 10-61 km over the long-term (3:5 years). Seasonal movements that coincide with seasonalshifts in freshwater input are observed in populations inhabiting the Sundarbans mangrove forest in Bangladesh. Irrawaddy Dolphins follow the salinity gradient, probably to keep up with movements of prey species. In the Mahakam River system, the Irrawaddy Dolphin migrates daily between the river proper and the Semayang Lake, sometimes swimming up to 1300 km upstream. During the high-water season, they spend more time in tributaries.

Status and Conservation. CITES Appendix I. Classified as Vulnerable on The IUCN Red List. The five freshwater subpopulations (Irrawaddy, Mahakam, and Mekong rivers; Melampaya Sound; and Songkhla Lake) of the Irrawaddy Dolphin are listed separately as Critically Endangered. Population sizes are generally low, ranging from the tens to low hundreds. In recent years, the Irrawaddy Dolphin has decreased in abundance, there have been reductions in its distribution, and incidental catch and habitat degradation are extensive. As such, a 30% reduction in its abundance is likely over the next three generations (45-48 years). Total abundance and trend estimates for the Irrawaddy Dolphin are currently unavailable, and only a few locations in its distribution have been assessed. There are 77 individuals in Malampaya Sound, 127 individuals in the Mekong River, 70 individuals in the Mahakam River, 58-72 individuals in the Irrawaddy River, 62-98 individuals in Chilika Lake, fewer than 50 mature individuals in Songkhla Lake, 5383 individuals in coastal waters of Bangladesh, and 451 individuals in mangrove forests of Bangladesh (Sundarbans). Because it has a limited distribution and inhabits environments close to shore, the Irrawaddy Dolphin is especially vulnerable to threats from human activities. The most serious are incidental catch and habitat degradation. Irrawaddy Dolphins have been caught incidentally in fisheries throughout most of their distribution. Between 2001 and 2005, 13 deaths occurred from gillnet entanglement in the Mekong River, and between 1995 and 2005, 32 similar deaths occurred in the Mahakam River. The Irrawaddy Dolphin also has died incidentally in elasmobranch fisheries off Bangladesh and in crab fisheries in Malampaya Sound. In the Irrawaddy River, fishing with electricity poses a severe threat to the small population of Irrawaddy Dolphins living there. Between 2001 and 2005, 29 individuals died in incidental catches in Malampaya Sound, and the population is expected to go extinct within seven years if fishing continues unmitigated. Damming of the Mekong River threatens the population there by blocking movements along the river and disrupting availability of deep pool habitats, which are vital to Irrawaddy Dolphins during the dry season. Upstream water withdrawals in Bangladesh are causing increased sedimentation in the Sundarbans mangrove forest, which is gradually reducing water depth and channel accessibility for resident Irrawaddy Dolphins. Deforestation is causing similar problems in the Songkhla, Chilika, and Semayang lakes. For example, between 1992 and 1997, Chilika Lake’s maximum depth was reduced from 3-4 m to 1-4 m, which caused a decrease in salinity and shrinking of its entrance channel. Songkhla Lake has 40,000 fixed fishing nets that create more than 8000 km of linear barriers along the river system. As such, movements of Irrawaddy Dolphins are severely restricted; some parts of the habitat are inaccessible. Fixed fishing nets in the Semayang Lake also restrict access into the Mekong River by a narrow, dredged channel subject to intense vessel traffic. The Irrawaddy Dolphin, which is rather charismatic in appearance and behavior,is also taken for captive display in aquaria. Because international trade in live Irrawaddy Dolphins is a possibility, it is included on CITES Appendix I. The Irrawaddy Dolphin is legally protected from direct catch in Bangladesh, Cambodia, India, Laos, Malaysia, Burma (= Myanmar), Thailand, and Vietnam, but there are no monitoring programs currently in effect for incidental catch. Habitats of Irrawaddy Dolphins have only been minimally protected so far. Parts of the Sundarbans Delta are included in Protected Forests and UNESCO World Heritage Sites, but there are no conservation initiatives in the area specifically for Irrawaddy Dolphins. Eight protected areas totaling 5721 ha and 190 km of the Mekong River have been drafted for protection by the Cambodian Department of Fisheries, and a 74km segment of the Irrawaddy River has been announced as a protected area by the Ministry of Livestock and Fisheries of Myanmar. In these areas, the Irrawaddy Dolphin may not be deliberately caught, electrofishing is prohibited, and nets are not allowed to obstruct the river course or be more than 100 m long or spaced less than 200 m apart. Fishermen are also obligated to release Irrawaddy Dolphins that are found entangled alive in fishing nets. Other proposed strategies for conserving the Irrawaddy Dolphin, especially the Critically Endangered freshwater populations, include subsidization of fishermen to reduce incidental catch, no-fishing zones for areas ofcritical habitat, establishment of long-term population monitoring programs, and promotion of the Irrawaddy Dolphin as a “poster species” for ecosystem-based conservation initiatives.

Bibliography. Baird & Beasley (2005), Beasley et al. (2005), Gonzales & Matillano (2008), Jefferson, Karczmarski et al. (2000), Jefferson, Webber & Pitman (2008), Kreb & Budiono (2005), Kreb & Rahadi (2004), Poulsen et al. (2002), Reeves, Brownell et al. (2009), Reeves, Jefferson et al. (2008a), Smith (2004, 2009), Smith & Beasley (2004a, 2004b, 2004c), Smith, Beasley et al. (2004), Smith, Braulik et al. (2009), Smith, Tun Mya-Than et al. (2009), Stacey & Hvenegaard (2002).

Figures

On following pages: 3. Peale’s Dolphin (Lagenorhynchus australis); 4. Hourglass Dolphin (Lagenorhynchus crucigen; 5. Pacific White-sided Dolphin (Lagenorhynchus obliquidens); 6. Dusky Dolphin (Lagenorhynchus obscurus); 7. Irrawaddy Dolphin (Orcaella brevirostris); 8. Australian Snubfin Dolphin (Orcaella heinsohni).

Distribution. SE Asian coastal waters and river mouths including Bay of Bengal, Ganges Delta, Strait of Malacca, Gulf of Thailand, Sunda Shelf, and Malampaya Sound (Palawan), also in Irrawady (= Ayeyarwady), Mekong, and Mahakam river systems, Chilika and Songkhla lakes. A population that once inhabited Tonle Sap Great Lake in Cambodia has been extirpated.