Orcaella brevirostris (Mahakam River subpopulation)
|Scientific Name:||Orcaella brevirostris (Mahakam River subpopulation)|
|Species Authority:||(Owen in Gray, 1866)|
This is a
freshwater subpopulation of Orcaella
brevirostris Owen in Gray, 1866, found in the
|Red List Category & Criteria:||Critically Endangered D ver 3.1|
|Assessor(s):||Jefferson, T.A., Karczmarski, L., Kreb, D., Laidre, K., O’Corry-Crowe, G., Reeves, R.R., Rojas-Bracho, L., Secchi, E., Slooten, E., Smith, B.D., Wang, J.Y. & Zhou, K.|
|Reviewer(s):||Brownell Jr., R.L. & Cooke, J. (Cetacean Red List Authority)|
This subpopulation is considered Critically Endangered D. Using a proxy value of 54% for the mature adult proportion of the total population (based on calculations from the age of first reproduction (8 years), interbirth interval (2.5 years) and oldest age of reproductive females (30 years) in Sotalia fluviatilis (see Taylor et al. 2007) – a species that lives in similar habitat (i.e., large river) and has similar, but better-known, life history characteristics), there are only 31-42 (total, all ages: 59-79) mature animals in the Mahakam subpopulation of Irrawaddy dolphins. Recent live-captures and ongoing bycatch in fishing gear are the factors likely most responsible for the subpopulation’s decline to such small numbers, and these threats still exist (Kreb et al. 2007).
|Previously published Red List assessments:||
|Range Description:||The subpopulation of Irrawaddy dolphins in the Mahakam River of East Kalimantan, Indonesia, ranges in the mainstem of the river from about 180 km above the mouth to 600 km upstream, seasonally inclusive of Kedang Kepala, Kedang Rantau, Belayan, Kedang Pahu, and Ratah tributaries, as well as Semayang and Melintang lakes (Kreb 1999, 2004). In the early 1980s, dolphins were still commonly reported in Samarinda, about 60km upstream of the coast, but in the early 1990s they rapidly disappeared and are now observed only upstream of about 180 km from the coast. The apparent 120 km range decline represents a loss of about 15% of their historic range (Kreb et al. 2007).|
|FAO Marine Fishing Areas:||
Pacific – western central
|Range Map:||Click here to open the map viewer and explore range.|
The most recent (2005) best estimates of total population size varied between 67 and 70 dolphins (CV = 10%; CL = 59-79), based on direct counts and Petersen mark-recapture analyses of photo-identified dolphins, respectively (Kreb et al. 2007).
|Current Population Trend:||Unknown|
|Habitat and Ecology:||
Significant differences have been recorded among encounter rates of dolphins in the Mahakam for eight 40-km long segments of the river mainstem and tributaries (χ² = 35.91, df = 7, P < 0.01). The three segments with the highest rates included particularly large numbers of confluences and appended lakes. Also, the confluence at Muara Pahu and another confluence about 10 km upstream, in the Kedang Pahu tributary, accounted for 89% of the sightings of newborns (N = 9). The majority of deaths (54%) with known location (N = 46) between 1995 and 2005 also occurred in confluences (Kreb et al. 2007).
The average daily home ranges of 27 groups, which were followed for more than six hours, was 10 km of river length (SD = 8.6, range = 1-45 km). River length ranges were also calculated for 53 photo-identified dolphins during 3.5 consecutive years. Individuals were identified on average 12.5 times (SD = 9.5, range = 2-39) and during 6.2 different survey days (SD = 3.7, range = 2-20). These dolphins moved within the river an average of 61 linear km (SD = 44, range = 4-181) (Kreb et al. 2007).
|Movement patterns:||Full Migrant|
The main threat to this subpopulation is undoubtedly gillnet entanglement, which accounted for approximately 66% of the 46 deaths documented between 1995 and 2005. Five of these dolphins were eaten by local people, and the skins of two were used as medicine for skin allergy. Dolphins in the Mahakam often are observed feeding in close proximity to gillnets and fishermen use the dolphins’ feeding patterns to determine the location and time to set their gillnets. Deliberate kills accounted for 9% of the documented deaths, occurring mostly in isolated areas where the animals were rarely found. Vessel strikes caused 7% of the deaths. Seven percent of the deaths were judged to represent fetal or neonatal mortality, and electro-fishing and hook-fishing each caused 2% of the deaths (Kreb et al. 2007).
From 1974 until 1988, 28
dolphins were live-captured and taken to Jaya Ancol oceanarium in
The high density of gillnets in Semayang and Melintang lakes physically obstructs dolphin movements, thereby reducing available habitat. This problem, together with high sedimentation caused by de-vegetation of the surrounding shorelines, has probably eliminated these lakes as primary areas of occupancy, as reported by Tas’an and Leatherwood (1984). Leaks of chemical wastes, including mercury and cyanide, from retention dams at gold mines in the upper reaches occurred in 1997 and resulted in a massive fish kill (D. Kreb, pers. comm.). Cleaning waste from coal mines enters the Kedang Pahu tributary during floods, and on two occasions dolphins have been observed there with changes skin pigmentation (Kreb et al. 2007). An additional threat is heavy vessel traffic, particularly large coal barges that operate in narrow tributaries and which the dolphins actively avoid (Kreb and Rahadi 2004).
The Action Plan for the Conservation of Freshwater Populations of
Irrawaddy Dolphins (Smith et al.
2007; also see Kreb and Budiono 2005) recommended that core conservation
zones be established in the Mahakam at key river confluences, including of about 10 km of river in both upstream
and downstream directions. The Action
Plan also recommended a strict ban on gillnetting in the proposed core
conservation zones, to be implemented on a step-wise basis as alternative gears
or employment options are provided. Outside the core conservation zones, current
regulations prohibiting the use of gillnets with a mesh size of 10 cm or greater
should be enforced (Kreb et al. 2007 found that most of the documented deaths
|Citation:||Jefferson, T.A., Karczmarski, L., Kreb, D., Laidre, K., O’Corry-Crowe, G., Reeves, R.R., Rojas-Bracho, L., Secchi, E., Slooten, E., Smith, B.D., Wang, J.Y. & Zhou, K. 2008. Orcaella brevirostris (Mahakam River subpopulation). The IUCN Red List of Threatened Species 2008: e.T39428A10237530. . Downloaded on 13 October 2015.|