Orcaella brevirostris (Malampaya Sound subpopulation)
|Scientific Name:||Orcaella brevirostris (Malampaya Sound 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. & Perrin, W.F. (Cetacean Red List Authority)|
The best estimate of total subpopulation size is 77 individuals. Guidelines for considering measurement error (in Annex 1: Uncertainty, of the Red List Criteria) suggest using plausible lower bounds, rather than best estimates, to determine population size. In the case of Malampaya dolphins, this implies that it would be appropriate to use the lower 95% confidence interval, which is 45 individuals. Also, the threshold of 50 mature individuals for listing a species or population as CR according to Criteria 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 Malampaya Sound is less than 50. Therefore, the population qualifies for listing as CR based on Criterion D. The population also qualifies as CR on the basis of Criterion C2a (i, ii) as there are certainly fewer than 250 mature individuals in the population, a continuing decline in the number of mature individuals is projected or inferred based on the ongoing threat from gillnet entanglement, 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 Lao PDR, Cambodia and Viet Nam. Irrawaddy dolphins also occur in completely or partially isolated brackish or freshwater bodies, including Chilka Lake in India and Songkhla Lake in Thailand. Irrawaddy dolphins were first documented as occurring in Malampaya Sound, Palawan, Philippines, during an investigation of dugongs (Dugong dugon) in 1986 (Kataoka et al. 1995). This is the only known subpopulation of the species in the Philippines and the nearest other known subpopulation is in northern Borneo, some 550 km to the south. The species occurs only in the inner portion of the sound, an area of about 134 km².|
|FAO Marine Fishing Areas:|
Pacific – western central
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||The first dedicated cetacean survey of Malampaya Sound, conducted in June-July 1999, recorded 17 sightings during 230 linear km of search effort with a mean encounter rate of 7.4 dolphins/100 km (S.E.=2.9) and a mean group size of 5.3 dolphins (S.E.=1.1; Dolar et al. 2002). All sightings were made in shallow waters of the inner Sound, although survey effort also included waters of the outer Sound. |
Line-transect surveys conducted in April-November 2001 covered 884 km of trackline in the entire Sound (total area 230.7 km²), and resulted in a total population estimate of 77 individuals (CV=27.4%), confined to the inner portion (133.7 km²; Smith et al., in press). Seasonally stratified estimates were calculated as 67, CV=38.6%; 78, CV=78.1% and 81, CV=31.7% for pre-monsoon, monsoon and post-monsoon seasons, respectively. The absence of Irrawaddy dolphin sightings in the outer Sound (see Dolar et al. 2002, Smith et al. in press, and unpublished reports of WWF-Philippines) and the close agreement among abundance estimates from surveys conducted during pre-monsoon, monsoon and post-monsoon seasons strongly suggest that the subpopulation is resident within the inner Sound.
There are no direct data on population trends. However, for small cetaceans generally, it is recommended that yearly removals should not exceed 1-2% of the overall population size (Wade 1998). Using a minimum estimate of two dolphins killed per year (see above), this represents 2.6% of the population, according to the best estimate of abundance made during line-transect surveys (77 dolphins). Using the minimum abundance estimate (45 dolphins), the estimated minimum yearly mortality rate would be 4.4% of the population size. Even this latter figure may be an underestimate because it considers only the two confirmed kills recorded during a 7-month study (Smith et al. in press). Considering that the small size of the Malampaya population already makes it vulnerable to extirpation 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:||The species inhabits deep pools of large rivers, sheltered inshore marine environments with substantial freshwater inputs, and partially isolated brackish or fresh- water lagoon or lake systems (Stacey and Leatherwood 1997, Stacey and Arnold 1999, Smith and Jefferson 2002).
During 48 sightings of Irrawaddy dolphins in Malampaya Sound, Smith et al. (in press) found significant differences between environmental characteristics in the inner and outer Sound (DF=158) for depth (F=164.54, Prob.=0.0000), salinity (F=19.27, Prob.=0.0000) and turbidity (F=21.53, Prob.=0.0000), with depth and salinity greater in the outer Sound and turbidity greater in the inner Sound, indicating the restricted environmental preferences of the Malampaya Irrawaddy dolphin subpopulation.
Between February and August 2001, two dolphins were accidentally killed in bottom-set nylon gillnets used to catch crabs - locally known as matang quatro nets. Unconfirmed reports from fishermen indicate that three additional dolphins may have been killed in gillnets during the same seven-month period (Smith et al. in press). Although the dolphins are not believed to be hunted in Malampaya Sound, their value for human consumption means that those found alive in gillnets may be taken for food rather than released.
Although gillnet entanglement is believed to be the dominant threat to the population (Smith et al. in press), other potential threats include incidental takes in fish corrals (baklads), liftnets (bukatot) and crab traps, prey depletion from increased fishing pressure and increased inputs of organic wastes leading to eutrophication (Dolar et al. 2002).
|Conservation Actions:||The Malampaya Irrawaddy dolphin subpopulation is a component of the Malampaya Sound Ecological Studies Project currently being implemented by Kabang Kalikasan Ng Pilipinas (World Wildlife Fund - Philippines). This project consists of research, monitoring and raising local awareness about the conservation significance of dolphins in the Sound.|
|Citation:||Smith, B.D. & Beasley, I. 2004. Orcaella brevirostris (Malampaya Sound subpopulation). The IUCN Red List of Threatened Species 2004: e.T44187A10858619.Downloaded on 25 September 2018.|
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