Orcaella brevirostris (Songkhla Lake subpopulation)
|Scientific Name:||Orcaella brevirostris (Songkhla Lake 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. & Taylor, B.L. (Cetacean Red List Authority)|
The extremely low numbers of dolphin groups detected during the 2001 and 2002 surveys and the absence of sightings during the 2003 survey indicate that the population in Songkhla Lake certainly contains fewer than 50 mature individuals. The evidence is particularly strong when these survey results are compared to the sighting rates recorded for the Critically Endangered Malampaya Sound Irrawaddy dolphin subpopulation (see Smith et al. in press). Therefore, the Songkhla subpopulation qualifies for listing as CR based on Criterion D. It also qualifies as CR on the basis of Criterion C2a (i,ii) as there are certainly fewer than 250 mature individuals in the subpopulation, a continuing decline in the number of mature individuals is projected or inferred based on the ongoing threat from gillnet entanglement and capture in fishing traps, 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 Laos, Cambodia and Viet Nam. Irrawaddy dolphins also occur in partially isolated brackish or fresh-water bodies, including Chilka Lake in India and Songkhla Lake in Thailand. Irrawaddy dolphins were first recorded in Songkhla Lake by Pilleri and Gihr (1974) who examined three stranded specimens from Thala Luang, north of Papayurn Island.|
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||During survey effort in May 2000 and February 2001 covering 545 km in the inner and middle portions of the lake (Thale Luang), north of Papayurn Island, Beasley et al. (2002) recorded only four sightings and calculated a sighting rate of 0.03 dolphins/linear km (mean group size = 4.3 dolphins, S.D. = 2.9, range = 1-8). All sightings were made in the inner portion, which is the deepest part of the lake (2.1-2.5 m). Due to shallow water and the extremely high density of fixed fishing gear, Beasley et al. (2002) proposed that dolphins were probably absent from the outer portion of the lake (Thale Sap) and the southern part of Thale Luang. If their supposition is correct, it means that dolphins do not move out of the lake into the Gulf of Thailand, or vice-versa, making the Songkhla population permanently and completely isolated. The same researchers presented additional evidence of the population's geographic isolation based on interviews with 86 local fishermen. All the fishermen who stated that they had seen dolphins in the lake (69%) fished in the inner portion of Thale Luang (where the sightings were made during the 2000 and 2001 surveys), whereas the remaining fishermen, who had not seen dolphins, fished either to the north or south. Geographic isolation of the population is also supported by observations made during the line-transect survey described below. |
In September 2003 a line-transect survey, covering 234 km in 21 hours of search effort, was conducted in Songkhla Lake (B. Smith, unpublished). Sighting conditions were good but no dolphins were seen. The intention had been to search the entire lake, following parallel track lines spaced 3.5 km apart. However, shallow water, dense sea grass and an extremely high density of fixed fishing gear prevented the survey vessel from following the designed track lines in the southern portion of Thale Luang and Thale Sap. Search effort was therefore effectively limited to the same area covered by Beasley et al. (2002). The 2003 survey was conducted using a similar vessel/observation platform and the same methods and number of observers as had been used for line-transect surveys for Irrawaddy dolphins in Malampaya Sound, Philippines (see Smith et al. in press). If the sighting rate in Songkhla Lake had been equivalent to that recorded in Malampaya Sound (0.0865 sightings/km; a subpopulation also proposed to be listed as Critically Endangered), 20 dolphin groups should have been detected. Even accounting for the difference in size between the area occupied by dolphins in Malampaya Sound (ca. 134 km²) and the area searched during the 2003 Songkhla Lake survey (ca. 755 km²), if the two water bodies supported approximately the same number of groups and individuals, 5-6 groups consisting of 27-32 individuals (based on the mean group size of 5.3 dolphins recorded in Malampaya Sound) should have been detected.
Currently, there is only a single connecting channel between the lake and the Gulf of Thailand. This channel is located at the southern tip of the lake and almost certainly inaccessible to dolphins due to the extremely high density of fixed fishing weirs (which remain set throughout the year) and gillnets (some of which are set throughout the year, others of which are removed when some fishermen practice other occupations during the monsoon season - May through December, peaking from September through November). Previously a second smaller connecting channel existed at the northern tip of the lake. This has been blocked by a closure dam constructed to prevent saline inputs, which supplies irrigation water to intensive agriculture practiced in the fields surrounding the lake. The published records of Irrawaddy dolphins geographically closest to the southern connecting channel are of two specimens in 1901 at Pattani (ca. 100 km to the south; Bonhote 1903 - not seen; cited in Pilleri and Gihr 1974) and one stranding in 1994 at Surat Thani (ca. 300 km to the north; Chantrapornsyl et al. 1996). The absence of records near the connecting channel (although this may also be explained by a lack of survey effort), the extremely high density of fixed fishing weirs that probably constitute a physical barrier to dolphin movement, and interview surveys indicating that dolphins do not occur in the southern portion of the lake imply that there is little, if any, demographic exchange between dolphins in Songkhla Lake and those in the Gulf of Thailand - almost certainly fewer than one successful migrant per year (the criterion for defining a subpopulation according to the IUCN Red List Categories and Criteria Version 3.1). Although we have no information on the geomorphic history of the lake, the dynamic sedimentation and erosion processes typical of marine appended lakes would suggest that there were previous primary (i.e., non-anthropogenic) isolation events. These may have allowed population-level genetic differences to develop between the dolphins in Songkhla Lake and those in coastal waters of the Gulf of Thailand, although no data are available to investigate this possibility. A precautionary approach is to consider the dolphins in Songkhla Lake geographically isolated, pending future evidence to suggest otherwise.
There are no quantitative data for estimating population trends. However, circumstantial evidence indicates declining numbers. Compared to the 2001-2002 surveys reported in Beasley et al. (2002), the 2003 survey used three rather than two observers and search effort was conducted from a raised platform about two meters higher above the water level. This should have improved searching efficiency. Even with no change in efficiency, the 2003 survey should have produced at least one or two sightings. It is possible that the difference in results simply reflects random variation in sighting biases, but a precautionary interpretation would be that the very small population that existed in 2001 and 2002 has been reduced even further. This interpretation is reinforced by the high mortality experienced by the population (as evidenced by the large number of recorded deaths; see above) in relation to its extremely low (although precisely unknown) population size.
|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 freshwater bodies. (Stacey and Leatherwood 1997, Stacey and Arnold 1999, Smith and Jefferson 2002).|
Beasley et al. (2002) listed 28 records of dolphins that stranded in Songhkla Lake between January 1990 and April 2001. At least 13 of them were judged to have died from net entanglement, based upon the presence of net scars on the carcass or the reports of local fishermen (Somserm Choorak, pers. comm.). Of the total strandings, at least nine were calves (i.e., one meter in length or smaller). Since that report, 15 additional strandings have been recorded, including nine calves (four of these in February 2003 and two in December 2003) and a pregnant female (Somserm Choorak, pers. comm). Several of those 15 animals were believed to have been killed accidentally in gill nets and fish traps set for sea bass, the carcasses having been discarded and then drifting ashore. Some of the calves were probably stillborn based on the presence of fetal folds and a non-erect dorsal fin. The pregnant female's flukes had been cut off, probably to extract her body from a gill net.
The increase in the number of recorded strandings, from 2.5 per year between January 1990 and April 2001 to 5.5 per year between May 2001 and December 2003 (see above), probably reflects an increase in reporting (due to the greater awareness of local people about the dolphins and efforts to conserve them; see below), the developing capacity of local agencies to respond to stranding reports, and possibly a rise in mortality within the population itself. It is unlikely that any real increase in mortality could be explained by an increasing population size, given the lack of recent sightings despite extensive survey effort (see above).
The proportion of stranded calves has also increased, which could mean that some adults are habituating to the presence of nets and fish traps, or that the calves are dying at an unusually high rate for altogether different reasons; the causes of death for these carcasses could not be determined (Somserm Choorak, pers. comm.). Although we have no supporting evidence, a possible explanation for the high proportion of stranded calves could be that they were stillborn or died shortly after birth due to high toxic loads from agrochemicals used intensively along the shores of the lake.
Although incidental mortality in gillnets and fish traps is the principal known threat to the population, prey depletion is another potential threat. This could be the result of overfishing and/or the sedimentation and eutrophication caused by shoreline development and deforestation. A closure dam across a narrow channel that previously connected Thale Luang to the sea has undoubtedly contributed to the cumulative effects of human activities (Beasley et al. 2002, Smith, unpublished). Year-round availability of fresh water after the dam's closure has allowed an increase in shoreline agriculture, which uses large quantities of pesticides and herbicides. The dam's construction also may have eliminated a movement corridor for dolphins between Songkhla Lake and the Gulf of Thailand.
|Conservation Actions:||The Irrawaddy dolphin has been adopted as the mascot of the Phattalung Province and local people have expressed great enthusiasm for its conservation. The Nue Ham La Ow Bang Teng Forestry Station, which is responsible for protecting the lake's wildlife, provides a government infrastructure for dolphin protection. A Royal proclamation by the Queen of Thailand on 3 October 2001 designated the Irrawaddy dolphins in Songkhla Lake as a Royal Protected Species. The proclamation makes harming the dolphins or possessing their body parts punishable by four years in jail and/or a fine of 40,000 Baht (ca. US$1,000). The Irrawaddy Dolphin Conservation Society was established by a local schoolteacher in Phattalung in 1996. The Society has conducted numerous awareness-raising activities and keeps detailed records of dolphin strandings; the compilations are reported directly to the Queen at regular intervals. In 2001, a collaborative research and conservation program with the Irrawaddy Dolphin Conservation Society was initiated by the Whale and Dolphin Conservation Society, UK. This project was continued and expanded in 2003, with the involvement of the Thailand Departments of Forestry and Fisheries and support from the Wildlife Conservation Society, USA.|
|Citation:||Smith, B.D. & Beasley, I. 2004. Orcaella brevirostris (Songkhla Lake subpopulation). The IUCN Red List of Threatened Species 2004: e.T44557A10919695.Downloaded on 23 June 2018.|
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