Orcaella brevirostris (Ayeyarwady River subpopulation)
|Scientific Name:||Orcaella brevirostris (Ayeyarwady River subpopulation)|
|Red List Category & Criteria:||Critically Endangered C2a(i,ii); D ver 3.1|
|Reviewer(s):||Reeves, R. & Taylor, B.L. (Cetacean Red List Authority)|
The best estimate of abundance for the Ayeyarwady population of Irrawaddy dolphins is 59 individuals, based on the December 2003 upstream survey from Mandalay to just above the Taping tributary confluence in Bhamo. Guidelines for considering measurement error (Annex 1: Uncertainty, in IUCN 2001) suggest using plausible lower bounds, rather than best estimates, to determine population size. In the case of dolphins in the Ayeyarwady, this means that it would be appropriate to use 51, the sum of minimum estimates of group size from the December 2003 survey. The threshold of 50 mature individuals for listing a species or subpopulation as CR according to Criterion 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 the Ayeyarwady River 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, and a continuing decline in the number of mature individuals is projected or inferred based on continuing threats from gillnet entanglement, electric fishing, and habitat degradation and acoustical disturbance caused by gold mining operations, and the potential threat from bioaccumulation of mercury, 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 southern 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. |
The earliest reference to dolphins in the Ayeyarwady River is from the New T’ang History (Chinese text from ‘about 800 A.D.’ as cited in Luce 1966), which mentions trade in ‘river pigs’ among the Pyu people. During surveys of the Ayeyarwady River between Rangoon [Yangon] and Bhamo, Anderson (1879) observed Irrawaddy dolphins no farther downstream than Prome [Pyay] (about 360 km from the sea) during the low-water season and Yenanyoung (about 540 km from the sea) during the high-water season. Upstream, the local Shan people reported to Anderson (1879) that dolphins were never found upriver of a point 30 m above Bhamo, where the course of the river was interrupted by rocks. They called the site Labine, or "Dolphin Point." Anderson (1879) also reported that the dolphins ascended larger tributaries, such as the Taping, Khyendwen [Chindwin] and Shuaylee [Shweli], when these were in flood. (See Figure 1 in the Supplementary Material for a map of the Ayeyarwady River and the locations used to describe the distribution of the dolphin).
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||Between 1996 and 2003 several visual boat-based surveys for Irrawaddy dolphins were conducted in the Ayeyarwady River. The most recent of these indicated that the dry season range of the species has declined dramatically and that the number of remaining animals is precariously small. Two surveys (details below) resulted in the same best estimate of 59 animals remaining.
During the November-December 2002 survey of the entire Ayeyarwady River sighting conditions were generally good. If the absence of sightings downstream of Mingun is interpreted to mean that this is the range limit for the species in the river system, this would indicate a range decline of 488 km in river length (or 56.7%) compared with the historical distribution reported by Anderson (1879) and that the distance from the nearest reported record of species in the delta is almost 1,000 km.
During the December 2003 surveys of the Bhamo to Mandalay segment sighting conditions were also generally good. The increase in the number of dolphins observed during the 2003 upstream survey, compared to the 2002 downstream survey, can probably be attributed to the slower speed of the survey vessel and the increased visual coverage of downstream facing tributaries and braided channel confluences (where the dolphins were often found) while surveying in an upstream direction. This hypothesis is supported by similar differences in the number of dolphin groups observed during surveys in upstream and downstream directions between Mandalay and the Shweli River confluence in December 1996 (Smith et al. 1997), five and two, respectively, and during the 2003 surveys themselves, 16 and 10, respectively (Smith 2003).
The abundance estimate from the December 2003 survey is probably close to the actual size of the Ayeyarwady dolphin population due to measures taken in the field to increase sighting efficiency (team of trained and mostly experienced observers, three looking forward and one backwards on the main vessel and two looking forward on the smaller vessel, all alternating between searching with 7x50 binoculars and naked eye and given sufficient rest so that vigilance remained high) and the upstream direction of the survey. The relatively narrow cross section of the main channel (mean=650 m; SD=342 m; range=175–2,200 m; determined by laser range-finder readings, when the distances from the survey vessel to both banks were less than about 600 m and there were suitable reflective targets [e.g., steep sand slope, defile walls], or by visual estimation when these conditions were not met) and the limited deep water area within the cross-section where the dolphins were typically found (and where the vessel’s survey path was confined) also ensured a high level of sighting efficiency.
Potential sighting biases were evaluated using distance estimates and dive time frequencies. According to distance estimation data recorded during the survey (n=16), the sighting frequency declined at around 400 m. Mean vessel speed was 7.8 km/hr or 2.2 m/sec, which means that on average it took 182 seconds to cover the 400 m distance where dolphins had a high probability of being detected (otherwise there would have been a decline in sighting frequency before this distance). Group dive times (n=416) were recorded during nine sightings (mean=20 sec, range=1–105, SD=20). A frequency distribution of these times indicates that 100% of dolphin groups within this distance increment would be at the surface, ‘available’ for detection, at least once and, on average, during about nine surfacings. The dolphins would also be available during the same number of surfacings in the 401–800 m and 801–1,200 m distance increments where the proportion of detections were half the number detected at 0–400 m. This analysis indicates that sighting efficiency during the survey was relatively high and therefore the abundance estimate reasonably unbiased.
A quantitative estimate of population trend cannot be made from the available data, but there is clear evidence of a major reduction in this population’s extent of occurrence and area of occupancy. As summarized above, the linear extent of occurrence in the Ayeyarwady appears to have declined by nearly 60% since the 19th century. Moreover, the best estimate of the total number of dolphins observed was the same (59) for surveys in 1998 and 2003 even though in the latter, the survey boat was traveling at a much slower speed in an upstream direction (7.8 km/hr) in comparison to the faster speed of the earlier downstream survey (13 km/hr). Distances between dolphin sightings (172 km, 124 km, and 33 km for the longest three during the November-December 2002 survey, and 120 km, 89 km, and 23 km during the upstream December 2003 survey) suggest that the remaining population is fragmented to some extent and that opportunities for demographic interaction among dolphin groups are limited, thereby contributing to projected future population declines.
|Current Population Trend:||Decreasing|
|Habitat and Ecology:||The species inhabits deep pools of large rivers, protected 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). During surveys in the Ayeyarwady River dolphin sightings were concentrated in geomorphologically complex reaches upstream and downstream of channel convergences (especially tributaries), islands, and defiles (where an alluvial channel becomes abruptly narrow and deep as it cuts through a mountain range, Smith and Hobbs 2002).|
Smith (2003) recorded a total of 5,701 fishing gears in the main channel of the Ayeyarwady during the November-December 2002 survey. Gill nets accounted for the majority of gear clusters (defined as a grouping of gears within 500 m of the first one observed; 57.4%) and, if sticks with multiple hooks are excluded, the majority of individual gears (53.5%). Gill nets were also the most widespread gears in terms of their distribution throughout the river and there was a significant positive relationship between gillnet encounter rates (i.e., number of gears observed each day) regressed against downstream progress on the survey (DF 19; fixed gillnets p=0.0176, F=6.8321, R2=0.2750; drifting gillnets p=0.0002, F=20.7149, R2=0.5351).
Wherever gillnets and cetaceans occur together there will be entanglements and mortality (see International Whaling Commission 1994). The fact that gillnets were present in higher frequencies in areas where dolphins were reported to occur historically but were not observed during the 2002 survey, implies that these fishing gears are at least partially responsible for the range decline of the species. For small cetaceans it is generally recommended that yearly removals not exceed 1–2% of the population size (Wade 1998) – the lower bound being more applicable to very small populations that are already vulnerable to extirpation due to demographic, genetic and other factors. If there are only about 60 animals in this subpopulation (‘best’ estimate of abundance from December 2003 survey), any more than a single death every one or two years may be unsustainable.
During the December 2003 survey electric fishing was occasionally observed during daylight hours and reported by local villagers to be practiced widely and surreptitiously at night. A local veterinarian reported that he had examined a stranded dolphin that was killed by electrocution (Smith, unpublished). Electric fishing has been cited as being responsible for the largest number of recent known deaths of the baiji (Lipotes vexillifer), a Critically Endangered dolphin in the Yangtze River of China, and has come to be regarded as the main anthropogenic threat to the survival of that species (Zhang et al. 2003).
Potential additional threats
Smith (2003) also recorded a total of 890 gold mining operations during 15 days of the 2002 survey. These were concentrated primarily in areas of reduced current, above and below defiles and near channel convergences – the same areas that constitute the preferred habitat of Irrawaddy dolphins (see above). Large boat dredges (15.8% of the total operations) and hydraulic land blasters (13.4% of the total operations) introduce, break up, and redistribute large quantities of gravel and fine sediments. This causes major changes in the geomorphological and hydraulic features of river channels that make them suitable for dolphins. These operations are also very noisy, which may interfere with the ability of dolphins to navigate, detect and catch their prey and communicate.
Gold mining operations use mercury to amalgamate the gold. Relatively high levels of THg and MeHg have been found in the muscle of 104 fish belonging to 22 different species sampled from fishermen's catches and fish markets along the entire length of the river. In the most widely distributed genus (Ompok) mean muscle Hg from multiple similar individuals was found to increase significantly in the downstream reaches, as compared to upstream reaches (Slotton et al. 2004). The Irrawaddy dolphin is a predatory species and the biomagnification potential of mercury makes pollution by this element a source of concern to the dolphin population. The risk may be increased by the animals’ affinity for counter-currents, where entrained metals may settle in higher concentrations than elsewhere, and tributary mouths fed by lower-order streams, where gold mining may be even more intense and dilution limited by lower flows relative to the mainstem. Although we have no information on the effects of mercury on Irrawaddy dolphins, a casual link has been suggested between liver disease and high levels of the metal in bottlenose dolphins (Tursiops truncatus) and long-finned pilot whales (Globicephala melas) (Bowles 1999).
|Conservation Actions:||The dolphins receive customary protection from directed killing or intentional disturbance by virtue of the positive role they play in a cooperative fishery with throw-net fishermen (Smith et al. 1997). The Wildlife Conservation Society and Myanmar Fisheries Department have plans to establish a long-term conservation program for the dolphins in the upper reaches of the Ayeyarwady River. These entail one or more protected areas where gillnetting would be eliminated or dramatically reduced, enforcing laws that prohibit electric fishing, and monitoring the population and threats to its survival.|
|Citation:||Smith, B.D. 2004. Orcaella brevirostris (Ayeyarwady River subpopulation). The IUCN Red List of Threatened Species 2004: e.T44556A10919593.Downloaded on 17 August 2018.|
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