The effective range of Irrawaddy dolphins in the Mekong River is a 190 km segment from Kratie (about 500 km upstream of the river mouth in Viet Nam) to slightly upstream of the Laos/Cambodia border at Khone Falls (or Lee Pee), which physically obstructs further upstream movement. During the high water season (June?October), anecdotal reports suggested that the dolphins ascended the Sekong River and its tributaries, the Houay Khaliang, Xepian (to Xepha Falls about 50 km above the Sekong confluence), Xenamnoi (to Tatkhek Falls about 8 km above the Sekong confluence), and Xekaman (to about 50 km above the Sekong confluence and including the Houay Twai tributary and possibly the Xepian). In the Sekong, the dolphins were reported to range as far upstream as Kalaum Town, Laos, about 280 km above the Mekong confluence near Stung Treng, Cambodia (Baird and Mounsouphom, 1997). Based on interviews conducted by Baird (1997) and Beasley et al. (2003), dolphins probably now only rarely, if ever, ascend these rivers. During interview surveys downstream from Kratie to Phnom Penh, children were unaware of the existence of dolphins, whereas adults reported that before 1975, dolphins were observed every day during both low and high water seasons (Isabel Beasley, pers. comm.).
Based on visual surveys conducted by Beasley et al. (2003), dolphins are frequently found during the low-water season in nine deep areas in the Kratie to Khone Falls segment. Approximately 2 km below the falls, dolphins regularly occur in a small (ca. 600 m diameter), deep (> 50 m during the high-water season) pool, known locally in Laos as Boong Pa Gooang and in Cambodia as Anlong Chiteal. Dolphins were observed daily in the pool during the dry seasons of 1992-93, generally in groups of 2?10; 17 were seen at least once (Baird et al. 1994). Using visual and acoustic methods, Borsani (1999) estimated that there were 8?10 dolphins present in Boong Pa Gooang in late March/early April 1998. Other pools occupied by dolphins in the Kratie to Khone Falls segment are at Koh Suntuk, Kang Kohn Sat and Tbong Klar in the Stung Treng Province, and Sampan, Khasak Makak, Gopidau, Chroy Bantey and Kampi in the Kratie Province. Kampi pool, located 15 km north of Kratie, is currently considered the most important dolphin habitat in the Mekong, due to the 100% reliability of sightings, as recorded during 165 visits to the pool over three years, and the relatively large number of animals observed during each visit (mean group size = 7, range = 1?19) (Beasley et al. 2003, Beasley, unpublished).
Dolphins previously inhabited Tonle Sap (Great Lake) (Lloze 1973) but apparently have been extirpated there. Fishing is extremely intensive within the lake and in the channel connecting it to the Mekong. Researchers conducting extensive water-bird surveys in the lake from 1999?2003 have not observed any dolphins (Federic Goes, pers. comm.). Moreover, researchers from the Water Utilization Program ? Finnish International Development Aid (WUP-FIN) Tonle Sap Modeling Project visited sampling stations throughout the lake every month from 2001?2003 and never observed dolphins (Juha Sarkkula, pers. comm.).
The only documentation of Irrawaddy dolphins in the Mekong of Viet Nam consists of a few records reported by Lloze (1973), skulls housed in whale temples near the delta and in the mouth of the nearby Dong River (Smith et al. 1997, Beasley et al. 2003) and a single carcass found in a fishing net in the Tien distributary near the Cambodian border in March 2002 (Chung and Ho 2002). During a survey of almost the entire length (224 km) of the two main distributaries of the Mekong, Tien and Hau Giang, in April 1996, Smith et al. (1997) were unable to find a single dolphin.
During March and May 1997, Baird (1997) observed 40 dolphins in the segment of the Mekong from Kratie to the Laos/Cambodia border. He estimated, on the basis of surveys and interviews, that the total population in the Mekong was roughly 100 individuals. Beasley et al. (2003) conducted 11 boat-based direct-count surveys, traveling upstream from Jum Neight (about 30 km downstream of Kratie) to the Laos/Cambodia border during January-May from 2001 to 2003. All navigable channels were surveyed (zigzagging when widths were greater than one km and transiting through the center when less than 1 km). Unsurveyed channels were either too shallow or unsafe to survey due to high-velocity currents, which also meant a low probability of dolphins occurring there; interview surveys of local people living along these channels supported this assumption. The largest number of dolphins observed during an upstream survey conducted at the height of the low-water season in April 2003 was 64. This number was based on the sum of best estimates of group size, with a range of 55?82, according to the sum of low and high estimates of group size, respectively. A slightly different method was used during 2002-2003, traveling downstream in the same river segment but stopping for 10?30 minutes in the nine deep pool areas where dolphins had been observed during previous surveys. The maximum number of animals recorded using the pool-count method was 69 individuals based on the sum of best estimates of group size, with a range of 57?84 individuals based on low and high estimates, during a survey in May 2003. Paired observation experiments, using land-based survey teams and concurrent boat-based observations, were conducted during each of the pool-count surveys in an attempt to assess the proportion of dolphins missed by the boat-based team. This resulted in a 100% match between the two methods in terms of the number of dolphin groups detected in each of the nine pools and very similar group size estimates when experienced observers were present on both teams (Beasley et al. 2003).
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