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.
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