IUCN Red List Assessment
Red List Category
Red List Criteria
1) the low probability that dolphins occur below Kratie and in the Sekong River during the low-water season, and in Tonle Sap (Great Lake) and its connecting channel at any time, as indicated by interview surveys and the observations by researchers conducting water-bird surveys and hydrological investigations;
2) the nearly comprehensive search coverage of navigable channels during vessel-based surveys; and
3) the 100% match in concurrent detections of dolphin groups and close agreement in group size estimates by land-based and boat-based observers.
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 Mekong dolphins, this implies that it would be appropriate to use 57, the sum of minimum estimates of group size from the May 2003 pool-count survey, as the estimate of abundance for this subpopulation. The threshold of 50 mature individuals for listing a species or population 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 Mekong River is less than 50. Therefore, the subpopulation qualifies for listing as CR based on Criterion D. The subpopulation also qualifies as CR on the basis of Criterion C2a (i,ii) as it certainly has fewer than 250 mature individuals, a continuing decline in the number of mature individuals is projected or inferred based on the continuing known threat of gillnet entanglement and potential threats from water development and navigation improvement projects, 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).
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