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Markedly reduced genetic variation and population collapse of the Irrawaddy dolphin (Orcaella brevirostris)

Krützen, K., Ludwig, A., Spencer, P., Beasley, I., Ackermann, C., Parra, G., Bejder, L. and Brownell, B. (2013) Markedly reduced genetic variation and population collapse of the Irrawaddy dolphin (Orcaella brevirostris). In: 20th Biennial Conference on the Biology of Marine Mammals, 9 - 13 December, Dunedin, New Zealand.


Irrawaddy dolphins Orcaella brevirostris are small cetaceans distributed in coastal, lake and riverine waters in Southeast Asia. Riverine populations include those in the Mekong River in Cambodia and southern Laos. All populations are small (<100 individuals) and listed on the IUCN Red List as Critically Endangered. We carried out the first genetic and demographic assessment of the Mekong River population using 68 individuals sampled between 2000 and 2009. We sequenced part of the hypervariable region I of the mitochondrial DNA (mtDNA) and genotyped 22 autosomal microsatellite loci. Bayesian phylogenetic reconstruction using mtDNA showed reciprocal monophyly for the Mekong Irrawaddy compared to other populations, suggesting a significant and long-lasting isolation of this population. We found four haplotypes and very low overall haplotype and nucleotide diversity (h=0.240,π=0.001) with no indication of significant population structure (φST=0.277). Only 10 out of 22 microsatellite loci genotyped were polymorphic, with few alleles (NE=1.819) and low levels of heterozygosity (HE=0.411). A Bayesian cluster algorithm did not detect any significant population structure. We reconstructed the most probable demographic and genealogical history of the Mekong population using a Markov Chain Monte Carlo simulation and found a strong reduction in effective population size (NE) starting 1MYA. Current NE is reduced by 99.93% compared to the ancestral NE, which is consistent with a founder effect when coastal Irrawaddy dolphins started to inhabit the Mekong River. Based on our mtDNA and microsatellite data, all Mekong Irrawaddy dolphins should be regarded as a single conservation unit. Although we could not detect a dramatic recent decline in population size using our genetic data, probably due to low statistical power, the observed levels of low genetic diversity combined with a high incidence of adult and calf mortality in this population highlight an immediate danger of extinction and the need for urgent conservation actions.

Publication Type: Conference Item
Murdoch Affiliation: School of Veterinary and Life Sciences
Notes: Oral presentation
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