Irrawaddy dolphin demography in the Mekong River: an application of mark–resight models


  • Gerard Edward Ryan,

    Corresponding author
    1. WWF Cambodia, 21, Street 322, Boueng Keng Kang I, Phnom Penh, Cambodia
    2. Office for Environmental Programs. The University of Melbourne, Parkville, Victoria 3010 Australia
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  • Verné Dove,

    1. Faculty of Veterinary and Biomedical Science. Murdoch University, Murdoch, Western Australia 6150 Australia
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    • Present addresses: Sea Shepherd Conservation Society Australia, Suite 7, 288 Brunswick Street, Fitzroy, Victoria 3065, Australia, and IUCN/SSC Conservation Breeding Specialist Group, 12101 Johnny Cake Ridge Road, Apple Valley, Minnesota 55124 USA.

  • Fernando Trujillo,

    1. Foundation Omacha, Calle 86A-23-38, Bogotá, Colombia
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  • Paul F. Doherty

    1. Department of Fish, Wildlife, and Conservation Biology. Colorado State University, Fort Collins, Colorado 80512 USA
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  • Corresponding Editor: T. O'Brien.


Riverine Irrawaddy dolphin populations are critically endangered and much uncertainty exists over the population status in the Mekong River of northeast Cambodia and southern Lao People's Democratic Republic (Lao PDR). We conducted 11 surveys over three years to estimate abundance at each survey as well as survival and the probability of individuals becoming unavailable for detection between surveys. We utilized novel mark–resight estimators to account for the detection process in estimating these parameters. Annual survival was 0.977 (0.040 SE) and movement in (0.060) and out (0.018) of an observable state was low. We estimated abundance at 84.5 (95% CI = 77.9–91.2) with little change over our surveys. We also estimated recruitment and population growth rate for the marked, and presumably older, individuals by estimating seniority using a reverse-time model. Seniority was estimated at 0.999 (0.028 SE), recruitment at 0.001 and population growth rate at 0.978. Although the population size appears to be stable, we believe this represents the slow disappearance of a long-lived animal with no recruitment. Along with the isolated nature of the population and reduced population size as compared to historical estimates, we believe this population is in serious threat of extirpation. We believe there may be as few as 7 or 8 animals in Lao PDR and that the species is at risk of extinction there in the short-term. Although recent management actions (e.g., outlawing of explosive fishing and some restriction on the use of gill-nets) have likely been beneficial we believe identifying population goals to work towards, identifying additional management actions to improve recruitment, and designing the survey methods to best estimate the success of these actions is needed.