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  • N. Richman
  • J.M. Gibbons
  • S.T. Turvey
    Zoological Society of London
  • T. Akamatsu
    National Research Institute of Fisheries Engineering, Fisheries Research Agency, Kamisu, Ibaraki, Japan
  • Benazir Ahmed
    University of Chittagong
  • E. Mahabub
    Bangladesh Cetacean Diversity Project, Wildlife Conservation Society
  • B.D. Smith
    Bangladesh Cetacean Diversity Project, Wildlife Conservation Society
  • Julia P.G. Jones
Detection of animals during visual surveys is rarely perfect or constant, and failure to account for imperfect detectability affects the accuracy of abundance estimates. Freshwater cetaceans are among the most threatened group of mammals, and visual surveys are a commonly employed method for estimating population size despite concerns over imperfect and unquantified detectability. We used a combined visual-acoustic survey to estimate detectability of Ganges River dolphins (Platanista gangetica gangetica) in four waterways of southern Bangladesh. The combined visual-acoustic survey resulted in consistently higher detectability than a single observer-team visual survey, thereby improving power to detect trends. Visual detectability was particularly low for dolphins close to meanders where these habitat features temporarily block the view of the preceding river surface. This systematic bias in detectability during visual-only surveys may lead researchers to underestimate the importance of heavily meandering river reaches. Although the benefits of acoustic surveys are increasingly recognised for marine cetaceans, they have not been widely used for monitoring abundance of freshwater cetaceans due to perceived costs and technical skill requirements. We show that acoustic surveys are in fact a relatively cost-effective approach for surveying freshwater cetaceans, once it is acknowledged that methods that do not account for imperfect detectability are of limited value for monitoring.
Original languageEnglish
Pages (from-to)e96811
JournalPLoS ONE
Issue number5
Publication statusPublished - 7 May 2014

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