title: To see or not to see: investigating detectability of Ganges River dolphins using a combined visual-acoustic survey.
abstract: 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.
authors.affiliation: Institute of Zoology, Zoological Society of London, Regent's Park, London, United Kingdom; School of Environment, Natural Resources and Geography, Bangor University, Gwynedd, United Kingdom.
authors.forename: Nadia I
authors.initials: NI
authors.lastname: Richman
authors.affiliation: School of Environment, Natural Resources and Geography, Bangor University, Gwynedd, United Kingdom.
authors.forename: James M
authors.initials: JM
authors.lastname: Gibbons
authors.affiliation: Institute of Zoology, Zoological Society of London, Regent's Park, London, United Kingdom.
authors.forename: Samuel T
authors.initials: ST
authors.lastname: Turvey
authors.affiliation: National Research Institute of Fisheries Engineering, Fisheries Research Agency, Kamisu, Ibaraki, Japan.
authors.forename: Tomonari
authors.initials: T
authors.lastname: Akamatsu
authors.affiliation: Department of Zoology, University of Chittagong, Chittagong, Bangladesh; Bangladesh Cetacean Diversity Project, Wildlife Conservation Society, Khulna, Bangladesh.
authors.forename: Benazir
authors.initials: B
authors.lastname: Ahmed
authors.affiliation: Bangladesh Cetacean Diversity Project, Wildlife Conservation Society, Khulna, Bangladesh.
authors.forename: Emile
authors.initials: E
authors.lastname: Mahabub
authors.affiliation: Bangladesh Cetacean Diversity Project, Wildlife Conservation Society, Khulna, Bangladesh; Ocean Giants Program, Wildlife Conservation Society, Bronx, New York, United States of America.
authors.forename: Brian D
authors.initials: BD
authors.lastname: Smith
authors.affiliation: School of Environment, Natural Resources and Geography, Bangor University, Gwynedd, United Kingdom.
authors.forename: Julia P G
authors.initials: JP
authors.lastname: Jones
mesh_terms: D000162:Acoustics; D000818:Animals; D001459:Bangladesh; D004289:Dolphins / Q000235:genetics* / Q000254:growth & development / Q000502:physiology*; D005618:Fresh Water; D011156:Population Density; D045483:Rivers