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PLoS One. 2014 May 7;9(5):e96811. doi: 10.1371/journal.pone.0096811. eCollection 2014.

To see or not to see: investigating detectability of Ganges River dolphins using a combined visual-acoustic survey.

Author information

1
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.
2
School of Environment, Natural Resources and Geography, Bangor University, Gwynedd, United Kingdom.
3
Institute of Zoology, Zoological Society of London, Regent's Park, London, United Kingdom.
4
National Research Institute of Fisheries Engineering, Fisheries Research Agency, Kamisu, Ibaraki, Japan.
5
Department of Zoology, University of Chittagong, Chittagong, Bangladesh; Bangladesh Cetacean Diversity Project, Wildlife Conservation Society, Khulna, Bangladesh.
6
Bangladesh Cetacean Diversity Project, Wildlife Conservation Society, Khulna, Bangladesh.
7
Bangladesh Cetacean Diversity Project, Wildlife Conservation Society, Khulna, Bangladesh; Ocean Giants Program, Wildlife Conservation Society, Bronx, New York, United States of America.

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.

PMID:
24805782
PMCID:
PMC4013050
DOI:
10.1371/journal.pone.0096811
[Indexed for MEDLINE]
Free PMC Article

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