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.
Highlights
Estimates of abundance, trends over time, and distribution are all important for conservation management of threatened species [1,2,3]
The use of methods typically used for monitoring marine cetaceans is largely precluded for freshwater cetaceans due to constraints arising from survey conditions in river systems, and from differences in freshwater cetacean morphology and surfacing behaviour [6]
Distance sampling using a visual line transect is commonly used to survey marine cetacean species including Sperm whales (Physeter macrocephalus) [7], Killer whales (Orcinus orca) [8], and Vaquita (Phocoena sinus) [9]. This method has been attempted with freshwater cetaceans, e.g. Ganges River dolphins (Platanista gangetica gangetica) [10], Yangtze Finless porpoises (Neophocaena asiaeorientalis asiaeorientalis) [11], and Amazon River dolphins (Inia geoffrensis) [12]) (Table 1)
Summary
Trends over time, and distribution are all important for conservation management of threatened species [1,2,3]. Distance sampling using a visual line transect is commonly used to survey marine cetacean species including Sperm whales (Physeter macrocephalus) [7], Killer whales (Orcinus orca) [8], and Vaquita (Phocoena sinus) [9]. This method has been attempted with freshwater cetaceans, e.g. Ganges River dolphins (Platanista gangetica gangetica) [10], Yangtze Finless porpoises (Neophocaena asiaeorientalis asiaeorientalis) [11], and Amazon River dolphins (Inia geoffrensis) [12]) (Table 1). The exceptionally small dorsal fin (or lack of one altogether in finless porpoises) and rapid surfacing behaviour of other freshwater cetacean species limits the feasibility of photoidentification, making mark-recapture generally impractical [6]
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