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.

The Endangered Ganges River dolphin Platanista gangetica gangetica in Nepal: abundance, habitat and conservation threats

“…We followed the survey methods recommended by Smith & Reeves (2000b) for narrow channel habitat. It was assumed that allowing a dolphin to surface at least once within our range of detection would avoid double counting, which was achieved by maintaining a boat speed of 5 km h −1 (Kelkar et al 2010, Richman et al 2014). Teams typically consisted of 6 individuals, representing researchers, park officials and NGO members on a single platform (eye height 1.5 m), with 2 primary observers and 1 rear observer.…”

Passive acoustic monitoring of coastally associated Hawaiian spinner dolphins, Stenella longirostris, ground-truthed through visual surveys

“…Bailey et al (2010) compared visual surveys with acoustic surveys for harbor porpoise and bottlenose dolphins for one to five hours per day over a period of six months. Richman et al (2014) compared acoustic and visual survey methods for Ganges River dolphins (Platanista gangetica gangetica) over a period of two months. Additionally, when compared to previous visual surveys for Hawaiian spinner dolphins, the coverage in this study is far greater.…”

“…Other studies comparing visual and acoustic survey methods for cetaceans exist in the literature (Akamatsu et al, 2001;Wang et al, 2005;Oleson et al, 2007;Kimura et al, 2009;Bailey et al, 2010;Richman et al, 2014). However, none have compared visual and acoustic survey methods for Hawaiian spinner dolphins, the focus of this study.…”

Toward a Harmonized Approach for Monitoring of Riverine Floating Macro Litter Inputs to the Marine Environment

“…The methodological approach considers regular short monitoring sessions (30-60 min) in the estuarine section of the river to account for inputs to the sea, using the tablet computer application described in Section The JRC Floating Litter Monitoring Application to perform visual observations from an elevated position (e.g., bridges, piers, pontoons, and others). As in visual observations in other research fields, e.g., cetaceans (Harwood and Joynt, 2009;Richman et al, 2014), marine birds (Titmus and David Hyrenbach, 2011) or jellyfish (Doyle et al, 2007), duration of monitoring sessions is limited in order to avoid observer's fatigue (EC JRC, 2013; Suaria and Aliani, 2014). Observation of the water from above allows an improved view into the surface water layer for identification of floating litter items.…”