Detection probability and density estimation of fin whales by a Seaglider

Abstract

A single-hydrophone ocean glider was deployed within a cabled hydrophone array to demonstrate a framework for estimating population density of fin whales ( Balaenoptera physalus) from a passive acoustic glider. The array was used to estimate tracks of acoustically active whales. These tracks became detection trials to model the detection function for glider-recorded 360-s windows containing fin whale 20-Hz pulses using a generalized additive model. Detection probability was dependent on both horizontal distance and low-frequency glider flow noise. At the median 40-Hz spectral level of 97 dB re 1 μPa2/Hz, detection probability was near one at horizontal distance zero with an effective detection radius of 17.1 km [coefficient of variation (CV) = 0.13]. Using estimates of acoustic availability and acoustically active group size from tagged and tracked fin whales, respectively, density of fin whales was estimated as 1.8 whales per 1000 km2 (CV = 0.55). A plot sampling density estimate for the same area and time, estimated from array data alone, was 1.3 whales per 1000 km2 (CV = 0.51). While the presented density estimates are from a small demonstration experiment and should be used with caution, the framework presented here advances our understanding of the potential use of gliders for cetacean density estimation.

Gliders for passive acoustic monitoring of the oceanic environment

“…For application to physical oceanography, PAM gliders provide information about the surface weather conditions (Cauchy et al, 2018;Cazau et al, 2019) co-located with collection of hydrographic profiles, allowing promising applications for storm monitoring and quantification of air-sea interaction processes such as wind-driven mixing or air-sea gas exchange. For bioacoustics, PAM glider observations are well adapted to long term population monitoring (Verfuss et al, 2019;Cauchy et al, 2020) and have the potential to provide density estimations (Fregosi et al, 2022). Onboard processing with real-time transmission of animal detection has reached operational level, used by Canadian and United States authorities to trigger slowdowns of marine traffic when North Atlantic right whales are detected (robots4whales.whoi.edu).…”

“…Such an assumption is largely violated in the case of a PAM glider observing mobile marine species, which could introduce an overestimation bias. Optimized statistical approaches need to be developed to extend standard density estimation methods to PAM glider surveys (Marques et al, 2013;Fregosi et al, 2022).…”