Integrating passive acoustic and visual surveys for marine mammals in coastal habitats

Dual-platform studies in confined marine habitats can contribute to the calibration of passive acoustic data for purposes of density estimation. We established a long baseline hydrophone array in northern coastal British Columbia, Canada, that consists of four synchronized, bottom-mounted, continuously recording hydrophones. Automated detectors have been developed for vocalizations of humpback whales, orcas, and fin whales. Visual surveys are conducted from an observation platform overseeing the same area of approximately 200 sq. km. Here we compare humpback whale detections between the visual and acoustic platforms for 78 days of surveys in 2018. Two call types were analyzed: bubble net feeding calls and miscellaneous other. Acoustic surveys yielded higher detection rates (15x) of bubble net feeding groups than visual surveys, but visual detection rates of humpback whales engaged in other behaviors were twice that of acoustic surveys. When summarizing data into 24-hour periods, we found strong correlations between visual and acoustic detection rates for both call types. These correlations were strongest when all visual detections were included, and weakest when we excluded distant sightings from our dataset. These preliminary results are an encouraging first step in the derivation of call rates and the estimation of local species densities using passive acoustics.