Illustrating passive acoustic monitoring's potential to characterize beaked whale foraging ecology: A workflow for estimating deep dive duration

Passive acoustics offers a lens into the foraging ecology of elusive, deep-diving beaked whales. As they dive—sometimes to nearly 3000 m—they echolocate continuously in pursuit of prey. Although 24 beaked whale species are recognized, only five have been tagged, including goose-beaked whale (Ziphius cavirostris) and Blainville’s beaked whale (Mesoplodon densirostris). Tagging studies reveal that each deep dive features a “vocal period,” beginning several hundred meters into descent and continuing throughout prey search at depth. Extended echolocation encounters can thus be functionally coupled with foraging effort. Passive acoustics often captures only a fraction of vocal periods due to click directionality, but offsets this limitation with extensive observational coverage, increasing the likelihood of recording near full-length vocal periods. In this study, we leverage this observational power by focusing on upper percentile echolocation encounter durations, as annotated in long-term spectral averages (LTSAs). We identify the percentiles that best match vocal period tag measurements and apply tag-derived ratios between vocal period and dive duration to (1) validate PAM-based dive duration estimates for previously tagged species and (2) extrapolate PAM-based dive durations for hitherto untagged species (i.e., Hubb’s, Stejneger’s, and Gervais’ beaked whales) currently categorized as Data Deficient on the IUCN Red List of Threatened Species.