Are they really not there? Using passive acoustics to overcome false absences in the study of vocal species that are rare, secretive, or distributed at low densities.

Abstract

Estimating abundance and spatial use behavior can be challenging for marine species that are rarely sighted. This situation is exacerbated in the polar regions due to the peculiar logistical difficulties of working in the pack ice, which makes survey effort enormously expensive. Presented is a simple approach for modeling sounds per animal over a unit time as a relative index for species where there is information on the production of vocalizations (acoustic behavior including seasonal calling patterns, diurnal calling patterns, inter‐individual stereotypy, inter‐sexual stereotypy, audience effect, and predictable calling rate over a unit of time) and the detection range of those vocalizations (survey distance—theoretical estimates calculated with call intensities). We focus on an Antarctic pack ice seal, the leopard seal, and Hydrurga leptonyx, as estimating abundance from survey effort faces challenges. Our case study shows that with the advent of more sophisticated marine engineering coupled with effort focused on the pertinent parameters of acoustic behavioral ecology we can open up the scope to study the behavior of rare, secretive, or low‐density species across a range of site occupancy studies, density, and habitat use at a time when we need to develop cost efficient tools to glean an understanding of what is happening in these most vulnerable locations.