Estimating density from passive acoustics: Are we there yet?

Abstract

In the last few years, there have been a considerable number of papers describing methods or case studies involving passive acoustic density estimation. While this might be interpreted as evidence that density estimates might now be easily and routinely implemented, the truth is that so far these methods and applications have been essentially proof-of-concept in nature, based in areas and/or species particularly suited for the methods and also often involved assumptions hard to evaluate. We briefly review some of the existing work in this area concentrating on a few aspects we believe are key for the implementation of density estimation from passive acoustics in a broader context. These are (1) the development of fundamental research addressing the problem of sound production rate, fundamental as it allows to convert estimates of density of sounds into density of animals and (2) the development of hardware capable of providing cheap deployable units capable of ranging, allowing straightforward implementations of distance sampling based approaches. The perfect density estimate is out there waiting to happen, but we have not found it yet.