Facilitating the conservation and management of Hawaiian odontocetes using an echolocation click type ‘library’

Passive acoustic monitoring is a valuable method for studying marine mammals because it is inobtrusive and can record continuous, year-round data. Echolocation clicks produced by odontocetes (toothed whales) are one of several vocalizations used in passive acoustic monitoring to study, manage, and conserve species of interest. However, species-specific acoustic features of clicks are often subtle and may vary dependent on an animal’s position relative to the recording device or its behavioral state, making species-level classification difficult. Using unsupervised machine learning, regional sets of echolocation click types can be identified that allow for automated labeling of bioacoustic data while accounting for within-type signal variability. This methodology derives labels for clusters of click types based on parameters including spectral shape, inter-click interval, and click duration.

Here we present classification results of echolocation clicks produced by Hawaiian odontocetes. Unsupervised methods were used to establish a set of click types for 10 years of recordings (200 kHz) collected from a bottom-mounted hydrophone off the coast of Kona, Hawaii. These automated labels were compared with manual labels for species with established and recognizable clicks, which allowed for ground-truthing of some click types identified by the unsupervised machine learning method. Towed array data (500 kHz) was used to assign click types to species with visual verification, expanding the set of known species-specific click types for the region. Particular focus was put on establishing a click type for the false killer whale, Pseudorca crassidens, which is a focus of current conservation and management interest due to the endangered status of the endemic Main Hawaiian Islands (MHI) insular stock of false killer whales. The click types and method established here will be useful in future analyses of density estimation, abundance, and distribution patterns of not only this species but also all other identified odontocetes, which is of interest to managers in the region.