Acoustic differentiation and classification of wild belugas and narwhals using echolocation clicks

Marie J Zahn,Marianne H Rasmussen,Kristin L Laidre,Frederick Archer,Jennifer L K Mccullough,Shannon Rankin,Jens C Koblitz

doi:10.1038/s41598-021-01441-w

Abstract

Belugas (Delphinapterus leucas) and narwhals (Monodon monoceros) are highly social Arctic toothed whales with large vocal repertoires and similar acoustic profiles. Passive Acoustic Monitoring (PAM) that uses multiple hydrophones over large spatiotemporal scales has been a primary method to study their populations, particularly in response to rapid climate change and increasing underwater noise. This study marks the first acoustic comparison between wild belugas and narwhals from the same location and reveals that they can be acoustically differentiated and classified solely by echolocation clicks. Acoustic recordings were made in the pack ice of Baffin Bay, West Greenland, during 2013. Multivariate analyses and Random Forests classification models were applied to eighty-one single-species acoustic events comprised of numerous echolocation clicks. Results demonstrate a significant difference between species’ acoustic parameters where beluga echolocation was distinguished by higher frequency content, evidenced by higher peak frequencies, center frequencies, and frequency minimums and maximums. Spectral peaks, troughs, and center frequencies for beluga clicks were generally > 60 kHz and narwhal clicks < 60 kHz with overlap between 40–60 kHz. Classification model predictive performance was strong with an overall correct classification rate of 97.5% for the best model. The most important predictors for species assignment were defined by peaks and notches in frequency spectra. Our results provide strong support for the use of echolocation in PAM efforts to differentiate belugas and narwhals acoustically.

Highlights

Belugas (Delphinapterus leucas) and narwhals (Monodon monoceros) are highly social Arctic toothed whales with large vocal repertoires and similar acoustic profiles
Passive Acoustic Monitoring (PAM) relies on the ability to distinguish between species solely based on their characteristic sounds, which means individual or a combination of specific sonic identifiers must be known for each species
Echolocation, sometimes referred to as biosonar, is characterized by the emission of high frequency, relatively broadband clicks of high directionality and listening for returning echoes; it is a dominant sense for odontocetes, much like vision is for humans[1,2]

Summary

Introduction

Belugas (Delphinapterus leucas) and narwhals (Monodon monoceros) are highly social Arctic toothed whales with large vocal repertoires and similar acoustic profiles. Passive Acoustic Monitoring (PAM) that uses multiple hydrophones over large spatiotemporal scales has been a primary method to study their populations, in response to rapid climate change and increasing underwater noise. When whistles and buzzes were absent, they used an increase in spectral power around the 20 kHz frequency band to identify narwhal mid-frequency c licks[17] Their comparison was narrow in scope as it was not their primary research objective. Narwhal echolocation has been characterized by clicks with frequency maxima between 30–70 kHz25,30,43,44, with some as low as 2–10 kHz and 7–14 kHz32 Still, among these studies, differences in study design, sampling equipment, and location make direct comparisons between beluga and narwhal click characteristics challenging. Underwater anthropogenic sounds from seismic surveys, drilling and oil production, military sonar, and motorized vessels can substantially disturb the acoustic environment which they depend o n53,55–59

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Conclusion