Beluga (Delphinapterus leucas) and narwhal (Monodon monoceros) echolocation click detection and differentiation from long-term Arctic acoustic recordings

Abstract

Beluga (Delphinapterus leucas) and narwhal (Mondon monoceros) echolocation signals have been described in numerous acoustic studies but reported characteristics of their clicks vary across studies. Here, a year of acoustic recordings was collected in the Chukchi Sea where belugas are abundant, and narwhals are not present. A second year was recorded in Eclipse Sound, Nunavut, where beluga sightings are rare and narwhals abundant. The same calibrated hydrophone was used at both locations to facilitate data comparison. Click detection and signal parameter measurements were carried out using a single analysis method. Peak frequency of detected clicks decreased with peak-to-peak received sound pressure level (RL) for both species. High RL beluga clicks (n = 23,946) and narwhal clicks (n = 25,433) had a modal peak frequency of 56 kHz. Lower RL modal peak frequency of beluga clicks (n = 688,601) was 53 kHz and for narwhal clicks (n = 616,536) was 22.5 kHz. Modal inter-click interval (ICI) for beluga clicks (n = 872,336) was 49 ms. Narwhal ICI distribution (n = 791,905) was bimodal and right skewed with modal values of 4 and 144 ms. Clicks of belugas and narwhals are distinguishable by frequency spectra and ICI distribution. These parameters provide a reliable way to discriminate between the monodontid species in large acoustic datasets. Received sound levels substantially influence measured frequency spectra and must be carefully accounted for in acoustic identification of monodontid echolocation. Frequency-dependent acoustic absorption of seawater results in longer propagation and detection distance predictions for narwhal clicks that show greater energy below 30 kHz than found in beluga click spectra.