Towards automatic detection and classification of orca (Orcinus orca) calls using cross-correlation methods

TL;DR

This study explores the use of cross-correlation methods for automatic detection and classification of orca calls, demonstrating potential and limitations in analyzing complex vocalizations for population monitoring.

Contribution

It introduces the application of cross-correlation techniques using the R package warbleR to orca sound analysis, a novel approach in this field.

Findings

Pearson cross-correlation was efficient for >85% of pairwise calculations.

One sound type matched Icelandic catalogue with high correlation (0.62-0.67).

Automatic classification faced challenges due to background noise and sound complexity.

Abstract

Orca (Orcinus orca) is known for complex vocalisation. Their social structure consists of pods and clans sharing unique dialects due to geographic isolation. Sound type repertoires are fundamental for monitoring orca populations and are typically created visually and aurally. An orca pod occurring in the Ligurian Sea (Pelagos Sanctuary) in December 2019 provided a unique occasion for long-term recordings. The numerous data collected with the bottom recorder were analysed with a traditional human-driven inspection to create a repertoire of this pod and to compare it to catalogues from different orca populations (Icelandic and Antarctic) investigating its origins. Automatic signal detection and cross-correlation methods (R package warbleR) were used for the first time in orca studies. We found the Pearson cross-correlation method to be efficient for most pairwise calculations (> 85%) but with false positives. One sound type from our repertoire presented a high positive match (range 0.62-0.67) with one from the Icelandic catalogue, which was confirmed visually and aurally. Our first attempt to automatically classify orca sound types presented limitations due to background noise and sound complexity of orca communication. We show cross-correlation methods can be a powerful tool for sound type classification in combination with conventional methods.