Automated detection of sperm whales (Physeter macrocephalus) creaks and association with depredation events.

Abstract

Sperm whales have been depredating black cod (Anoplopoma fimbria) from demersal longlines in the Gulf of Alaska for decades, but the behavior has now become pervasive enough that it is starting to affect government estimates of the sustainable catch, motivating further studies of this behavior. In July 2007 and June 2009, bioacoustic tags were attached to adult sperm whales off Southeast Alaska under both natural foraging conditions and situations wherein the animals were depredating sablefish from commercial longlining vessels. Tags remained attached for a cumulative hours, yielding more than 100 h of depth, orientation, and acoustic data. Results show that when depredating, sperm whales often dive at depths shallower than 50 m, compared to natural foraging depths of 300–400 m in the area. Dive durations, pitch, and roll rates are also distinctively different. Techniques such as spectrogram cross-correlation and cepstral analysis are used to automatically detect creak events, which have lower intensities and narrower frequency ranges than conventional clicks. Acoustic measurements such as creak production rate, duration, and intensity are then extracted. Correlations are derived between creak events, sudden changes in body orientation and depredation rate, with a goal of determining an acoustic measure of depredation activity and efficacy. [Work conducted under the SEA SWAP Program, supported by the North Pacific Research Board and the National Geographic Society.]