Long-term variations in calls produced by a potential blue whale species from the central Indian Ocean

Abstract

The Comprehensive Nuclear-Test-Ban Treaty Organization hydrophones at Diego Garcia record many marine mammal sounds from the central Indian Ocean. A peculiar call between 20 and 40 Hz from a potential blue whale species, referred to as the Diego Garcia Downsweep (DGD), is regularly recorded in the dataset. The call consists of a set of tones that resemble a comb, followed by a downsweep. Long term spectral averages suggest that the call frequencies are drifting. Tracking the change in frequencies by mere visual observations from the spectrograms is however challenging because of interference from anthropogenic noises and other biophony. This talk presents signal processing methods to track the call-frequencies, across years 2002 to 2019. Energy detectors are initially used to identify, and then exclude acoustic interference from shipping and outliers from electronics. Following that, to isolate the DGDs, and cancel other biophony, the presentation constructs subspace detectors. Finally, the talk uses the spectrograms of the detections to track the change in frequencies for each day. The observations show that while the frequencies of the comb steadily increase, the downsweep decrease. There are new frequencies that appear, and some disappear. These changes are unlike behavior observed from other whales in the region.