Long-term frequency changes of a potential baleen whale call from the central Indian Ocean during 2002-2019.

The multiple baleen whales of the central Indian Ocean use distinct calls, with their acoustic signatures marking their respective geographic distributions. This paper uses observations from Diego Garcia to track long-term changes (2002-2019) in calls produced by an unidentified whale. The calls around 20-45 Hz consist of closely spaced frequency tones that resemble a comb, followed by a downsweep. The observations show that while the average comb frequencies steadily increase, the downsweep portion decreases. Some frequencies disappear, while new ones appear. These frequency-observations are different from similar studies of other baleen whales in the region, which mostly show a decrease.