Two new whale calls in the southern Indian Ocean, and their geographic and seasonal distribution over five sites and seven years

Abstract

Since passive acoustic is widely used to monitor cetaceans, unidentified signals from biological sources are commonly reported. The signal's characteristics and emission patterns could give keys to identify the possible sources. Here, we report two previously unidentified signals found in acoustic records from five widely spread sites in the southern Indian Ocean and spanning seven years (2007, 2010 to 2015). The first reported signal (M-call) consists of a single tonal unit near 22 Hz and lasting about 10 s. The second signal (P-call) is also a tonal unit lasting about 10 s, but at a frequency near 27 Hz. The latter has often been interpreted as an incomplete Antarctic blue whale Z-call (Balaenoptera musculus intermedia). From a systematic analysis of our acoustic database, we show that both signals have similar characteristics as blue whale vocalizations, but with spatial and seasonal patterns that do not resemble any of the known populations dwelling in the southern Indian Ocean. M-calls are recorded only in 2007, while P-calls are present every recording year, with an increasing abundance with time. P-calls may co-occur with but are clearly distinct from Z-calls. The sources of the two new calls have yet to be visually identified.