USING PASSIVE ACOUSTICS TO STUDY OMURA’S WHALES OFF THE COAST OF MADAGASCAR

The Omura’s whale (Balaenoptera omurai) is a small rorqual species first described in 2003, and first studied in situ off northwest Madagascar from 2011-2014. Here, we present the first efforts to study the acoustic behavior and spatiotemporal distribution of B. omurai using passive acoustic monitoring. Acoustic recorders were deployed at four sites across 40km of known habitat in the Nosy Be region of Madagascar from October 26 to November 15, 2014. Digital sound spectrograms were manually browsed to characterize call type and temporal patterns at each site. Two distinct call types were identified and attributed to B. omurai: a repetitive, low frequency, pulsive song phrase further categorized as either mono- or bi-syllabic, and a short, non-repetitive downsweep. Presence of song varied across time and sites, with relatively consistent presence across all deployment days at the most northern site, and periods of absence over multiple days at the southern site. These data indicated that B. omurai exhibited movement within and likely beyond the range of the study area, suggesting that the region serves as critical habitat for this potentially non-migratory species. In addition, variation in hourly call presence provides preliminary indication that B. omurai song follows a diel trend, with more calls observed during daylight hours (the reverse of the typical diel trend found in other baleen whales). Collectively, the results of this study provide the first description of B. omurai acoustic behavior and spatiotemporal distribution and will inform ongoing long-term research efforts of this species in NW Madagascar.