Mapping cetacean sounds using a passive acoustic monitoring system towed by an autonomous Wave Glider in the Southwestern Atlantic Ocean

Abstract Passive acoustic monitoring techniques provide a useful alternative to visual surveys for monitoring of marine mammals since they are less affected by adverse weather conditions. An autonomous unmanned Wave Glider, equipped with a towed hydrophone and recording system was used to collect acoustic data in the Brazilian offshore waters of the Southwestern Atlantic Ocean between 6 and 25 February 2016. Nearly continuous data were obtained during this period (light and dark hours) for the 750 km traveled. There were 165,130 high-frequency detections in 31 encounters, including 1789 whistles, 389 burst-pulse sounds, and 162,952 echolocation clicks. A total of 705 low-frequency tonal signals were recorded in 5 encounters. Although high-frequency detections occurred at all hours of the day, the majority occurred during dark hours. Low-frequency detections were not evenly spread through all hours of the day, with the majority of them occurring during dark hours. High-frequency and low-frequency detections represented 26% and 3% of the recording hours, respectively. Duration, the number of emissions, and ocean depth varied among acoustic encounters. Encounters composed by high-frequency sounds were separated into seven different groups, probably different species. One of the encounters presented whistles that were previously recorded in the Rio de Janeiro Coast for Steno bredanensis (Rough-toothed dolphins). All of the low-frequency encounters were composed of a type of Balaenoptera brydei (Bryde's whales) call. This type of data collection is un-precedented for the Southwestern Atlantic Ocean and highlighted the use of the sampled area by delphinids on different days and different times of the day, including the dark hours.