Mapping delphinid occurrence using a passive acoustic monitoring system towed by an unmanned surface vehicle on the Southeast Coast of Brazil

The use of autonomous passive acoustics is an efficient technique to monitor vocally active marine animals such as dolphins. Coupling autonomous acoustic recorders with unmanned vehicles allows data sampling through varying time scales and an increase in the sample area in relation to fixed recorders. In the present study, an autonomous acoustic recorder was towed by an unmanned surface vehicle, type Wave Glider, in the offshore waters of the Southeast Coast of Brazil through a period of ten days, sampling continuously along a distance of 377 km. Three sound detectors were used to scan all recordings in search of whistles, burst-pulsed sounds and echolocation clicks produced by delphinids. Consecutive detections within an interval shorter than one hour between them were considered as belonging to a detection event, meaning an encounter with a group of delphinids. Intervals larger than one hour between detections were considered a separate event. Detection events were quantified and mapped to obtain the spatial distribution of encounters in the sampling area. There were 99,849 positive detections in all of the sampled days, distributed through 14 detection events. Of all detections, 100,212 were echolocation clicks, 1,029 were whistles, and 202 were burst-pulsed sounds. There was no clear time pattern distribution, but the majority of detections occurred during night hours. Mean local depth of delphinid encounters was of 960 m. The abundance of detections through the entire sampling period indicates that the area around the shelf break is an important area for delphinid occurrence. Furthermore, it was evidenced that autonomous recorders coupled to glider technology can adequately register delphinid presence.