The behavior of sotalia guianesis during boat traffic in Sepetiba Bay, Southeastern Brazil

Abstract

Sepetiba Bay has an industrial and harbor district, where dredging processes were developed in order deepen the channel. The bay has depths between two and twenty-five meters, where several species inhabit, including the estuarine dolphin (Sotalia guianensis), a threatened species, according to Brazilian environmental agency. Bioacoustics consists in the study of sounds emitted by animals, which represents signs of communication and therefore plays a fundamental role in species behavior, as used by the estuarine dolphin. In order to capture and record vocalizations in presence of vessels and search for any anthropic interference in their communication in this region, the monitoring occurred from December 2015 to December 2016, with a fifteen-day interval between field work. Underwater noises were obtained with three recording systems in fixed buoys along the channel spaced approximately one kilometer. Recorders and hydrophones were installed in buoys and in each field work four-hour recordings were made, totalizing 269h36m of sample effort. While records were made, sightings were made in order to identify, count and visualize behavior of dolphins. Data were processed and analyzed with Raven Pro 1.4. After one year of monitoring period, it was possible to analyze the acoustic intensity in the environment in presence of private boats and twenty cargo ships. Also the dolphin behavior was analyzed in presence and absence of such boats and coexistence. Noises caused by boats and ships navigating in the channel were 90 dB (± 4,44), reaching 100 dB in isolated cases. 104 dolphins were sighted and communication sounds were all in different frequencies from vessels. The acoustic data show an environment with great intensity of anthropogenic noises, manly, low frequency ones. It also shows a region of intense sound disturbance due to currents and tides, where sound intensity often reaches 60 dB only with ambient noise. The presence of estuarine dolphins with hunting and navigating behaviors demonstrates the, even in regions with greater acoustic disturbance, they keep their communication and behavior unaltered by noise.