Effects of vessel noise on underwater vocalizations of bottlenose dolphins, Tursiops truncatus, in the Sado Estuary

• Main Author: Sobreira, Filipa Alexandra Veiga
• Publication Date: 2017
• Format: Master thesis
• Language: eng
• Source: Repositórios Científicos de Acesso Aberto de Portugal (RCAAP)
• Download full: http://hdl.handle.net/10773/21964
• topic: Golfinhos - Rio Sado (Portugal); Percepção auditiva; Barcos - Ruído

Maritime traffic is an important source of disturbance for coastal cetaceans, especially for local and resident populations, like the bottlenose dolphins (Tursiops truncatus) population in the Sado estuary. Vessel noise might mask important signals such as communication calls. To compensate masking effects, animals may change their vocal behavior by shifting vocal rate, call intensity, call type, call frequency and duration. To evaluate the potential impacts on the acoustic behavior of this population, abundance and acoustic characteristics of whistles, echolocation signals and burst-pulsed sounds were analyzed in relation to boat traffic. The samples used were obtained in field recordings of dolphin vocalizations made from March 2014 to April 2017. Boat traffic operating within a 1000 m radius was listed as absent or present. Vocal elements were classified according to visual graphical and aural characteristics in: whistles, slow-click trains, short-burst pulses, creaks, squawks, variable rate click trains, bangs, gulps, squeaks and grunts. Analysis of emission rates was based on the number of recognizable units per minute for all vocal elements. In the presence of vessels, differences in call rates were not significant for all types of vocal elements. For selected vocal elements, different acoustic parameters were examined, using a nonparametric MANOVA, and modifications between vessel presence and absence were found for the following vocal elements: whistles (X2 (7) = 56.42; N = 620; p < 0.001), creaks (X2 (8) = 19.53; N = 94; p = 0.012), grunts (X2 (8) = 80.968; N = 339; p < 0.001), gulps (X2 (7) = 58.76; N = 260; p < 0.001) and squeaks (X2 (10) = 25.894; N = 121; p = 0.004)). These results show modifications in acoustic behavior in the presence of vessels, suggesting that bottlenose dolphins in this population might adjust their vocal frequencies and produce shorter signals to maintain communication. This study shows that although resident bottlenose dolphins in Sado estuary seem to display some tolerance to the noise generated from boats in their habitat, it probably causes significant changes in their communication behaviors.