Bottlenose dolphins’ acoustic communication in an Argentinian aquatic park: influence of temporal and anthropogenic factors

The marine mammal species most kept under human care is the bottlenose dolphin (Tursiops truncatus). Dolphins’ life in captivity is scheduled through daily training sessions and open shows to the public, with potential changes to the natural circadian rhythms. The aim of this work is to study the sounds emitted by a group of bottlenose dolphins, held in the ‘Aquarium Dolphin Discovery’ in Mar del Plata (Buenos Aires, Argentina), and analyze their temporal patterns. 253 hours of audio recordings have been collected from 2017 to 2020 in the framework of collaborative projects involving researchers from the Italian National Research Council and the Consejo Nacional de Investigaciones Científicas y Técnicas and University of Mar del Plata (Argentina). I manually screened 11968 files to detect click trains, burst pulses and whistles. Furthermore, click trains have been counted and their parameters are characterized in 9771 files with an automatic software. I examined the signal occurrence in relation to the dial cycle, the year and the aquarium condition (opening with the presence of show, closure and forced closure for pandemic issues) to investigate possible changes due to human care. I used the Mann-Whitney and Kruskal-Wallis tests to compare the number of detected signals and acoustic parameters between the daytime, studied years and aquarium conditions. Moreover, to assess the combined effects of the year, the aquarium condition and the hours of the day, Generalized Additive Models were performed, and the best model was chosen through the AIC value. In addition, I studied the signature whistles through the SIGID method (Janik et al., 2013) in order to blind identify the signal of the adults present in the pools. Results suggested that bottlenose dolphins emitted more click trains in the early morning and in the evening, while burst pulses and whistles were emitted all along the 24 hours. Furthermore, click trains parameters changed in relation to the year, the time slot, the hour and the aquarium condition. However, since the values of the explanatory variables of the best fitting GAMs were low, other factors could contribute to this variability. These outcomes reveal that signals used for communication, such as whistles and burst pulses, constantly occurred with little effect related to the aquarium condition, while click trains probably serve more functions and are affected by several factors, possibly linked to the captivity. This study can be used as a baseline for assessing and monitoring the possible effect of human care and shows on dolphins and evaluating dolphins’ welfare.