Changes in the behavioural complexity of bottlenose dolphins along a gradient of anthropogenically-impacted environments in South Australian coastal waters: Implications for conservation and management strategies

The susceptibility of bottlenose dolphins (Tursiops sp.) to disturbance within South Australian coastal waters is of particular importance due to both the ever increasing impact of anthropogenic activities on these waters and their semi-enclosed nature. Currently, little is known about the ecology of dolphins in this region, in particular in relation to anthropogenically-driven disturbances. This study investigates the level of stress experienced by bottlenose dolphins from the complexity of their temporal patterns of dive durations recorded along a gradient of environment types defined as a function of the intensity of anthropogenically-driven pollution and disturbances, including urban development and recreational boating. Dive durations were opportunistically recorded from land-based stations scattered across South Australian coastal waters both in the absence of boat traffic, and the potential for boat-related disturbance was investigated when a motorized vessel was within 100m from a traveling individual to infer the effect of indirect exposure to boat disturbance. This approach fundamentally differs from more standard assessments of the behavioural effect of direct exposure to boat disturbance, for instance when dolphins chase fishing vessels, flee from motorboats or bow ride. Subsequent analyses were based on nearly 12,000 behavioural observations. No significant differences were found in dive durations measured in the absence of boats and when boats were present. In contrast, fractal analysis consistently identified significant differences in the complexity of dive duration patterns as a function of environment and exposure to disturbance. Specifically, bottlenose dolphins occurring in environments with less anthropogenic pressure exhibit a higher behavioural complexity. This complexity consistently and significantly decreases both within each environment and between environments with increasing anthropogenic pressure. These results further show that the relative changes in bottlenose dolphins' behavioural complexity increase in environments less impacted by anthropogenic activities. These results are discussed in the general context of the adaptive value of fractal behaviour, the susceptibility of bottlenose dolphins occurring in distinct environments to anthropogenic disturbance, and how behavioural properties identified with our fractal methods can be used to establish baseline information that can be used for the design and implementation of conservation and management strategies.