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Anthropogenic noise can alter marine mammal behaviour and physiology, but little is known about cetacean cardiovascular responses to exposures, despite evidence that acoustic stressors, such as naval sonars, may lead to decompression sickness. Here, we measured heart rate and movements of two trained harbour porpoises during controlled exposure to 6–9 kHz sonar-like sweeps and 40 kHz peak-frequency noise pulses, designed to evoke acoustic startle responses. The porpoises initially responded to the sonar sweep with intensified bradycardia despite unaltered behaviour/movement, but habituated rapidly to the stimuli. In contrast, 40 kHz noise pulses consistently evoked rapid muscle flinches (indicative of startles), but no behavioural or heart rate changes. We conclude that the autonomous startle response appears decoupled from, or overridden by, cardiac regulation in diving porpoises, whereas certain novel stimuli may motivate oxygen-conserving cardiovascular measures. Such responses to sound exposure may contribute to gas mismanagement for deeper-diving cetaceans., Summary: Trained porpoises respond initially to sonar sound exposure with intensified bradycardia, but habituate rapidly, whereas 40 kHz pulses consistently evoke a startle, but elicit no other behavioural or heart rate changes.

Model results and addition figures

THE ESM consists of the following: Figure S1. Horizontal movement trajectory of four harbour porpoises exposed to short-term noise pulses from a single airgun noise in the inner Danish waters. A detailed description of harbour porpoise capture, handling, and tagging procedure. Figure S2. Overview of fieldwork protocol. Figure S3. Complete dive profile of four harbour porpoises exposed to airgun pulses in the inner Danish waters. Figure S4. Results of generalized additive mixed model (GAMM) analyses showing predicted effects of hour of the day on three horizontal movement parameters of four harbour porpoises captured in the inner Danish waters and fitted with a GPS-unit. Figure S5. Results of generalized additive mixed model (GAMM) analyses showing predicted effects of hour of the day on four vertical movement parameters of four harbour porpoises captured in the inner Danish waters, and fitted with TDR-units. Figure S6. Overview of the full dataset for each horizontal movement parameters. Figure S7. Overview of the full dataset for each vertical movement parameter. Figure S8: Overview of the temporal autocorrelation in piecewise regression model residuals of each horizontal movement parameter. Figure S9: Overview of the temporal autocorrelation in piecewise regression model residuals of each vertical movement parameter. Table S1: The mean (SE) values of breakpoints (h since release) as estimated by the best piecewise-regression models used to quantify timing of behavioural alterations in horizontal and vertical movement parameters of five harbour porpoises captured and tagged in the inner Danish waters and exposed to short-term noise pulses from a single airgun. Table S2: The coefficient (SE) of each regression line as estimated by the piecewise-regression models to quantify timing of behavioural alterations in horizontal and vertical movement parameters of five harbour porpoises captured and tagged in the inner Danish waters and exposed to short-term noise pulses from a single airgun.