Abstract
ABSTRACTAnthropogenic 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.
Highlights
Naval sonar use has been linked to mass-strandings of beaked whales (e.g. Frantzis, 1998) and harbour porpoises (Phocoena phocoena) (Wright et al, 2013)
While diving, increased peripheral vasoconstriction and proportionally decreased heart rate conserve blood oxygen, mainly for the brain and heart (Davis, 2019; Scholander, 1940). This dive response is influenced by dive duration and exercise (Davis and Williams, 2012; McDonald et al, 2018; Williams et al, 2015), and is under anticipatory and volition control
We predicted a behavioural response as seen for a range of other anthropogenic noise sources at similar or lower loudness
Summary
Naval sonar use has been linked to mass-strandings of beaked whales (e.g. Frantzis, 1998) and harbour porpoises (Phocoena phocoena) (Wright et al, 2013). We investigate the fH and motor-response of porpoises exposed to both mid-frequency sonar-like sweep and startling noise pulse to understand the implications for a diving animal. Even without a behavioural or movement response, the first sonar-like sweep exposure of each porpoise [RL, Freja: 109 dB
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