Narwhals react to ship noise and airgun pulses embedded in background noise

Anthropogenic activities are increasing in the Arctic posing a threat to species with high seasonal site-fidelity, such as the narwhal Monodon monoceros. In this controlled sound exposure study, six narwhals were live-captured and instrumented with animal-borne tags providing movement and behavioural data, and exposed to concurrent ship noise and airgun pulses. All narwhals reacted to sound exposure by reduced buzzing rates, where the response was dependent on the magnitude of exposure defined as 1/distance to ship. Halving of buzzing rate, compared with undisturbed behaviour, and cessation of foraging occurred at 12 and ~7-8 km from the ship, respectively. The effect of exposure could be detected > 40 km from the ship. At distances > 5 km, the received high-frequency cetacean weighted sound exposure levels were below background noise indicating sensitivity of narwhals towards sound disturbance and demonstrating their ability to detect signals embedded in noise. Further studies are needed to evaluate the energetic costs of disrupted foraging due to sustained disturbance but the observed sensitivity should be considered in the management of anthropogenic activities in the Arctic. The results of this study emphasize the importance of controlled sound exposure studies in the wild to explore the auditory capabilities of odontocetes.