Abstract
One of the last pristine marine soundscapes, the Arctic, is exposed to increasing anthropogenic activities due to climate-induced decrease in sea ice coverage. In this study, we combined movement and behavioral data from animal-borne tags in a controlled sound exposure study to describe the reactions of narwhals, Monodon monoceros, to airgun pulses and ship noise. Sixteen narwhals were live captured and instrumented with satellite tags and Acousonde acoustic-behavioral recorders, and 11 of them were exposed to airgun pulses and vessel sounds. The sound exposure levels (SELs) of pulses from a small airgun (3.4 L) used in 2017 and a larger one (17.0 L) used in 2018 were measured using drifting recorders. The experiment was divided into trials with airgun and ship-noise exposure, intertrials with only ship-noise, and pre- and postexposure periods. Both trials and intertrials lasted ∼4 h on average per individual. Depending on the location of the whales, the number of separate exposures ranged between one and eight trials or intertrials. Received pulse SELs dropped below 130 dB re 1 μPa2 s by 2.5 km for the small airgun and 4–9 km for the larger airgun, and background noise levels were reached at distances of ∼3 and 8–10.5 km, respectively, for the small and big airguns. Avoidance reactions of the whales could be detected at distances >5 km in 2017 and >11 km in 2018 when in line of sight of the seismic vessel. Meanwhile, a ∼30% increase in horizontal travel speed could be detected up to 2 h before the seismic vessel was in line of sight. Applying line of sight as the criterion for exposure thus excludes some potential pre-response effects, and our estimates of effects must therefore be considered conservative. The whales reacted by changing their swimming speed and direction at distances between 5 and 24 km depending on topographical surroundings where the exposure occurred. The propensity of the whales to move towards the shore increased with increasing exposure (i.e., shorter distance to vessels) and was highest with the large airgun used in 2018, where the whales moved towards the shore at distances of 10–15 km. No long-term effects of the response study could be detected.
Highlights
Anthropogenic activities such as shipping, seismic exploration, pile driving, dredging, ice breaking, and sonar and military activities introduce underwater noise pollution in both coastal and open ocean areas (Hildebrand, 2009)
The North Atlantic is frequently affected by wide-ranging seismic surveys (Nieukirk et al, 2012), some of which can be detected in high Arctic areas where anthropogenic noise is rarely encountered (Moore et al, 2012b; Ahonen et al, 2017)
Three and six whales were instrumented with Acousonde recorders in 2017 and 2018, respectively, before the arrival of the seismic vessels
Summary
Anthropogenic activities such as shipping, seismic exploration, pile driving, dredging, ice breaking, and sonar and military activities introduce underwater noise pollution in both coastal and open ocean areas (Hildebrand, 2009). Even in high Arctic areas, local seismic surveys are periodically a concern for endemic marine mammal populations (HeideJørgensen et al, 2012; Martin et al, 2017; Kyhn et al, 2019) These surveys are conducted during the ice-free season when Arctic whales are either in coastal areas or migrating between summer and winter grounds. The Atlantic Arctic generally has lower background noise levels at low frequencies compared to equatorial regions (Haver et al, 2017) This is mainly due to the dampening effect of seasonal ice cover on wave action, but during summer, after the noisy melting and disintegration of sea ice, offshore Arctic background noise levels increase due to wind, rain, and anthropogenic activities (Klinck et al, 2012). They abandon the increasingly noisy offshore areas and move into summer grounds with presumably lower noise level
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