Behavioral Response Study on Seismic Airgun and Vessel Exposures in Narwhals

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.

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

“…High-frequency cetacean (HF) weighting [ 23 ], which provides a better estimate of actual levels perceived by the whales, lowered the airgun pulse SELs and background noise SELs less than 10 km by 28–61 and 9–32 dB, respectively, compared with unweighted values ( figure 1 ). Near the ship, HF-weighted background SELs approached unweighted values (minimum difference was 6 dB) in part due to the presence of the MBES signals ( figure 1 , see [ 9 ] for more details). Within approximately 3 km of the source, MBES signals were therefore part of the sound exposure the whales were experiencing, but beyond this distance, the whales were presumably reacting to a combination of airgun pulses and ship noise ( figure 1 ).…”

“…All individuals ceased foraging within approximately 7–8 km of the ship at received HF-weighted airgun pulse SELs below background noise levels of 107 dB (interquartile range 95–97 dB) re 1 μPa 2 s ( figure 2 and table 2 ). At this distance, noise from the ship was also buried in the background, but was difficult to quantify (see [ 9 ]); the unweighted airgun pulse SELs were less than 135 dB re 1 μPa 2 s ( table 2 ). Compared with undisturbed behaviour, a 25% and 50% decrease in buzzing rate occurred at 16 and 12 km from the source, respectively ( figure 2 ).…”

“…Background noise levels, measured 10–45 km from the ship, consisted of 1 s samples (10 Hz–48 kHz bandwidth, three-term Blackman–Harris window, NFFT 96000, 50% overlap) selected 3 s before the actual onset of each airgun pulse, as long as the pulses were detectable, and every 80 s thereafter. Airgun pulses were also analysed from Acousonde records on the whales when possible ( figure 1 ; see electronic supplementary material for details on the analyses; see [ 9 ] for more information).

Figure 1 Received SELs of sound from airgun and MBES pulses as compared to background levels, as a function of distance from the sound source.

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“…Time–depth records were down-sampled to 1 Hz and time-synchronized with GPS positions. Additional GPS positions were created for each second between successive positions through linear interpolation [ 9 ]. Buzzes were used as a proxy for foraging attempts [ 6 ] and were detected from the Acousonde acoustic data using a custom-written detector (Matlab, The MathWorks Inc., USA) and verified manually.…”

“…Controlled sound exposure studies in the wild in which received sound levels are recorded by animal-borne sensors can be used to determine sound exposure thresholds for behavioural responses. Since the received level at the animal depends on a number of factors including the environment's sound speed profile, and the depth and behaviour of the animal, measuring the received level can be challenging, but studies of behavioural responses can still be used as invaluable indicators of signal detection [ 8 , 9 ].…”