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. 2020 Jun 16:9:e56760.
doi: 10.7554/eLife.56760.

Vessel noise levels drive behavioural responses of humpback whales with implications for whale-watching

Kate R Sprogis  1   2 , Simone Videsen  1 , Peter T Madsen  1
Affiliations

Vessel noise levels drive behavioural responses of humpback whales with implications for whale-watching

Kate R Sprogis et al. Elife. .

Abstract

Disturbance from whale-watching can cause significant behavioural changes with fitness consequences for targeted whale populations. However, the sensory stimuli triggering these responses are unknown, preventing effective mitigation. Here, we test the hypothesis that vessel noise level is a driver of disturbance, using humpback whales (Megaptera novaeangliae) as a model species. We conducted controlled exposure experiments (n = 42) on resting mother-calf pairs on a resting ground off Australia, by simulating whale-watch scenarios with a research vessel (range 100 m, speed 1.5 knts) playing back vessel noise at control/low (124/148 dB), medium (160 dB) or high (172 dB) low frequency-weighted source levels (re 1 μPa RMS@1 m). Compared to control/low treatments, during high noise playbacks the mother's proportion of time resting decreased by 30%, respiration rate doubled and swim speed increased by 37%. We therefore conclude that vessel noise is an adequate driver of behavioural disturbance in whales and that regulations to mitigate the impact of whale-watching should include noise emission standards.

Keywords: anthropogenic noise; behavioural response; cetacean; controlled exposure experiment; ecology; humpback whale; unmanned aerial vehicle.

Plain language summary

Whale-watching is a multi-billion-dollar industry that is growing around the world. Typically, tour operators use boats to transport tourists into coastal waters to see groups of whales, dolphins or porpoises. There is, however, accumulating evidence that boat-based whale-watching negatively affects the way these animals behave and so many countries have put guidelines in place to mitigate activities that may disturb the animals. These guidelines generally stipulate the boat’s angle of approach, how close the boat can get and the speed at which it can pass by the animals. In general, these guidelines are based on the assumption that the animals are disturbed by the closeness of the whale-watching boats. However, whales, dolphins and porpoises have very sensitive hearing, and only have a short range of vision underwater. Therefore, it seems plausible that the animals hear whale-watching boats long before they see them and so the loudness of underwater noise from the boats may be enough to disturb these animals' behaviour. To test this hypothesis, Sprogis et al. performed experiments where they simulated a whale-watching vessel approaching humpback whale mothers and calves who were resting off the northwest coast of Australia. A small motorised research boat travelling at a low speed passed different mother-calf pairs at a target distance of 100 meters, which is a common whale-watching distance guideline in many countries. The boat had an underwater speaker that played recordings of the boat noise at different volumes, while a drone with a video camera flew overhead to record the whales’ behaviours in detail and to identify individual animals. These “controlled exposure experiments” showed that the quiet boat noise did not appear to disturb the mothers and calves. However, compared to when the quiet boat passed the animals the louder boat noise decreased how long the mother whale rested on the surface by 30%, made her swim 37% faster, and doubled the number of breaths she took per minute. If there are many disturbances from humans, then it can negatively impact the energy the mother and calf have available for nursing, fending off males and predators, and migrating back to their feeding ground nearer the Earth’s poles. Based on these findings, it is shown that the loudness of the underwater noise from boats can explain why whales may be disturbed during whale-watching activities. To help reduce this disturbance, Sprogis et al. recommend that noise emission standards should be added to the current whale-watching regulations such that boats should be as quiet as possible and ideally around the volume of the ambient background noise. This would allow operators to approach the animals in a responsible, sustainable manner and offer tourists a view of undisturbed wildlife.

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Conflict of interest statement

KS, SV, PM No competing interests declared

Figures

Figure 1.
Figure 1.. Controlled exposure experimental design to measure behavioural responses of humpback whales to different vessel noise levels.
(A) Schematic showing simulated whale-watch approaches of resting mother-calf pairs in the during phase, at the same distance and speed, with different vessel noise playback levels (control/low 124/148 dB re 1 μPa, medium 160 dB re 1 μPa, high 172 dB re 1 μPa; not to scale). (B) Set up of the vessel and underwater acoustic transducer (circled). (C) Aerial photograph of a mother-calf pair resting on the surface, from the view of the unmanned aerial vehicle during data collection.
Figure 1—figure supplement 1.
Figure 1—figure supplement 1.. Study area in Exmouth Gulf, Western Australia, and research effort tracks.
Figure 1—figure supplement 2.
Figure 1—figure supplement 2.. Spectral signatures of whale-watch vessel noise and research vessel noise.
Figure 1—figure supplement 3.
Figure 1—figure supplement 3.. Spectral components of the vessel noise recording compared to playback noise.
Figure 2.
Figure 2.. Examples of controlled exposure experiments of the research vessel simulating a whale-watch vessel approach.
Three focal mother-calf pairs (dashed line) in before, during and after phases with the research vessel approaching (solid line) at 800 rpm (~1.5 knts) during control, medium and high noise treatments. Details of these CEEs are: (A) control: mother resting/logging the entire duration drifting with the incoming tide (flow from ~north to south), closest point of approach 97 m, (B) medium: mother logging before, mother logging during, mother peduncle dove underwater after, high tide, closest point of approach 95 m, (C) high: mother logging before, mother logging during until closest point of approach then peduncle dove underwater and swam away slowly just under the surface of the water, mother logging after, outgoing tide (flow from ~south to north), closest point of approach 95 m.
Figure 2—figure supplement 1.
Figure 2—figure supplement 1.. Absolute body length of humpback whale mother and calves.
Figure 3.
Figure 3.. Received levels in noise treatment calibration quantified as third-octave level bands in dB re 1 μPa RMS.
RLs are across third-octave band frequencies 400 Hz-20 kHz. The vessel was transiting at 800 rpm at ~100 m (range 85–113 m) distance to the SoundTrap, in 14 m water depth. 50th percentiles of control (purple line), low (yellow line), medium (red line) and high (blue line) vessel noise. Ambient noise 18th August (dark green): humpback whale male whale song absent. Ambient noise 18th September (medium green): peak whale season with song. Ambient noise 18th October (light green): towards the end of the whale season (integration bins 0.125 s). The transparent area around each percentile line corresponds to the 25th and 95th exceedance levels. Self-noise of the SoundTrap plotted as dashed black line.
Figure 3—figure supplement 1.
Figure 3—figure supplement 1.. Noise treatment received levels quantified as third-octave level bands (TOLs) in dB re 1 μPa RMS in 6.8 m depth.
Figure 3—figure supplement 2.
Figure 3—figure supplement 2.. Ambient noise recorded in Exmouth Gulf, Western Australia.
Figure 4.
Figure 4.. Short-term behavioural responses of humpback whales.
(A) Proportion of time resting for the mother, (B) proportion of time resting for the calf, (C) probability of instantaneous behavioural events for the mother, (D) respiration rate for the mother, (E) mean heading change for the mother, and (F) mean swim speed for the mother. Representation of the model (back-transformed from the logit scale A, B, C). Vertical lines represent standard errors. Asterisks indicate significant differences among phases and/or treatments.
Figure 4—figure supplement 1.
Figure 4—figure supplement 1.. Coefficient plots of model outputs.
See this image and copyright information in PMC

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