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Geophysics and whales: noise pollution in the "Silent World"

Whales communicate through their "song" within an increasingly noisy underwater sound landscape, where the airguns used for geophysical research can be heard thousands of kilometers away. What is the space and time overlap of these two sources ?

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For a half-century, military, scientific and industrial activities have been emitting in the oceans an increasing amount of low-frequency sounds (<1000 Hz), so that concern is growing about the impact of this kind of pollution on whales, which are acoustically sensitive and use these frequencies for their own communication.

Offshore oil exploration and marine seismic surveys are the noisiest activities at sea. They use airguns emitting low-frequency (2–188 Hz), short-duration (<0.1 s) and high-energy pulses. Airguns are fired every 10–60 s for days or weeks at a time. Although fin whales (Balaenoptera physalus) produces numerous sounds, the most common of them is also very short (0.5 to 1 s) but emitted in a narrower frequency range (18-25 Hz); series of this pulse make the "calls" of male individuals. In the North Atlantic Ocean, the sounds produced by the estimated 50,000 fin whales are a significant contributor to ambient noise.

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An example of co-occurrence of airgun (fine and long regularly spaced vertical marks, between 10 and 50 Hz) and fin whale sounds (thick and short marks between 20 and 25 Hz). This spectrogram covers a duration of about twelve minutes

 

To assess the potential effects of airgun sounds on whales, the knowledge of their physiology and behaviour must be complemented by a description of the temporal and geographical occurrence of this sound and the distribution of species that are potentially impacted. In this paper, the authors analyzed a 10-year series of recordings by a network of underwater autonomous hydrophones which were designed to detect the acoustic signals of undersea earthquakes but also receiving the sounds produced by a variety of others sources like icebergs, ships, airguns and whales.

From 1999 on, hydrophones were moored by geophysicists on both sides of the Mid-Atlantic Ridge, between 15 and 55°N, in fixed positions at ~900m below the surface. This is the depth of the "SOFAR channel", an ocean layer where the speed of sound is the lowest and the sound waves are guided with little dissipation. The hydrophones detect sounds from sources located several thousand kilometres in the case of airguns or only about 70 km for whales.

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Locations of 12 hydrophone moorings (stars) and approximate areas of seismic airgun activity (dotted boxes)

 

More than 246,000 hours of acoustic data were recorded during this 10-year period. The analysis of the same noises recorded by several hydrophones made possible to localize the areas where airguns were shot. The vocal activity of whales was quantified using a "fin index" based on the daily average energy emitted in the frequency range of their "calls".

Airgun noises were recorded on every site during most of the year (9 months out of 12) and during more than 80% of the days over periods exceeding one year. They came from three main regions situated off Newfoundland, Brazil and Mauritania. Conversely, 20-Hz noises are produced by whales on a clear seasonal basis, between August and April with a maximum in December and January. The vocalization index reached higher values in the north of the studied area than in the south, and showed an increasing trend along the 10-year period.

The two low-frequency sound sources thus emit with a clear seasonal overlap: even if the airgun pattern is quite variable along the year, their noises are always produced during the vocalization period of fin whales.

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Temporal overlap of airgun pulses (black bars) and fin whale vocalizations (red line). N.B. left- and right-axis scales are not comparable and do not reflect sound intensities.

 

Many studies have shown that the airgun sounds have a direct or indirect negative impact on whale physiology or behaviour. The migrating whales whose calls were recorded near the Mid-Atlantic Ridge were only occasionally exposed to the sounds of airguns shot in the same area: most of those which reached them were produced several thousand kilometers away, thus without a likely direct effect.

The most important impact of the seismic noise is to decrease the effective range of communication among whales. The ambient noise of airguns is emitted in a wide range of frequencies including the 20-Hz whale calls; if this background noise is too loud, it can mask the vocalizations. In spite of sound attenuation with distance, this possibility is realistic because the two sources clearly overlap through time, particularly at the end of the year. Owing to the physical properties of sea water (temperature, pressure), the airgun noises are guided within the SOFAR channel at depths of several hundred meters, but a part can escape through refraction and can reach whales in the shallower layers where they emit and receive their calls.

This acoustical study provides important elements to the understanding of the oceanic soundscape, but other questions still need to be addressed to measure the actual impact of this noise pollution on whales. How far do they communicate? How is the perturbation of sound influenced by the distance between individuals and their distance from the airguns? Is the impact greater for populations reduced by whale hunting, where individuals are more scattered? Can the vocal adaptation observed in some species compensate the increase of ambient noise?

 

The paper

S. L. Nieukirk, D. K. Mellinger, S. E. Moore, K. Klinck, R. P. Dziak, J. Goslin, 2012. Sounds from airguns and fin whales recorded in the mid-Atlantic Ocean, 1999–2009. Journal of the Acoustical Society of America, 131(2) : 1102-1112.
See the first page

 

The authors

This paper is the result of a collaboration between researchers from France (Laboratory Domaines océaniques of IUEM) and the USA (Oregon State University and NOAA)

 

The journal

Since 1929 the Journal of the Acoustical Society of America has been the leading source of theoretical and experimental research results in the broad interdisciplinary subject of sound. The Journal serves physical scientists, life scientists, engineers, psychologists, physiologists, architects, musicians, and speech communication specialists.

 

Contacts

Authors : browse IUEM staff directory
Communication and outreach service: communication.iuem@univ-brest.fr


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