Are we making too much noise?
Belugas depend to a large extent on their sense of hearing to communicate, to orient themselves and to hunt. Their habitat is becoming more and more noisy, to the point that we often use the term noise pollution when referring to their environment. But, what is really going on?
To go through the looking glass
Scientists use hydrophones and sound projectors to characterise noise levels and measure sound propagation in various St. Lawrence beluga whale habitats. Radio tracking of individual beluga whales—carried out since 2000—combined with visual tracking of pods since 1986 will enable researchers to determine the amount of time that beluga whales spend near loud noises.
Initial results have demonstrated that there is a real risk of permanent damage to beluga whale ears in certain habitats and at certain times of the day. Sound recordings will allow researchers to extend the analysis and create maps of the beluga whale’s sound environment in both space and time. Beluga whale tracking data will be used to “swim” the whales through these sound maps and evaluate with greater precision the risks that they face when exposed to noise.
Peter Scheifele, University of Connecticut, Robert Michaud and Pierre Béland, SLNIE and GREMM, Véronique Lesage, François Saucier and Ian McQuinn of Fisheries and Oceans Canada’s Maurice Lamontagne Institute and James Theriault of the Department of National Defence in Halifax.
World Wildlife Fund Canada, Canadian Humane Society, Croisières AML Cruises and Saguenay—St. Lawrence Marine Park.
I want to know more
Shush, I can’t here anything!
by Peter M. Scheifele, University of Connecticut
Translated from the document “Résumés des projets de recherche scientifique” produced by the Saguenay-St. Lawrence Marine Park and WWF Canada, 1998
The beluga “sees” with its ears, hence, the importance of being able to hear well. In fact, the noise of boat motors in certain areas of the St. Lawrence is so loud that, if humans were exposed to the same sound levels, they would be required to wear safety equipment. Unlike chemical pollution, we are just now beginning to question whether noise pollution might have a negative impact on aquatic life. It is difficult to evaluate with certainty the effects of background noise on beluga hearing. But if sound generated by boats is impairing their hearing, it could have an effect on their ability to orient themselves, communicate and avoid boats.
Sound levels were recorded at three separate locations: the mouth of the Saguenay, the head of the Laurentian Channel and off Alouette Shoal. From 1995 to 1997 inclusively, 3600 sound samples were recorded during the summer. Noise levels were recorded at various hours of the day at four separate frequencies: 500, 1000, 10 000 and 40 000 hertz (Hz). The 500 and 1000 Hz frequencies were chosen because the noise produced by boat motors is mostly concentrated within this band. The 10 000 and 40 000 Hz frequencies were added because belugas often use these frequencies for communication and echolocation. What is more, 40 000 Hz is the frequency at which they hear best: their peak sensitivity. At this frequency the auditory threshold (minimal intensity at which belugas perceive sound) is as low as 40 decibels (dB).
The head of the Channel, louder than a city street
In over 90% of cases, belugas were within 750 m of the sound source. Low-frequency sound (500 and 1000 Hz), which was on average 150 dB, turned out to be the loudest. This means that belugas were exposed to sound levels louder than those of a city street, but not quite as loud as a jackhammer. At lower and middle frequencies, the head of the Laurentian Channel was the loudest area, followed by the mouth of the Saguenay and the site off Lark Reef. But, for reasons not yet understood, the Saguenay was the loudest area in the 40 000 Hz range. This does not take away from the fact that, by and large, middle- and high-frequency sound levels were weaker than low-frequency sound levels. This was predictable since high-frequency sounds do not travel as far as low-frequency sounds.
So, how did these sound levels affect beluga hearing? It is hard to say. We have determined the auditory thresholds of belugas at different frequencies by training them to react in a specific way each time they hear a sound. But we do not know at what intensity level the ears of this marine mammal begin to sustain damage. However, this data is available for humans. The maximum sound level authorized in the United States by the Occupational Safety and Health Administration is set at 90 dB for a period of eight hours. If beluga ears work as well as humanís, we have estimated that this animal would probably experience hearing loss.
From the human ear to the beluga ear
In acoustics, for practical reasons, depending on whether sound is travelling through water or air (we must subtract 62 dB from sound measured in water), and depending on whether the subject is a beluga or a human, the base level is different. Consequently, the 90 dB norm was converted to sound heard by belugas. The value obtained for each of the four frequencies was compared to the level of noise recorded in the belugaís habitat. The probability that the belugaís ears were damaged by 1000 Hz sounds varies between 0% and 33% depending on the area and the time of day. The highest value was recorded at the head of the Laurentian Channel at 1:00 p.m., corresponding with the peak in the number of boats on the observation sites. Nevertheless, the probability is nil for the three other frequencies, regardless of the sampling area or the time of day.
Human sources generally produce low-frequency sound. Belugas, on the other hand, use high frequency sound. Were it not for the fact that low-frequency sound tends to easily mask high-frequency sound, the two would not enter into conflict. This masking effect could inhibit beluga communication and echolocation.
Hearing is a vital component of the belugaís sonar. Their exposure to noise generated by humans could affect not only their ability to perceive sound, but may also permanently damage their hearing. The detailed examination of the ears of the recovered carcasses of stranded belugas should allow us to shed some light on this question.
Faucher, A. 1988. The vocal repertoire of the St. Lawrence Estuary population of beluga whale (Delphinapterus leucas) and its behavioral, social and environmental contexts. M.Sc. thesis, Dalhousie University, Halifax.
Lesage, V., C. Barrette, M. C. S. Kingsley et B. Sjare. 1999. The effect of vessel noise on the vocal behavior of belugas in the St. Lawrence river estuary, Canada. Marine Mammal Science 15 (1): 65-84.
Turl, C. W., R. H. Penner et W. W. L. Au. 1987. Comparison of target detection capabilities of the beluga and bottlenose dolphin. The Journal of the Acoustical Society of America 82 (5): 1487-1491.
Scheifele, P. M. 1997. Impacts of low-frequency noise on the auditory system of the beluga of the Saint Lawrence River Estuary. Ph.D. thesis, University of Connecticut.