Università degli Studi di Pavia

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Effects of underwater noise on marine mammals

Gianni Pavan

In the Mediterranean Sea marine life is threatened by habitat degradation due to human activities such as fisheries, ship traffic, pollution, and coast anthropization. Other than being affected by chemical pollution, which may contaminate the whole marine food web, cetaceans can also be affected by noise pollution.

The underwater environment has its own acoustic peculiarities and cetaceans are extraordinarily well adapted to them. In these mammals, acoustic communication has acquired a privileged role compared with other communication channels. Marine mammals live in a medium which poorly transmits light but through which sound propagates very well, even over long distances. Marine mammals heavily rely on sound to communicate, to exploit and investigate the environment, to find prey and to avoid obstacles.

The effect of anthropogenic noise on the marine environment is a new serious concern for scientists.
Noise can influence the physical and psychic well-being of man, and directives and laws set limits on potentially damaging acoustic emissions. Our knowledge about the impact of noise on the marine environment is still limited, but lately the problem has been taken more and more into consideration.
Underwater noise and vibrations produced by man may come from many sources: ship traffic, touristic boats, seismic surveys, seabed drilling, sonars, telemetry devices, oceanographical experiments, underwater explosions, vibrations propagating from the coast, etc. All these can variously interfere with animal life. The marine environment itself includes sources of noise: movement of water, wind, rain and earthquakes, are acoustic sources which may have an impact on animals. However, animals have adapted to most of them by elaborating suitable schemes of communication.

Exposure to sound can produce a range of effects on marine mammals. A low level sound can be audible to animals without resulting in any visible effect. At increased levels the sound may disturb animals and induce avoidance and other behavioral changes. If animals for any reason can’t avoid a noise source, they may be exposed to acoustic conditions capable of producing negative effects, which may range from discomfort and stress to physical acoustic trauma. Exposure to very loud sounds, explosions at short range for example, can produce damage to many organs in addition to hearing.

The basic effect of an acoustic trauma is a decrease in hearing ability, or worsening of sensitivity threshold. Hearing losses are classified as temporary (TTS) or permanent (PTS) threshold shifts. As intensity and duration can act synergistically to broaden the loss, long or repeated exposures to TTS levels may produce permanent hearing loss (PTS). This means that both level and duration of exposure may lead to acoustic damage.
Exposure to noise may have an effect even if below the levels required to produce hearing loss. Such low level noise exposure may produce a variety of potentially, difficult to evaluate, disruptive behavioral effects that may have a long-term impact on marine mammal populations.
As with reduced hearing capabilities, an increased background noise may severely affect the animals’ ability to perceive the environment, to communicate, to hear each other, and to perceive the weak echoes of their biosonar pulses.

These topics recently began to be widely discussed. It was 1995 when the Italian Navy started to face this problem and considered, based on the precautionary principle, to limit sonar exercises in the Ligurian Sea Cetacean Sanctuary.

In May 1996, a mass stranding of Cuvier’s beaked whales occurred in the Kyparissiakos Gulf, on the west coast of Ionian Greece. This stranding was near, and in temporal coincidence with, a NATO sonar test, and focused attention on potential impacts of high power active sonars. Further episodes (Bahamas, March 2000) increased the concern about this problem.

However, we have to remember that sources other than sonars emit loud sounds underwater, and a census of all sources is required to have a complete picture of the situation. In addition, we must consider that vibrations may propagate from the coasts, and in certain conditions the noises of construction works on a coast propagate underwater for tens of miles.

We usually consider two main types of noise pollution. One is acute pollution, produced in a location for a defined period of time, for example a sonar test or a geophysical exploration.The other is diffuse pollution, which is a generalized background noise increase due to the sum of a large number of sources.

Ship traffic is an example of diffuse pollution that may affect very wide areas. A monitoring of ships’ underwater noise is required to model their diffusion and the impact on the underwater environment. In relation to this, cumulative effects due to multiple and prolonged exposures must be studied. Ship traffic noise can be reduced by lowering the noise irradiated by engines and propellers, and by modifying ship tracks to avoid sensitive areas such as breeding grounds, feeding grounds and migratory corridors.

Acute pollution seems to be more manageable. The Acoustic Risk Mitigation Policies under development and testing by military and civilian institutions are aimed at finding ways to minimize effects of irradiated noise by carefully choosing specific areas and periods for potentially harmful activities. Constant verification that no animals are in the area to be ensonified or approaching it can further minimize harm. This can be achieved by combining visual observation and passive listening to underwater sound with suitable instrumentation.

These kinds of protocols are necessarily based on a strong knowledge of marine mammals and their environment in order to predict animal distribution and density, and once in the field, to be able to detect, classify and track animals within critical ranges. At the NATO Saclant Undersea Research Center, the SOLMAR (Sound Oceanography and Living Marine Resources) Project is aimed at developing a policy to safely operate high power sound sources.
However, the determination of safe exposure levels, not only related with PTS or TTS effects, but also with short and long term behavioural disruption is still a debatable question.
At present, there are no studies providing reliable threshold shift data for marine mammal species, and there continue to be a wide range of arguments presented about acoustic trauma susceptibilities in marine mammals.

A number of studies have demonstrated behavioral responses to man made sounds. These studies show how effects may widely vary depending on the nature of the sound, on local sound propagation conditions, and on the animals’ sensitivity, which varies according to species, behavior, social context, and other factors.

The US Office of Naval Research (ONR) currently funds many research projects on this topic. ONR's aim is to increase knowledge about marine mammals and their sensitivity to sound exposure, to allow Navies to operate within safeguard limits, and to comply with the Marine Mammal Protection Act. Research on these topics is also funded by oil companies, which need to mitigate the effects of airgun operations for petroleum search.
Even if the number of studies on this topic has increased in recent years, the scarce availability of baseline scientific information about cause-effect relationships prevents us from understanding long-term effects and adopting appropriate conservation policies.

In the last few decades, underwater acoustic technology has become available for civil research institutions, thus opening a new window on marine mammals studies. However, acoustic behavior of only a small number of marine mammals species has been described. Nowadays, other than expanding the number of studied species, the interest of researchers is shifting to more comprehensive research topics such as environment use, critical habitat identification, and analysis of the impact of human activities.

Very little is known about marine mammals’ critical habitats. The lack of adequate knowledge about cetacean population distribution, size, trends, dynamics, reproductive cycles, migratory habits, sensitivity to human activities, ecological roles and communication abilities, dramatically limits our ability to develop strategies and policies for their conservation. This makes all cetacean species vulnerable to increased disturbance and habitat degradation, particularly in the long term.

Other than general principles of environmental protection and precautionary principles, more funding for research and strong measures are urgently needed in order to increase our awareness about critical habitat requirements and to reduce direct and indirect impacts caused by human activities. Coordinated scientific research is the basic tool to achieve this goal.

Gianni Pavan
Centro Interdisciplinare di Bioacustica e Ricerche Ambientali
Università degli Studi di Pavia, Italia

Paper published on the ACCOBAMS Bulletin, number 4, 2002.

Selected References

Au, 1993. The sonar of dolphins. Springerl-Verlag
Urick, 1983. Principles of underwater sound. McGraw Hill
Richardson et al., 1995. Marine Mammals and Noise. Academic Press
AA.VV., 2000. Marine Mammals and Low-Frequency Sound: Progress Since 1994. National Academic Press
Gisiner (Editor), 1998. Proceedings of the workshop on the effects of anthropogenic noise in the marine environment. Available in pdf format at
AA.VV., 2003. Ocean Noise and Marine Mammals. The National Academies Press, Washington DC: 1-192.


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