Università degli Studi di Pavia
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Effects of underwater noise on marine mammals
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.
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.
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.
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
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.
Paper published on the ACCOBAMS Bulletin, number 4, 2002.
Au, 1993. The sonar of dolphins. Springerl-Verlag
Created August 2006, updated September 2006