Sounding out the secrets of the sea
Key text
This topic is sponsored by The Australian Acoustical Society.
The increasing use of sound by humans to explore the seas has raised questions about its potential impact on marine life.
Sound under water is very important for animals. It allows them to navigate, to hear approaching predators and prey, and is a way of communicating with other members of the same species.
Humans too make use of sound to explore the underwater world. Sound is used for many reasons including geological and biological surveys and to locate oil and gas fields. To understand how the human use of sound can impact on animals we need to take a closer look at the use of sound by animals in the ocean.
How do animals hear under water?
Sun light does not penetrate water very well and visibility can be poor, even in shallow water. The properties of sound make it an ideal way to communicate under water (Box 1: Comparison of the properties of sound in air and water), so many animals use sound to communicate with each other and to ‘observe’ objects in a marine environment.
Marine animals have evolved a variety of ways to detect and make sound in water. Most fish, apart from sharks and stingrays, have sensory hair cells lining a small cavity in the ear that is filled with viscous liquid. Attached to the hairs (technically known as stereocilia) and suspended in the liquid of each ear, there is a small stone of calcium mineral called an otolith or ‘ear stone’. When a sound wave passes through the water and the body of the animal, the otoliths tend to stay still, relative to the movement of the fish. The inertia of the otoliths stimulates the stereocilia to convey a message to the brain.
Related site: What is an otolith?
Explains what otoliths are and how they are used to determine the age of fish.
(Bedford Institute of Oceanography, Canada)
Some fish have a lateral line of pores that open to a continuous canal along the length of the body. The canal contains structures called neuromasts that detect sound waves. The motion of water by the sound moves the hairs a tiny amount, and the supporting cells transmit a message to connecting nerve cells.
Another method uses enclosed pockets of air: either the lungs of mammals such as dolphins and whales or swim bladders in fish. The air in the swim bladder is easily compressed by sound pressure waves, which are converted to vibrations, allowing the fish to detect sound as well as vibrations. The sensitivity of fish to noise and vibration differs depending on the proximity of the swim bladder to the inner ear in different species.
How animals make sounds under water
Crustaceans such as crabs and lobsters which have an exoskeleton make sounds by hitting or scraping one part of their body against another to make it vibrate, similar to the way insects make sounds. The ‘snapping’ sound made by many such animals gives a series of sharp sound pulses that can be heard at a great distance. Because there are so many of these small creatures, they produce much of the background noise in oceans.
Related site: Listen up
Provides recordings of sea animals including shrimp, whales and herrings.
(Ocean Link, Canada)
Fish that have soft skin cannot produce sound in this way and must actively vibrate some part of their body. Some vibrate the air in their swim bladders to make sounds which then radiate into the surrounding water.
Another way of making sound is typified by whales and dolphins. They generally make sounds by moving air from one body cavity to another through some sort of valve with a vibrating ‘lip’. Because the density of the flesh of a sea animal is very similar to that of water the sound radiates efficiently, is quite loud and travels long distances.
Echo, echo, echolocation
Some animals such as whales and dolphins have evolved to use sonar (SOund NAvigation and Ranging) or echolocation to produce and detect sound. This compensates for the lack of visual information available in the ocean. The animal produces a very short high frequency ‘click’ by passing air through vibrating ‘lips’ in their head. The sound wave is directed mainly forward like a sonic headlight, being focussed by an organ in the head that contains fats. The sound is partly reflected by objects such as a rock or a fish, and is then transferred to the ear drum via their lower jaw, which includes an area filled with fats. The time delay for the pulse journey is only about 1.5 milliseconds per metre travelled, but this is long enough for the animal to determine the position of the object. Repetitive pulses help the animal to reduce the influence of background noise or clicks from other animals, and the exact frequency of the returning click gives information about the movement of the object.
Related site: Echolocation
Explains how marine mammals use echolocation.
(Ocean Link, Canada)
Depending upon the size of the target, sonar is useful over tens of metres or even more. Some animals have developed remarkably sensitive powers of discrimination: a dolphin can detect the difference between a solid and a hollow metal ball the size of a baseball at a distance of 20 metres.
Oceans of noise
Background noise in the ocean is produced by breaking waves, wind, rain and by the huge number of small crustaceans and other animals. A typical background noise level is about 100 decibels (dB), which is about the same in energy terms as 40 dB in air. Wind and waves in storms, and choruses from fish and invertebrate can increase this level to about 120 dB (Box 2: Measurement of sound levels).
Related site: A collection of sounds from the sea
Provides recordings of ships, airguns, earthquakes, volcanoes and whales.
(Ocean Explorer, National Oceanic and Atmospheric Administration, USA)
Measurements show that the Pacific Ocean is still relatively quiet and that most of its background noise is produced by wind and by marine creatures. This is in contrast to the Atlantic Ocean, where most of the background noise is from the churning propellers of ocean-going ships.
Relative noise levels from man-made and animal sources
The diagram below shows the frequency distribution of pressure levels for natural and man-made sources of sound in Australian waters. The decibel figures are low because they show the sound pressure levels in individual bands only one hertz wide at each frequency. The contributions of all these bands must be added together to get the total sound pressure.
Within one metre of a typical sonar transducer the sound power level can be as high as 180 dB, which is comparable to the peak level for animal calls. But human sonar transmitters can be arranged in long lines so that the signal maintains a high level of power at considerable distances (Box 3: Use of sonar in the sea).
The calls of the loudest animals in the ocean, such as seals and whales, have levels as high as 190 dB at a distance of one metre, which is about the same sound power level as a loud human shout in air at the same distance. Some echo-locating clicks can reach peak levels as high as 230 dB, though for only very short times.
Other sounds in the ocean, including undersea earthquakes and seafloor volcanic eruptions, have been recorded at levels that exceed that of close echo-locating clicks, reaching levels beyond 240 dB over very large areas.
Is the use of sonar by man harmful to ocean animals?
The potential impacts of sonar on marine animals are similar to those of humans exposed to noise and include:
- behavioural changes;
- temporary or permanent hearing loss or tissue damage; and
- physiological stress responses.
The species affected by the sound depends on the frequency and sound pressure level. For example sound sources such as airguns produce more noise in the hearing range of baleen whales than mid-frequency echo sounders, which produce more noise in the ranges used by seals and toothed whales. Sounds that disturb one species appear to have no effect on others.
The potential risks to animals posed by the use of sound by humans result from a combination of the energy level, frequency and local acoustical effects due to the underwater ‘landforms’. Instruments of sufficiently low power and high frequency pose a minor risk. The equipment with the highest risks are airgun arrays and low frequency, high-power transducers with wide beam angles.
How can we tell if an animal is affected by sound?
Related site: Effects of sound: How do you determine if a sound affects a marine animal?
Lists a series of questions to be answered to determine whether a sound affects an animal.
(University of Rhode Island, USA)
It is not always easy to determine whether an animal is harmed by exposure to a sound. Biologists and engineers have developed a new digital tag that provides information about whale behaviour, including how deep they dive, what they hear, and what sounds they make to communicate. The tags make it possible to do controlled experiments rather than one-off incidental observations of animal behaviour. CT scans and 3-D imaging are also being used to study the structure of the ears of marine mammals and how they might be injured by exposure to sound sources (Box 4: Disturbing beaching events).
Safe and sound
Several studies have been done to investigate the potential impact of the human use of sonar for research purposes, especially in the naturally quiet waters around Antarctica. International guidelines have been developed for the use of sonar, particularly when long arrays are used, to minimise the possibility of damage or confusion to sea animals. Strategies include using the minimum sound energy level required, providing an escape route for animals in the area, and minimal use at times when animals are more sensitive to disturbance, such as breeding or mating times. Researchers record details of acoustic activities to allow retrospective assessment of the cause of any future changes to the distribution, numbers or breeding patterns of animals. Until we know more about the impact on marine life of the use of sound, caution should prevail.
Boxes
1. Comparison of the properties of sound in air and water
2. Measurement of sound levels
3. Use of sonar in the sea
4. Disturbing beaching events
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Posted August 2007.