Università degli Studi di Pavia

Centro Interdisciplinare di Bioacustica e Ricerche Ambientali

Via Taramelli 24 - 27100 Pavia - Italy
e-mail : cibra@unipv.it

Marine Bioacoustics

The first investigations on underwater sound were made by the Navies to detect and locate ships and submarines of the enemy by listening to the noise of their engines and propellers. The same equipment developed for military needs allowed to listen to unexpected sounds that for many years puzzled the experts and opened up new scientific research branches: acoustical oceanography and marine bioacoustics.

In the aquatic environment the acoustic communication among animals has a very important role because the high speed of propagation (1500 m/sec - almost five times greater than in air) and the scarce attenuation with the distance allow an efficient transmission of the sounds.
Many species use sounds not only to communicate but also to explore the environment around them. The sight, that is limited to few meters of distance in water and that can’t be used in the dark oceanic depths, is thus replaced by the use of sound.
Marine bioacoustics is the study of the sounds produced by marine animals, to understand their behaviour and their relationships with the marine environment. Beyond studying individual features of each species for their biological and ecological significance, marine bioacoustics is also concerned with the development of practical applications for the management and preservation of the environment. The sea animals can’t be seen when they dive, some cetacean species make dives up 40-50 minutes long, or when they are too far from the observer. But the sounds these animals emit travel so well in water that is possible to listen to them at a distance of many kilometres, in some cases up to tens of kilometres and more.
By listening to those sounds it is possible to know which species inhabit a given area, and also to make censuses, that is to count or estimate the number of individuals living in a given area, and also to understand what they are doing there and how they exploit the available resources.

In the aquatic environment the acoustic communication has a very important role because the high speed of propagation (1500 m/sec - almost five times greater than in air) and the scarce attenuation with the distance that allows an efficient transmission of the sounds.
Many species use sounds not only to communicate but also to explore the environment around them. The sight, that is limited to few meters of distance in water and that can’t be used in the dark oceanic depths, is thus replaced by the use of sound.

Many aquatic organisms produce sounds. Invertebrates (mostly crustaceans), fishes, marine mammals (ceteaceans and pinnipeds) produce sounds with frequencies ranging from infrasounds to ultrasounds. All these acoustic signals are emitted in a complex noisy environment to which man and its activities contributes significantly.
Sound productions in crustaceans and fishes is quite common but poorly studied; in Teleost fishes more than 50 families include species that use sounds to communicate, normally with frequencies below few kHz and low sound intensity that limits communication range to short distances. On the contrary, cetaceans use sound extensively, with sound levels that allow communication over long ranges, and ultrasonic acoustic pulses that allow accurate echolocation over ranges of many hundred of meters.

Marine Mammals

The zoological group of marine mammals includes animals who live underwater as well as terrestrial animals who spend only part of their time into water, for feeding, for example. Sirenians, Pinnipeds, Otters and Cetaceans spend all or most of their life into water and use sound extensively.


Cetaceans are divided into two sub orders, the Odontocete, or toothed whales, and the Mysticete, or baleen whales, each one with peculiar behaviours and acoustic features.

The Odontocetes are active hunter that chase and capture their prey using a variety of senses. They may range in size from 1.4 meters to 18 meters (sperm whale).
They feed on fish and squids and different species exploit different ecological niches. Some animals such as the harbour porpoise, hunt in the very shallow water of the surf zone whilst at the other extreme animals such as the sperm whale and the beaked whales may dive at depths of 1000 metres and below. Few species live in fresh waters of intertropical rivers.
In order to locate and capture their prey in waters of limited visibility (light penetrate water for no more than 30-40 meters and vision is further more limited) these animals have developed echolocation. This consists in the emission of short wideband pulses, called clicks, covering up to four octaves with frequencies ranging up to 150 kHz. The mechanism by which the high power pulse is produced is still not fully understood and few speecies have been studied so far. The sound is produced just below the nasal plug and it is then focused by a combination of reflection off the skull and passage through a lens mechanism formed by the melon, a mass of fatty tissues in the forehead of the animal. After emitting a pulse they listen for echoes reflected by the surrounding environment, by obstacles, and by preys too. Their receive system appears to use the usual mammal ears, but assisted by waveguides in the lower jaw formed from fatty channels and possibly by the lower teeth that may work as an acousic antenna.
Other than echolocation clicks, most odontocetes produc tonal whistles for communication. They can also produce a third type of sound, which can best be described as a squawk, a moan or a buzz. It is produced by emitting low-power echolocation clicks at high speed. Repetition rates of well over 1000/sec have been recorded.

The Mysticetes are filter feeders feeding on plankton and small fish and are primarily the larger whales, ranging in size from the minke whale (8 meters on avg) up to the blue whale (28 meters and more), the largest living animal. They feed by capturing large quantities of water within their mouths and then expelling this water through filters formed by the baleen plates, which take the place of teeth. The filters capture the plankton and small fish, which are then swallowed. Because they do not need to chase the prey they have not developed echolocation in the same manner as the Odontocetes. The sounds they make are primarily low frequency tonals for inter-animal communication although there is some evidence that the fin and blue whales can transmit a FM sweep that can be potentially used to get a large scale "acoustic image" of the surrounfing environment for whole ocean basin navigation. As low frequencies propagate well into water, baleen whales sounds may propagate for hundreds of kilometers.

Marine mammals and sound

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 channels of  communication. Auditory mechanisms and sound producing organs are highly evolved and diversified with the acquisition of the ability to echolocate (biosonar, or biological sonar), which is peculiar to the Odontocetes and among other animals has only reached an equivalent level of sophistication in bats (Chiroptera).
The production of acoustic signals is extremely varied, ranging in frequency from the very low frequencies of the large baleen whales to the ultrasonic pulses of the echolocating dolphins. The short biosonar signals ("clicks") of the echolocating small odontocetes have peak frequencies ranging from 70 kHz to more than 150 kHz, with Source Levels up to 230 dB re 1µPa / 1m. The signals for social communication are usually below 25 kHz in the Odontocetes and below 5 kHz in the Mysticetes, with Source Levels ranging from 120 to more than 190dB re 1µPa / 1m.
The fin whale Balaenoptera physalus, the only mysticete constantly present in the Mediterranean Sea, emits mostly infrasonic signals (20-40 Hz) with Source Level up to 186 dB re 1µPa / 1m, which are emitted in long sequences and can be detected at large distances as the low frequencies propagate into water with little attenuation.
In the Odontocetes the propagation range of high-frequency echolocation signals was reported to reach 350 meters, while modulated whistles, with frequencies generally lower than 25 kHz, are usually detectable within 1-5 km. The sharp, broadband impulsive signals (clicks) emitted regularly by sperm whales (Physeter macrocephalus) while diving, containing frequencies ranging to more than 30 kHz with an estimated Source Level of 160-180 dB re 1µPa / 1m, are, instead, detectable within 10-15 km.
The distance of detection of these sounds varies widely, according to the frequency (for a given source level the propagation range is inversely proportional to frequency) and acoustic structure of the signal, the power of the source, the propagation pattern, and the environmental noise, mainly due to hydrodynamic and meteorological phenomena, microseismic phenomena of the sea bottom, and other biological sources, but also to the noise generated by everyday increasing human activities.

Underwater sound and its analysis

Hydrophones are the transducers that transform sounds propagating underwater into electrical signals. They are usually omnidirectional (receive from all directions at once) and may cover a wide range of frequencies, from a few Hz to more than 100 kHz. More complex hydrophone systems consisting of multiple transducers are also used. These hydrophone arrays are more directional and sensitive, typically used to locate acoustic sources.
In marine bioacoustics, hydrophones are mostly used in two ways: stationary, for the monitoring of a single area, or towed for continuous monitoring during navigation. Hydrophones may be electrically connected to data analyzers/recorders by wire or radio; e.g. a stationary hydrophone can be connected to a floating buoy with a radio that transmits the detected sounds to a receiving station on the coast, on a ship or also on an airplane.
For some applications, they may be packaged with a recorder and batteries to operate autonomously for periods of time extending from few hours to months. These devices can be deployed on the sea bottom, or, if small enough, attached to an animal.
Cetacean sounds detected by hydrophones may be visualized and analyzed in real time, and/or recorded for later processing.

Sound analysis can be based on dedicated digital systems, or can be carried out with standard computers equipped with suitable analog-to-digital converters and specific Digital Signal Processing (DSP) software.
Sounds can be displayed as oscillograms, to show the amplitude of the signal over time and its waveform. But spectrographic analysis is the most used as it shows the composition in frequency of the signals versus time; it is essential for the analysis of non stationary signals typically emitted by animals, i.e. sounds characterized by fast variations.
Results of sound analysis are generally displayed graphically as a spectrogram. Spectrograms make easy to correlate sound features to species, behaviours, and situations.

The spectrogram is a graph that shows the structure of an acoustic event, either audible or inaudible, in the time-frequency plan. In other words, the spectrogram shows the sound decomposed in its frequency components versus the time. On the x axis it is represented the time and on the y axis the frequencies; the intensity of the various components in the time-frequency plane is given by colours or by different grey levels. In origin produced by expensive and slow analogical equipment, the spectrogram can be now generated by a computer with a software that processes the sound recorded in digital format.
With this technique it is also possible to visualize and to study sounds that our ear cannot hear, as the ultrasounds of echolocating dolphins or the low frequencies of the large whales.

A digital spectrogram can be generated in real-time to allow an immediate visualization of the sounds received by an hydrophone, or in post-processing on sound files previously recorded. Using hydrophones (towed arrays, single hydrophones, sonobuoys and other types) we can listen to the sounds in the ocean. This technology allows us to detect presence of ships, and also of marine mammals.
Each species produces distinct sounds that can be used to detect and identify them from many kilometers away. The possibility of detecting and recording the sounds of marine mammals depends on the sensitivity, the bandwidth (frequency range) of the sensors, and the instruments they are connected to. The ambient noise also affects detection ability; for example, the presence of a noisy ship can limit or prevent detecting marine mammal sounds.

To acoustically detect the presence of marine mammals it's necessary to employ a system that is sensitive to the range of frequencies emitted by the animals. Each species emits its own set of signals. Some animals' sounds appear similar to those emitted by other species; other species are easy to identify because of their profoundly different sounds.
Listening systems limited to frequencies audible to humans, or even smaller ranges, like in many military systems, can detect only a small fraction of the possible sounds emitted by marine mammals. For a given frequency band, it is possible to define which species should be completely detectable, which can be detected partially, and which cannot be detected at all.Increasing the size of the band means increasing the probability of detecting various species. For example, for the frequency band ranging from 40 kHz to 100 kHz, one can detect the echolocation signals of dolphins, which normally have an energy peak from 30 to 100 KHz and are rarely detectable below 20 KHz. To be sure of detecting every species' emissions, a detection system (the hydrophone plus all the connected electronics) must have a frequency band from 10 Hz to 150 Hz.

To know more about underwater (bio)acoustics and required equipment, visit the page on underwater bioacoustics.

Spectrogram of Humpback whales' song

Marine Mammals of the Mediterranean Sea

In the Mediterranean Sea, 19 species of cetaceans can be encountered; 8 of them are considered common (Fin whale Balaenoptera physalus, Sperm whale Physeter macrocephalus, Striped dolphin Stenella coeruleoalba, Risso's dolphin Grampus griseus, long finned Pilot whale Globicephala melas, Bottlenose dolphin Tursiops truncatus, Common dolphin Delphinus delphis, Cuvier's beaked whale Ziphius cavirostris), while 4 are occasional (Minke whale Balaenoptera acutorostrata, Killer whale Orcinus orca, False killer whale Pseudorca crassidens, Rough toothed dolphin Steno bredanesis), and 6 accidental, alien to the Mediterranean, but occasionally sighted  in the last  120 years (among them the Humpback whale Megaptera novaeangliae occasionally appears in italian waters); moreover, we have to consider the presence of a small population of Harbour porpoise Phocoena phocoena in the Black Sea.

Among the occasional species, the Steno bredanensis has been observed frequently in recent years and thus it could be considered a common species rather than occasional.

Particular areas, like the Ligurian Sea, have proved to be of special conservational interest. It is a primary concern to scientists to produce abundance estimates of cetaceans in these areas and to discover the environmental parameters which affect their presence in order to calibrate study and conservational efforts.

The Mediterranean Monk Seal (Monachus monachus) is the only pinniped to be found within the Mediterranean Sea. It is now very rare and listed as an endangered species. The only known colonies are in the Alboran Basin and in the Aegean Sea. It is very unlikely that any animals will be encountered around Sicily or Malta.

Marine Mammals and noise

The possible negative impact of anthropogenic noise on the marine environment is now an important concern for scientists. The noise and vibrations produced by human activities, that may be defined as "acoustic pollution", may interfere in various ways with animal life.
The environment itself is certainly a source of noise: the swell, the wind, the rain, the microseism of the sea bed are all sources of acoustic signals with different features, but to which animals have adapted in the long course of evolution by developing suitable signals and communication schemes.
The most representative sources of noise are the sea traffic, the sonars, the underwater explosions, some military activities, some geophysical and oceanographical prospecting activities, the drilling of the sea bed for oil search.
Each of these sources has its own noise characteristics. Other than producing direct damage to hearing apparata (temporary and permanent hearing losses), noise can interfere with communication processes among animals, limiting their ability to communicate, to call and recognize each other during the reproductive period, for example, or their ability to identify obstacles and preys by means of their biosonar. Noise may therefore cause behavioural changes, decrease the reproductive rate, or induce the animals to abandon certain areas or avoid certain migration routes, thus causing serious ecological problems.

To know more about this problem visit the pages about the impact of noise on marine mammals and about current research projects on mitigation

Example of noises that can be heard by lowering an hydrophone into the Liguria Sea. Sonar pings at 4.8 kHz from a far military ship, a fast-ferry turbine at about 1.4 kHz and, below 1 kHz, jack hammers working on the coast at more than 30 miles of distance.

Selected Bibliography

AU W.W.L., 1993. The Sonar of Dolphins. Springer-Verlag: 1-277.

NOTARBARTOLO DI SCIARA G., DEMMA M., 1994. Guida dei mammiferi marini del Mediterraneo. Franco Muzzio Ed., Padova: 1-262.

PAVAN G., BORSANI J.F., 1997. Bioacoustic research on cetaceans in the Mediterranean Sea. Mar. Fresh. Behav. Physiol., 30: 99-123.

RICHARDSON W.J., GREENE C.R. JR, MALME C.J., THOMSON D.H., 1995. Marine Mammals and Noise. Academic Press: 1-576.

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Created August 2005; updated September 2006