The first to listen to sounds and noises below the sea surface were the military, in order to identify boats and submarines by the noise of their propellers. Yet the sea also has a much more pleasant sound, made of animal voices communicating by means of a complex music, like the songs of male humpback whales (Megaptera novae angliae) calling their females during their reproduction period with melodious songs that may be heard hundreds of kilometres away.
The science studying animal sounds is bioacoustics and, in close connection with ethology, it tries to understand how do animals regulate their individual and social behaviour through acoustics messages. Sound Libraries are a fundamental part of bioacoustics research: their are national or international institutions, somehow similar to museums, which collect, file and document recordings of animal acoustic signals coming from all over the world to make them available both to the scientific community and for educational activities.
Acoustic communication plays a basic role in aquatic environments: a high propagation speed of about 1.500 metres per second, five times higher than in air, and a scarce attenuation due to distance enable in fact an effective sound transmission.
Several aquatic organisms produce sounds and noises, invertebrates (mostly crustaceans), fishes and aquatic mammals (cetaceans and pinnipeds), with emission frequencies ranging from infrasonic waves to ultrasounds. Their acoustic signals integrate with the natural sound of the environment forming a complex acoustic system increasingly influenced by man as well. Underwater new sounds intertwine more and more: they are signals, sonorous and ultrasonic, produced by new human technologies for navigation, to spot boats and submarines, to look for wrecks, to convey information, to sound the bottom of the sea, to measure sea temperature, to spot shoals of fish, for oil or mining prospecting. The hydrophones are acoustic transducers picking up the sounds and vibrations conveyed by water, they are usually omnidirectional and may cover a wide range of frequencies, from a few Hertz to over 100 Kilohertz.
Underwater bioacoustics research makes use of two main types of fixed mono and multi-hydrophonic systems for the control of given areas, and multi-hydrophonic towed arrays for continuous sensing during navigation. The signals sensed by the hydrophones are either displayed and analyzed in real-time or recorded by means af appropriate systems to be filed and processed later on.
Sound analysis enables to describe and graphically represent the features of signals in order to understand their structure and link them to the species, to the individuals, to behaviour and to situations that have been studied. The most widespread kind of analysis is the spectrographic analysis which is digitally carried out by means of computers supplied with appropriate numeric conversion devices of analogue signals. It displays the frequency composition of the signals according to time. It also reveals to be necessary for the analysis of non-stationary signals, that are characterized by rapid modulations of the spectrum according to time; these are signals typical of animals.
In crustaceoans and in fish the production of sounds is widespread but of which little is known; in Telcosteous fish more than 50 families include species producing sounds, generally with low intensity and frequencies lower than 2 kHz pertaining to short distance communication.
In Cetaceans acoustic communication has gained ground as compared to other forms of communication. Their organs for sound reception and production have evolved and diversified, acquiring the echolocation function (biosonar, or biological sonar) typical of Odontoceti, which has however developed, in aerial environment, in bats as well. The production of acoustic signals is quite varied in Cetaceans, both for echolocation in Odontoceti, with frequencies over 25 kHz ranging to more than 150 khz and acoustic pressures over 200 dB SPL (ref. 1 m Pa/1m) and for lower frequencies communication signals, generally lower than 25 kHz in Odontoceti and lower than 5 kHz in Mysticeti (toothed and baleen whales respectively). The sensing distance for those sounds varies a lot: it depends on frequency (transmission improves with the decrease of frequency), signal structure, signal source power, propagation features and background noise, both natural and due to human activities.
In dolphins echolocation signals transmission range may reach 350 metres, while modulated whistles, having a frequency usually lower than 25 kHz, like those released by the striped dolphin (Stenella coeruleoalba), very common in the Mediterranean, may be easily detected within one kilometre. The sperm whale (Physeter catodon), the largest Odontocete, present in the Mediterranean Sea but very difficult to observe, is a basic animal for bioacoustics research. Very little is known of its behaviour and of its population in our seas. It makes long dives, often lasting 40 to 50 minutes, but sometimes even more, and it re-surfaces, barely visible, betrayed only by its typical forward bent spout. When diving it emitted peculiar impulsive sounds, called clicks, with a frequency extension over 30 kHz and high intensity, repeated in long sequences which may be sensed up to 15 kilometres away. The propagation distances of the low frequency Mysticeti signals are on the contrary longer, of about some dozens Km, even over 100 for the humpback whale. The sperm whale (Balenoptera physalus) is the only Mysticete constantly present in the Mediterranean Sea and it has been recently shown, thanks to genetics research, that it belongs to a local population. It produced sounds up to 1 kHz, and very low frequency signals, about 20-40 Hz, emitted in long sequences and that may be sensed even at a very long distance.
Their sounds have long been regarded has a mystery by military sonar operators, and today military technologies are made available to the scientific community in order to study the behaviour of marine mammals on a large scale. In 1993 the US Navy actually launched a programme for the use of its underwater monitoring equipment located in the Atlantic Ocean (IUSS - Integrated Undersea Surveillance Systems) to take a census of whales and study them: this project has already reached considerable results. In the framework of 1995 ENCY (European Nature Conservation Year), the Italian Military Navy as well has launched a co-operation project with the Institutions in charge of research and development concerning Cetaceans in the Mediterranean Sea.
In the Mediterranean Sea 19 species of Cetaceans were reported, out of which only 8 are to be regarded as common (Balenoptera physalus, Physeter catodon, Stenella coeruleoalba, Grampus grisens, Globicephala melas, Tursiops truncatus, Delphinus delphus, Ziphius cavirostris), 4 are only occasionally present (Balaenoptera acutorostrata, Orcoms orca, Pseudorca crassidens, Steno bredanensis) and the rest are only present accidentally, being alien to the Mediterranean Sea but occasionally reported over the last 120 years. A minor presence of Phocoena phocoena is moreover reported in the Black Sea.
The study of Cetaceans in the Mediterranean Sea has greatly developed over the last few years. Starting from 1989, the Multidisciplinary Centre of Bioacoustics of the University of Pavia and the Tethys Institute in Milan have been organising cruises to carry out studies about the sounds produced by Cetaceans, in the framework of wider biogeographical projects. The Sound Library collecting all the recordings made until now is to be considered as unique in Europe.
Bioacoustics has a leading role within the activities aiming at protecting those animals and the marine environment: the fact of recognising the typical signals of each species allows specific identification and some times the sensing and census taking of animals even at a long distance or without direct observation, by night for example. Acoustic sensing therefore integrates traditional techniques giving the chance to detect and approach species otherwise difficult to observe.
Studies focusing on the fine echolocation skills that are a peculiar feature of Odontoceti, are carried out in order to improve the performances of electronic sonars and, as far as the serious problem of the accidental capture of Cetaceans in fishing nets is concerned to develop devices to be used to warn dolphins about the presence of such nets, or devices that make the nets themselves more perceptible by means of the biosonar they naturally have.
Moreover the possible negative impact of noise on marine environment and on Cetaceans in particular is not to be disregarded.
The noise and vibrations produced by human activities and that may be defined as "acoustic pollution" may interfere in various ways with animal life. The environment itself certainly is a source of noise: the swell, the wind, the rain, the microseisms on the sea bottom are all sources of acoustic signals having different features, but to which animals have become well adapted in the course of evolution by creating suitable communication schemes. Noises produced by man can interfere with communication processes among animals, limiting their ability to communicate, to call and recognise each other during their reproductive period, for example, or their ability to signal danger or to identify obstacles by means of their biosonar. Noise might therefore cause behavioural changes, decrease the reproduction ability or induce the animals to desert certain areas, thus causing serious ecological unbalances. A further study of these aspects will be of paramount importance in order to formulate new and more precise rules for navigation and for all those activities which are potentially dangerous for the marine environment and in particular for those areas which are protected as Marine Parks and Reserves.
Marine bioacoustics research has opened new exciting cultural and scientific horizons, giving also practical hints for the protection of marine environment: notwithstanding its huge extension and dimensions, it results to be a heritage of increasing fragility that needs all possible efforts so that it may be well-known and protected. Only rather recently, has research been started in the Mediterranean Sea, and it mainly aims at preservation and scientific purposes: they will certainly give a basic contribution to a further knowledge, exploitation and protection of the Mediterranean Sea fauna, environment and biological balance.
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