4.2. Noise pollution
Sound is a very important component of marine environment. The light is limited underwater as it gets attenuated with water depth and turbidity, and chemical and tactile information exchange is restricted to animals in closer proximity. Due to its density, water is a great media of sound conduction. In the sea, sound travels faster than in the air and many marine organisms are using sounds as the most important mean of communication. Due to this, many marine animals have developed their auditory capabilities and rely on the sound to overcome the many difficulties that living in the sea implies. However, the advantages of using sound in the sea (to communicate, locate prey and navigate) have been jeopardized in recent times, as the world seas got noisier. Therefore, interferences in natural sound transmissions can cause serious damage to marine ecosystem.
Biological sources refer to the animals that produce sound underwater. Many marine organisms produce and perceive sounds to fit their environment, some use them in “passive” mode, others in “active” mode. Passive use of sound is when an animal does not actively generate sound impulses but only responds to them with a particular behavior – identification of predators, capture of prey, direction change etc. Active use of sound allows animals to: communicate during mating, search for food, fight for territory and social disputes, navigate over long distances, stun and catch the prey, produce alarm signals, distract a predator to escape from it.
Sound produced by marine organisms can be included in a very variable range of frequencies: from 0.1 Hz to over 200 kHz. Some ways to produce sounds include mechanical flapping of teeth or plates; rubbing bones, teeth or valve of shells; compression and decompression of the bladder through specialized muscles; the oscillation of the body; the distribution of fluids or gases within the body through sound-producing organs; the forced emission of fluids or gases outside the body. Marine invertebrates, such as shrimps, lobsters and crabs produce communicative sounds. Sound production has been revealed in more than 800 bony fish species and marine mammals create sounds to communicate the presence of danger, food, and exchange information on their position, identity, and territorial or reproductive status. Therefore, it is very important to keep the background noise in the sea at a level that ensures the continuous exchange of information between marine organisms.
The anthropogenic sources of underwater sound refer to the humanly generated noise that is generally categorized as impulsive (of short duration) or continuous (of long duration). Marine traffic, naval sonars, military exercises, construction of offshore and coastal works, explosions to demolish offshore structures, airgun for geospatial prospecting are all sources of underwater noise pollution. Anthropogenic noise was found to affect a wide range of marine species. The responses to noise depend on the received sound levels, its frequency and duration of animals’ exposure to it.
There are particularly sensitive marine species such as Cetaceans. The noise pollution detected in the Mediterranean is in the order of 190-230 dB peak and studies have identified acoustic pressure levels that induce Cetaceans to abnormal reactions from 120 dB, to the loss of temporary hearing sensitivity around 160 dB and the definitive loss of hearing sensitivity at levels above 180 dB.
Nowadays, sounds are recognized as indicators of diverse ecological processes, and when integrated across a landscape, are defined as a “soundscape”. The use of the non-invasive Passive Acoustic Monitoring (PAM) enables us to distinguish between healthy and deteriorating environments, so many scientists stress the importance of acoustic monitoring for distinguishing altered soundscapes from healthy underwater environments. The underlying assumption is that complex soundscape and signal diversity reflects healthy ecosystems, whereas a disturbed ecosystems’ soundscape would show gaps at frequencies where species had been lost.
4.2.1 Underwater communication and orientation: sound vs. light � main physical characteristics
Sound, as well as light, it is a wave phenomenon, it is a mechanical wave caused by pressure oscillation, that is a compression inside a substance. In the water sound propagates much faster in the air. Water is less compressible than air because it is a liquid and its molecules resist compression and they transmit the fastest wave. In the air the wave travels at a speed of around 1.200 kilometres for hour, covering a kilometer in just three seconds, while in the water it reaches over 5.300 kilometers for hour and in those same three seconds it would have already travelled about 4.5 kilometres. The speed depends on two factors: pressure and temperature, with the increase of these, the transmission speed also increases. The sound travels faster in the warm superficial waters and slower in the cold and deep waters. So when we are under water we cannot perceive where a noise comes from and above all we do not feel it perfectly. For the light there are two phenomena, refraction and reflection. When we are immersed in water we consider only the phenomenon of the refraction, based on the inclination of the sun's rays, that is, if they are not perpendicular, the objects will look a third larger than their original size. The angle formed by the rays, with the perpendicular, is called "angle of refraction". These are the reasons we see or feel differently when we are under water compared to when we are on the surface. But the animals are able to communicate because they use different methods.
4.2.2 Name and explain three activities in which sea animals relay onsound.
The transmission of sound through the water is fast and fluid because of its density. Marine animals have developed various forms in order to communicate effectively. The sound allows marine animals to communicate,locate prey and navigate. These animals produce sounds or ultrasounds,that can travel at high distances,which allow them to communicate with each other,but also to use them as an echolocation tool to search for prey or avoid possible obstacles. Some use low-frequency sounds similar to whistles that serve to identify an individual who is part of their group. Each group develops a different sound in order to facilitate communication and recognize who is part of it. Dolphins,for example,use echolocation,they emit sounds called clicks. This animals can know how far the objects are,as well as understanding their shape and density. These whistles, inaudible by human beings,collide against objects,returning an echo perceivable by dolphins even in very noisy environments. Thanks to this communication system they avoid becoming the predators' meal. Some scholars argue that dolphins have a specific sound system to alert them to the presence of a problem or food presence. They have also shown that when dolphins meet,they greet each other with their own vocabulary,which varies for each herd. Whales,on the other hand,are capable of producing strong melodic notes and tones commonly called whale songs. Only males produce these long and complex melodies. They are often produced in the mating season,so it is thought to be a way of letting prospective companions understand the desire to mate. When they are together they sing in harmony with each other. The males that have separated from the formation, even at long distances,can continue to sing the same melody remaining in tune with the other specimens. The songs can last from 5 minutes to half an hour. These marine animals also sing in other circumstances during hunting periods,when they have lost an element of the group.
4.2.3 What is passive use of sound?
Bioacustics studies the sounds and noises produced and perceived by animals and by humans.
The perception of sound and acoustic communication are widespread in the animal kingdom.
Acoustic ecology is a discipline created to study sound landscapes from a scientific point of view, even to study the acoustic components of an environment and their relationships even considering the noise of the physical environment and the increasing noise invasion.
Nowadays it has been demonstrated that anthropogenic noise has a significant impact on fauna both in natural environments and in anthropized environments.
The acoustic ecology also studies the soundscapes to evaluate in a objective way some parameters that can provide useful indications to the ecologist, also regardless of the recognition of the individual component species. For this reason, we are studying indices that can give a measure of the biological richness and biodiversity of an environment through the analysis of the sound landscape, to compare different environments, or to study the evolution of an environment in time to evaluate its evolution when subjected to external factors (biochemical changes, anthropization, introduction of alien species, noise pollution).
The subject of underwater bioacoustics is relatively recent and is destined to important future developments. The fear that noise could have a negative effect was born in the seventies.
Man-made noise overlaps with the frequency bands used by marine species and can interfere with their behaviour. Man's production categories of noise range from high-powered snap sources such as SONAR naval, military exercises, explosions to demolish structures or offshore extractions, blasting of war devices, airgun used in geo seismic outbreaks, which can be lethal at a short distance, to more discontinuous sources such as the construction works on the coast and naval traffic.
4.2.4 What is active use of sound?
The active use of sound is one of the two modes that marine animals use to orient themselves underwater.
It differs from the passive sound because the latter is used to identify predators, capture prey or change direction of travel, without generating sound pulses.
The use of active sound instead allows the animals to communicate underwater, for example they can send alarm signals or they can stun a prey or escape from an enemy. They can also use active sound to make long distances or to mate.
About 800 marine species are able to communicate, for example by rubbing bones or teeth, or by compressing and decompressing the bladder, with frequencies ranging from 1hz to 200khz.
Among the most famous cetaceans that use the active sound we have the humpback whale, whose song is very complicated and full of acoustic elements.
The songs are emitted especially during the mating period, when the males compete with each other for the females, singing up to 10 hours a day.
Moreover, recent studies have shown that whales also sing to look for food and to warn other members of their group.
Dolphins also communicate by emitting sounds, which reach up to 200khz, and can change their tone.
It is believed that theirs is a real language and is used to communicate between various groups.
The sound propagates under the surface of the water more efficiently, quickly and over much greater distances than in the air.
The main sources have been identified in navigation, in the activity of extracting gas and oil from the seabed, in that of researching the relative deposits and, above all, in the use of active sonar by military and commercial ships.
These sounds cause damage to marine animals and their sound propagation.
4.2.5 Name five different ways that sea animals are using to produce sounds!
Whale song consists of a series of sounds produced by whales in order to communicate.
The sounds are produced by passing air through a structure present in the head, similar to the nasal cavity of humans, called "phonic lips". When the air passes through this narrow passage, the membranes of the phonic lip are sucked together, causing the surrounding tissues to vibrate.
During the mating period, in spring, shoals of fish move to lay eggs. The males, to start the mating, court the females producing a sort of continuous croak, similar to the sound of a machine gun amplifying the sound thousands of times. The result is similar to that of a packed stadium, and the noise reaches up to 150 decibels.
The sea lion is a very noisy animal, in particular the males emit real roars and other vocalices to defend their territory and their group of females.
Dolphins (family Delphinidae) are mammals that live in water that produce whistles and a succession of intense sound pulses even at very high frequencies, up to 200 kilohertz (that is, about ten times greater than the sounds perceivable by man), even modifying them in tone. They have a "lexicon" that contains 15,000 distinct sounds.
The orcas emit a great variety of sounds to communicate, and each pod produces particular noises that the respective members recognize even in the distance. These animals use echolocation to communicate and hunt, emitting sounds that travel underwater until they meet other objects, at this point they bounce back and forth, revealing their position, size and shape.
4.2.6 Name five main anthropogenic sources of underwater noise!
Underwater anthropogenic noise is generated by fishing, shipping (with frequencies between 10 and 100 Hz), the exploration of the seabed, wind turbines, drilling and the sonar technology (Sound Navigation And Ranging, that produces a good 240 dB).
These noises can cause stress and even physical damage to the fish and cause them to change their behavior, which is why, if not properly regulated, the noises produced by humans can pose a threat to aquatic fauna.
For large boats, it has been calculated that the only sound produced by the propeller cavitation can spread to a radius of hundreds of kilometres around the ship that generated it. However, the noise generated varies greatly depending on the type, size, propulsion, design and cruising speed of the boat.
Underwater noise is recognized as a form of pollution that has a negative impact on marine mammals and the entire marine environment. The research on this matter also highlights areas of concentration of noisy activities and their adjacency and overlap with areas important for the survival and conservation of marine mammals. The overlap of multiple sources of noise can produce complex negative, synergistic and cumulative effects, which we can hardly evaluate. The research indicates the need for an overview of the multiple human activities that produce underwater noise and emphasizes the difficulty of obtaining information that is appropriate to the need to implement policies for the protection and conservation of the marine environment.
In the Marine Environment Directive (2008/56 / EC) the European Commission has explicitly included, among the forms of pollution, also underwater noise, inviting the Member States to conduct noise level assessments to define the "good environmental status" and "to want to take appropriate measures to try to counter it.
4.2.8 Cetaceans and noise pollution
Noise pollution is a new form of pollution that in recent years is gradually developing more and consists of
prolonged exposure to sounds and noises of high intensity. Mostly it is present in the cities, but this strange type of pollution is spreading always more even in natural environments. Although it is veryharmful to humans, we are not the only ones to suffer the consequences of this phenomenon. For example, cetaceans suffer from noise pollution, in particular dolphins. It has been studied that these frightening noises can cause, in dolphins, an increase in their energy use of 30.5%, thus leading to an increase in the metabolic rate and the necessity to feed to propel the beating of their fins of the animals on the run. It is therefore considered that many stretches could be due to cetaceans’ are exhaustion and disorientation after hours of escaping from an anthropogenic disturbance or from a loud noise. Whales are also affected, because of the high noise pollution in their habitats they find more and more difficult mutual communication about the location of food, the presence of predators and the care of children.
Often the areas with the greatest noise pollution are in or adjacent to areas important for the survival and conservation of marine mammals such as, for example, the Cetacean Sanctuary "Pelagos", the Ligurian Sea, the Sicilian Channel, and parts of the Hellenic trench and waters between the Balearic Islands and Spain. Another example of death caused by the acoustic pollution is that of the 9 giant squid found stranded near the Spanish coasts in the arly 2000s. In this area there are various oil companies that emit low frequency sounds (less than 100 hz) thus damaging the hearing of the squid that, disoriented, would have gone into hot water areas and died suffocated. Countries like USA have already imposed severe restrictions on the use of low prospecting systems frequency while Europe is raising awareness on the subject.
4.2.9 The Passive Acoustic Monitoring (PAM)
Passive Acoustic Monitoring (PAM) in the acquatic environment is the use of underwater microphones to monitor, detect and vocalising marine mammals. The word "passive" refers to the fact that we have only to listen and do not make noise into the environment: that's why this method should not be confused with active acoustic systems, such as Acoustic Deterrent Devices, Acoustic Harassment Devices, and sonar used for navigational and other purposes.
The sounds produced during some surveys might have physical, physiological and behavioural effects on underwater life, such as marine mammals like dolphins, porpoises, whales and seals.
Quite the opposite of what we think, the man-made underwater noise is often carefully regulated.
The noises in the ocean can be divided in three types: anthropogenic noise (man made, shipping, pile driving), noises from ocean fauna (whales, fishes) and natural non biological noises (wind, rain, waves, seismic events).
Passive Acoustic Monitoring has now been used in several large projects that have produced results that could not be achieved by other methods within realistic budgets, but it is a young method with multiple challenges.
For example, in 2008, 5 suspected fish sounds were identified in passive acoustic recordings from fixed recorders and autonomous underwater vehicles in the eastern Gulf of Mexico. Data were collected in depths ranging from 4 to 984 m covering approximately 39000 km2. The goals of this research were to map the spatial and temporal occurrence of these sounds. Sound production was correlated to environmental parameters like water depth, lunar cycle, and dawn and dusk, to understand the variability in seasonal calling.