Underwater noise

blue whale in the oceanClick to enlarge
Blue whales migrate thousands of kilometres a year to build up fat reserves in the Antarctic.
Source: L. Lehnert/ITAW

Whales and seals depend on their sense of hearing for survival. Man-made underwater sounds can impair their hearing in both the short and long term, displace them from vital habitats, cause a change in important patterns of behaviour and thereby deteriorate the survival capacity of these marine mammals.

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Underwater noise - a man-made problem

Most whales and seals, many fish and even some invertebrates such as squid rely on acoustic signals for a great number of basic activities, including communication, mate selection, location of prey, protection against predators, or navigation. A change in ambient noise can have a negative impact on the biological fitness of individual animals or even entire populations.
The oceans around Antarctica are of utmost importance to many whales and seals. Not only do they find excellent feeding grounds in the Antarctic summer, it is also one of the few regions in the world with little exposure to noise pollution from human activities.
Man-made background noise pollution is increasing in all of the world's oceans; it has doubled and even increased fourfold over the last 50 years in some regions.

Anthropogenic underwater noise is caused mainly by shipping, the use of various types of sonar, during construction and operation of offshore installations such as wind energy farms or oil and gas platforms, or during seismic activities which occur during oil and gas exploration and research activity. Depending on the source sound level and frequency, the acoustic and geometric properties of the body of water and the ocean floor, underwater sound can travel across great distances. The oceans of the southern hemisphere are quieter than the rest of the world. There is far less shipping traffic, which is why the predominantly low-frequency background noise it creates is more negligible.

Man-made underwater sound in the Southern Ocean

The background noise from shipping traffic in the southern hemisphere is 20 dB quieter than elsewhere in the world. There is no exploration for mineral resources in the Antarctic or construction of offshore installations such as wind farms. In addition, only few ships are equipped for travel in the oceans near the Antarctic continent. However, shipping traffic is highly concentrated during the short period of the Antarctic summer and in specific regions, for example the Antarctic Peninsula which is visited frequently by tourists and researchers.

Most people travel to the Antarctic by ship. There were at least 95 tourist vessels, 40 research vessels and 46 fishing vessels in the waters around the Antarctic during the 2012/2013 season. In addition, there are yachts and a few illegal fishing boats, most of which make several journeys per season. Every one of these ships imports mainly low-frequency, continuous sound into the ocean. Research vessels also use airguns or sonars which emit sound wave impulses with significant sound levels. Airguns used for scientific purposes can reach peak sound levels of up to 250 dB, which is 1,000 times louder than a ship. The majority of this noise is in the low-frequency range of up to 300 Hz, which is also the main range of communication between baleen whales like the blue and humpback common to the Southern Ocean.

How does underwater sound impact marine mammals?

A number of different effects are possible depending on the intensity, length and context of sonication. Extreme sound events, for example underwater detonations, can result in massive injuries and even death of animals. Less intensive sound events such as the use of airguns to explore the ocean floor can also damage the hearing system (acoustic trauma) of a marine mammal and result in short-term or even permanent loss of certain hearing abilities. The natural patterns of behaviour of marine mammals may be altered if the animal is further away from the noise source or the sound source is quieter, e.g. shipping noise. Acoustic disturbances may have a number of different effects: many whales will leave an area if it is too loud (flight reflex); others will become louder in a louder environment while still others cease to communicate entirely (altered communication behaviour). Marine noise can distract animals in such a way that they detect prey or a dangerous predator either too late or not at all (acoustic masking).

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Hearing damage from underwater sound

Sound, and sound impulses in particular, can damage the hearing ability of marine animals or cause temporary threshold shifts (TTS) or even permanent threshold shifts (PTS) in hearing, that is, hardness of hearing. High-intensity sound overstimulates the auditory cells in the inner ear (and their supportive structures). The development of hardness of hearing depends on many factors which include the length of exposure to noise, sound level, frequency spectrum of the impacting noise, the existence of possible noise intermissions, predisposition, and other individual factors.

TTS is temporary hearing fatigue which is triggered by biochemical and physical processes and is generally reversible. The hearing threshold usually returns to normal in a matter of hours or days. Nevertheless, it may lead to a neuronal degeneration of the synaptic contacts between auditory cells and nerve tissue in the long term and delayed hardness of hearing in old age. There is only very little research on this topic in relation to marine mammals and none at all on great whales. Little is known about the effects of repeated incidences of TTS. Research on mice and guinea pigs indicates that repeated episodes of TTS can cause cumulative damage to hearing. Tests on a harbour porpoise, the only whale species native to Germany, triggered TTS in an animal exposed to one single airgun burst with a peak sound pressure of 194 dB measured near the porpoise.

The BMUB has published a Sound Protection Concept for harbour porpoises in Germany which incorporates the UBA recommendation to apply twin criteria in noise protection: pile driving activities at offshore wind farms may not expose harbour porpoises to  sound levels which could impair hearing (TTS). At a distance of 750 metres from the sound source, a single sound event may not exceed a sound exposure level (SEL) of 160 dB and a peak sound pressure level (SPLp-p) of 190 dB, if the current state of knowledge can rule out harm to harbour porpoises. There are not yet any comparable protection guideline values for whales in the Southern Ocean. UBA is working to ensure that an international sound protection concept is developed for the 20 species of marine mammals (whales and seals) native to the Antarctic.

 

Interference from underwater noise

Man-made noise, even if it causes no immediate injury, can have a negative impact on the ability of individuals or entire populations to survive. The low-frequency sound waves emitted by ships and research equipment are not absorbed as much as higher frequency sounds and are thus perceived as louder and at greater distances in the ocean. The sound blasts from airguns, for example, can be heard at distances of more than 1,000 kilometres across an entire ocean basin.

Anthropogenic, or man-made, underwater noise can be so disruptive that it displaces marine mammals from their habitats, interrupts important behaviour or creates a change in communication strategy. This noise can also lead to "acoustic masking" of other ambient signals: if acoustic signals are masked – essentially diminishing a marine mammal's 'field of vision' – this can impair the biological fitness of animals which are already endangered, such as, for example, the blue and fin whales.

  • Changes in behaviour as a result of acoustic signals can be quite varied. The most easily recognisable effect of noise is avoidance: noise disturbances can result in animals leaving the area to escape the noise. Acoustic disturbance can also cause animals to interrupt their feeding and necessary inactive phases, affect avoidance behaviour when in the vicinity of predators, prevent migration or otherwise interrupt the feeding and rearing of their young.
  • When biologically important sounds are "masked", they can, for example, reduce the acoustic range necessary for communication or for finding food, or it may reduce the information which ambient signals contain. These effects depend in particular on the sound level and on the frequencies of the masked sound source.

Quantification of these effects is complex and efforts are now only in their infancy. Work is being done at international level to develop models which can evaluate and quantify the impact of disturbances on populations of marine mammals. One such model is the Population Consequences of other Disturbances (PCoD).

The UBA commissioned a research project to analyse the effects of airgun signals at distances of up to 2,000 km in order to better assess the potential for masking in marine mammals exposed to underwater noise from airguns. The models developed in the study show that airgun signals at distances of 1,000 km and more are received as continuous sound and can reduce the natural communication distance of blue and fin whales in the Antarctic, for example, to 1%.

The Federal Ministry for the Environment, Nature Conservation and Building (BMUB) developed a sound protection concept for the North Sea to reduce the impact of harmful noise which can cause injury or otherwise disturb harbour porpoises in Germany. The protection concept is based on studies which reveal significant avoidance behaviour at sound exposure levels (SEL) around 140 dB.

There is as yet no corresponding sound protection concept for whales in the Southern Ocean. UBA is campaigning at international level for the development of a sound protection concept for the 20 marine mammals (whales and seals) native to the Antarctic.