Principle of an active sonar on any source, animals or humans are illustrated above © Georg Wiora CC-BY-SA-2.5.
The ocean is mostly dark, as light only penetrates a few hundred feet, so seeing is of limited use. Sound waves travel in darkness and very far, and sound is the absolute major information channel of many marine organisms, especially marine mammals. Whales and dolphins for example use hearing for orientation and navigation. They use sounds, emitting and receiving them, to find prey, avoid predators, communicate with each other for maintaining family and social bonds. One essential advantage of sound is that animals are able to hear from all around them, no matter where their attention is focused. But in order to be able to use sounds to communicate and orientate, be alerted and find food (echolocation), marine mammals need to be able to hear the returning sounds .
Oceans are filled with sounds. They can be natural, like those from wave activity and marine life such as marine mammals, fish, some invertebrates. They can be generated by human activities (anthropogenic). Sound is classified as chronic – low in intensity but more constant such as from shipping (motor and sonar) and industrial activities, or acute – high in intensity and shorter in duration such as from explosion, pile-driving, seismic exploration and military sonar activity.
In just the last 100 years, human activities have increased along coasts, further offshore, and in deep ocean environments. The resulting increases and changes in ocean noise levels can have major and even fatal impacts on marine life. Collectively they raise the background noise, creating a continuous background rumble and cacophony which may obscures animal hearing and reduces the ability of animals to communicate, avoid predators, find food and navigate.
Single noise/acute sources, such as pile-driving activities, underwater detonations or low- and mid-frequency active sonar can cause disturbance and may create temporary or permanent reduced hearing or hearing loss. It may occur suddenly or develop gradually over time. At high exposure levels, single noise can create physical injury and even death.
Low frequency, active (both transmitting and listening) sonars used by the military (LFA sonar) to detect submarines has the potential to kill, deafen and/or disorient whales, dolphins and all marine life, as well as humans. It is the loudest sound ever put into the world’s oceans. Military sonar activities have been linked to the mass stranding of some deep diving cetaceans all over the world. Necropsies have revealed internal injuries such as haemorrhages and tissue damage in ears, brain and lungs, as well as symptoms of decompression sickness.