The ocean is full of both natural and anthropogenic sources of sound. Much attention has recently been focused on anthropogenic sources of sound in the ocean and their potentially harmful effects on marine animals. This has become a topic of increasing controversy, especially regarding marine mammals. Only by understanding how marine animals hear can researchers address the question of potential impacts.
Hearing is important because animals are able to detect sounds generated all around them, no matter where their attention is focused. Many species of blind amphibians, reptiles, fishes and mammals are known, but no naturally profoundly deaf vertebrate species have been discovered. Although hearing is important to all animals, the special qualities of the undersea world emphasize the utility of sound.
Sound travels far greater distances than light under water. Light travels only a few hundred meters in the ocean before it is absorbed or scattered. Even where light is available, it is more difficult to see as far under water as in air, limiting vision in the marine environment. It is similar to looking through fog on land. Sound travels much farther and faster underwater than in air. The sounds produced by many marine mammals travel for miles. Underwater sound allows marine animals to gather information and communicate with each other at great distances and from all directions.
Hearing is the universal alerting sense in all vertebrates. Marine vertebrates, i.e. fishes and marine mammals, rely on sound to acoustically sense their surroundings, communicate, locate food, and protect themselves underwater. In addition, by emitting clicks, or short pulses of sound, most toothed whales can listen for echoes and detect prey items, or navigate around objects. It is clear that producing and hearing sound is vital to marine mammal survival. (Please visit the DOSITS Audio Gallery to hear marine mammal sounds.)
Sound is also important to fishes. They produce various sounds, including grunts, croaks, clicks, and snaps, that are used to attract mates as well as ward off predators (Please visit the DOSITS Audio Gallery to hear fish sounds.) For the oyster toadfish, sound production is very important in courtship rituals. Sound is produced by the male oyster toadfish to attract the female for mating and is especially important in the murky waters that oyster toadfish inhabit where visibility is limited.
Marine invertebrates also rely on sound for mating and protection. Little research has been done on sound-producing marine invertebrates, but available research indicates that sound is very important for survival against predators for shrimp and lobsters.
Marine mammals evolved from mammalian ancestors that had air-adapted ears. Many of the structures of the ear in both land and marine mammals, therefore, are similar. Hearing is the result of the combined activity of the ear's three basic divisions: (1) the outer ear, which collects and directs sound, (2) the middle ear, which filters and amplifies the acoustic energy to the inner ear, and (3) the inner ear, which transforms the acoustic energy to electrical signals (neural impulses) that are processed by the brain.
The most dramatic changes in hearing among mammals can be found in the evolution of cetaceans (whales, dolphins and porpoises) and the sirenians (manatees and dugongs). Like some pinnipeds (seals, sea lions, and walruses), cetaceans have no external pinnae. But, unlike pinnipeds, the ear canals of cetaceans are not thought to be functional. In odontocetes (toothed whales) the ear canal is narrow and plugged with debris and dense wax. Additionally, the ear canals do not attach to the tympanic membrane (eardrum). In mysticetes (baleen whales) , the narrow ear canal is terminated by a waxy cap. If the ear canals do not attach to the tympanic membrane, how is sound getting to the middle ear? In cetaceans, sound is channeled from their environment to the middle ear through fats around the lower jaw.
Fish have developed special sensory adaptations for detecting and interpreting sounds. Two independent but related sensory systems used by fish to "hear" are the inner ear (the auditory system), and to a lesser extent, the lateral line system, which is generally used to detect vibration and water flow.
The Inner Ear
The bodies of fish have approximately the same density as water, so sound passes right through their bodies, which move in concert with the traveling sound wave. Fish have bones in the inner ear, called otoliths, which are much denser than water and the rest of the fish's body. As a result, these ear bones move more slowly in response to sound waves than does the rest of the fish. The difference between the motion of the fish’s body and the otoliths bend or displace the cilia on the sensory hair cells that are located in the inner ear. This differential movement between the sensory cells and the otolith is interpreted as sound. Otoliths are made of calcium carbonate and their size and shape is highly variable among fish species.
The Lateral Line
Sound passing through water also creates particle motion close to the source of the sound. Fishes have organs called neuromasts on their skin or in canals below the skin’s surface. These are composed of hair cells, like the inner ear. They detect the relative motion between themselves and the surrounding water. Fishes can use the lateral line system to detect acoustic signals at short range, over a distance of one to two body lengths, and at low frequencies (lower than 160 to 200 Hz).
Many marine arthropods (crabs, lobster, shrimp) have special sensory organs known as chordotonal organs, which are a type of internal mechanoreceptor. These organs sense pressure, movement, and tension. They detect cues generated from vibrations that may be associated with sound. Most marine invertebrates spend most of their time on some type of substrate rather than swimming about in the water. Stimuli produced by other organisms (e.g. predators or prey) can be transmitted through the substrate and detected by chordotonal organs.
Marine mammals face threats from many different human activities, including fishing, habitat destruction, ship strikes, whaling, and underwater sound production. Under certain conditions, sound sources such as Navy sonars, oil and gas airguns, and construction noises can affect marine animals. Of these threats, fisheries bycatch kills the most marine mammals. Globally, it is estimated that more than 650,000 marine mammals are killed annually by being accidentally caught in fishing nets. Although underwater sound can affect marine mammals, it is very rare for marine animals to die from underwater sound. Stranding events involving multiple beaked whales have been reported that coincided closely in time and space with military activities using sonar, with only a small number of deaths. The more serious concern is the increase in background noise from human activities that may reduce the distances at which animals can hear.
Research indicates that increased background noise and specific sound sources can impact marine animals in several ways (What are the effects of sound on marine animals?). The effects vary depending upon the intensity and frequency of the sound, and other variables including the hearing sensitivity of marine animals.
The potential impacts of underwater sound include:
- causing marine animals to alter their behavior
- preventing marine animals from hearing important sounds (masking)
- causing hearing loss (temporary or permanent) or tissue damage in marine animals
A number of factors affect the impact of sounds on marine animals. These include the sound level, frequency, and other characteristics of the sounds; the hearing sensitivity, age, sex, and behavior of the animals; and the environmental conditions under which the animals experience the sound. It is not clear how important these impacts are to the well being of the animals and their populations. Research is often limited to observing short-term changes in behavior or hearing, but it is very difficult to determine whether there are long-term effects to breeding, feeding, or survival.
Current knowledge about the effects of sound on marine animals is based on research on a small number of animals and is complicated by differences between the individual animals. It is not appropriate to extrapolate scientific results from one study to animals of other species, animals engaged in different behaviors, or animal groups with different age or sex structures. It is difficult and expensive to study marine animals. Much more scientific research is needed to fully understand the effects of anthropogenic sounds on marine animals.
A consortium of U.S. federal government scientists developed a research plan to address the effects of anthropogenic sound on marine life (Southall et al. 2009 http://www.sea-inc.net/resources/mmnoise_IATF_finalreport.pdf). They prioritized scientific studies that are most needed to better understand this problem. The following topics were classified as high importance:
- Improve ability to identify and understand biologically-significant effects of sound exposure in order to improve effectiveness and efficiency of efforts to mitigate risk.
- Hearing, physiological, behavioral, and effects data (e.g., controlled exposure studies) for key species of concern (baleen whales, beaked whales, Arctic and endangered species).
- Develop new technologies (e.g., acoustic monitoring) to detect, identify, locate, and track marine mammals in order to increase the effectiveness of detection and mitigation.
- Develop and validate mitigation measures to minimize demonstrated adverse effects from anthropogenic noise.
- Support the development, standardization, and integration of online data archives of marine mammal distribution, abundance, and movement for use in assessing potential risk to marine mammals from sound-producing activities.
- Long term biological and ambient noise measurements in high-priority areas (e.g., Arctic, protected areas, commerce hubs).
- Test/validate mitigating technologies to minimize sound output and/or explore alternatives to sound sources with adverse effects (e.g., alternative sonar waveforms).