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Acoustic Disturbance

Acoustic disturbance due to dredging, blasting, shipping, testing, recreational activities and other activities may affect humpback whales and their behavior.

Why is sound important to marine animals?
What are the effects of anthropogenic (human-generated) sound on marine animals?
Behavioral Changes
Strandings
Hearing Loss

Why is sound important to marine animals?

Hearing is the universal alerting sense in all vertebrates. Sound is important because animals are able to hear events all around them, no matter where their attention is focused. Although hearing is important to all animals, the special qualities of the undersea world emphasize the use of sound. The undersea world presents very different conditions for hearing. Sound travels much farther underwater than in air. The sounds produced by many marine mammals can project far beyond the horizon. Strong echoes are always present underwater, because sound travels without much loss and there are many underwater surfaces that reflect sound.  This sense of hearing provides an opportunity for underwater animals to detect events at very large distances. The speed of underwater sound is five times faster than sound traveling in the air and because sound travels much further underwater than in air, marine animals can perceive sound coming from much further distances than terrestrial animals.

Marine animals rely on sound for communication, navigation and feeding. Some marine mammals, such as toothed whales, use sound to identify objects such as food, obstacles, and other whales. Marine mammals also use sound to communicate over long and short distances. Communication over long distances is usually associated with reproduction, territoriality, and maintenance of group structure. Some whales and dolphins travel in large groups and over great distances, so it is important for them to use sound to communicate with one another to maintain the group. Communication over short distances among marine mammals is used in social interactions involving aggression, individual identification, and to maintain mother-offspring contact. Most marine mammals use sound to regain contact when members of a group are separated. Baleen whales may also use low frequency sound to aid in navigation. It has been suggested that large whales may be able to orient or navigate in relation to features on the seafloor using returning echoes of their low frequency vocalizations.

Research on how animals use sound in the sea continues to provide new information that begins to reveal how different sounds may be used in communication.  The NOAA Fisheries Ocean Acoustics Program is investigating all aspects of marine animal acoustic communication, hearing, and the effects of sound on beahvior and hearing in protected marine species.

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What are the effects of anthropogenic (human-generated) sound on marine animals?

How does anthropogenic (human-generated) sound affect marine animals? The ocean is full of both natural and anthropogenic sources of sound and attention has recently been focused on anthropogenic sources of sound in the oceans and their potentially harmful effects on marine animals. Researchers suggest that increased background noise and specific sound sources might impact marine animals in several ways. The effects vary depending upon the intensity and frequency of the sound, and other variables.

The potential impacts include sounds that:

  1. cause marine animals to alter their behavior
  2. prevent marine animals from hearing important sounds (masking)
  3. cause 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 also not clear how important these impacts are to the well being and long-term health of the animals and their populations.


Behavioral Changes

Some sounds may not cause any observable responses, while other sounds may cause subtle changes in diving, surfacing, or vocalization patterns or more significant changes in habitat use. Behavioral responses to sound vary greatly and depend on a number of factors. An individual animal's hearing sensitivity, tolerance to noise, exposure to the same noise in the past, behavior at the time of exposure, age, sex, and group composition all affect how it may respond. For example, sounds of the Surveillance Towed Array Sensor System Low Frequency Active (SURTASS LFA) sonar were played to singing humpback whales off Hawai‘i (Miller, P.J.O., Biassoni, N., Samuels, A. and Tyack, P.L. 2000. Whale songs lengthen in response to sonar. Nature 405(22 June 2000): 903). During nine of the eighteen playbacks, the whale stopped singing. Of these nine, four stopped when joined with another whale, which can be normal whale behavior. The other five may have stopped singing in response to the sound source, although whales stop singing without joining under normal conditions too. Other responses that were found include: the whale songs were 29% longer during transmissions and remained 10% longer up to two hours after exposure (Fristrup, K.M., Hatch, L.T. and Clark, C.W. 2003. Variation in humpback whale (Megaptera novaeangliae) song length in relation to low-frequency sound broadcasts. Journal of the Acoustical Society of America 113(6): 3411-3424).

Not all changes in behavior are cause for concern. Some marine animal responses are momentary inconsequential reactions, such as the turn of a head. Other responses are short-term and within the range of natural variation in these behaviors. These types of behavior change are probably not cause for concern.  

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Strandings

Marine mammals are known to strand, and unfortunately many stranded animals die. Strandings occur worldwide. Approximately 40,000 stranded marine mammals were reported in the United States alone by the National Marine Fisheries Service stranding network over the decade 1990-2000, with an average of 3,600 strandings per year. Most often the cause of death cannot be determined because of decomposition and other problems. One controversial issue is the relationship between anthropogenic (human-made) sounds and the stranding of cetaceans, particularly beaked whales. Beaked whale strandings are relatively rare, with 17 beaked whale strandings reported in the U.S. in 1999 and 5 strandings in 2000. Strandings of more than one beaked whale at the same time are very uncommon.

Over the past ten years four stranding events involving multiple beaked whales have been reported that coincided closely with military activities using sonars. Investigating these strandings is very difficult, because scientists must try to reconstruct what happened after the event, based on limited information. There are currently only limited publications describing and discussing these four strandings, and most of these publications have not undergone independent scientific review. Although these strandings are closely related in time and space to the operation of military sonars, the mechanism by which the sonars might have caused the strandings is still a mystery.

Hearing Loss

The basic mechanism of sound detection is the same in all mammals, whether they live on land or in the sea. Sound waves are transformed to neural impulses in the inner ear, which are interpreted by the brain as sounds. Exposure to loud sounds can interfere with this process and cause hearing impairment or loss.

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