Anthropogenic (human-made) sounds may affect the behavior of marine mammals. 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. For example, ringed seals showed no change in population density during the construction and drilling for an oil field (1 & 2), whereas sperm whales and pilot whales stopped vocalizing during transmissions of the Heard Island Feasibility Study sound source (3). In order to understand how anthropogenic sounds may impact marine life, the animal's reaction to known sounds must first be measured. Observations of normal behavior, "control" or "baseline" data, provide the reference points for measuring any changes occurring during or after sound exposure. It is important to obtain baseline data that describe both the typical value of the measurements and the range of natural variability. (see How do you determine if a sound source might affect a marine animal?`

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 (4). For example, sounds of the Surveillance Towed Array Sensor System Low Frequency Active (SURTASS LFA) sonar were played to singing humpback whales off Hawaii (5). During nine of the eighteen playbacks, the whale stopped singing. Of these nine, four stopped when joined with another whale, which is a normal 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 were found: the whale songs were 29% longer during transmissions (6) and remained 10% longer up to two hours after exposure (7).

The graphs show the distribution of humpback whale song length (in minutes) during control periods when no sounds were being played (top) and during experimental conditions when LFA sounds were being played (bottom). The two graphs look similar and show that there is considerable variation in the length of humpback whale songs. The songs were slightly longer during LFA playbacks. (8)

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 are also probably not cause for concern. Bowhead whales avoided seismic airgun operations by swimming away from the source. They also reduced dive times, the amount of time spent at the surface, and the number of blows per surfacing, but increased intervals between successive blows (9). In response to increased levels of Acoustic Thermometry of Ocean Climate (ATOC) sounds, humpback whales slightly increased their average dive time and travel distance. Longer dive times and travel distances were observed as the received level was increased, however these changes were well within the range of dive times and distances observed in the absence of the transmissions. This small response was seen in two separate experiments (10, 11, 12). Over a broader spatial scale, there was no difference in sighting rate (number of animals seen per kilometer searched) or distribution of humpback whales between a year when the source was off and two years when the source was on (13).

Humpback whales slightly increased their dive time and travel distance in response to increased levels of acoustic thermometry of ocean climate (ATOC) sounds. (14, 15, 16) Photo ©Tsuneo Nakamura.

In other cases, more significant changes in behavior have been observed. For example, beluga whales stopped feeding and swam rapidly away from approaching ice-breaking vessels. The whales traveled up to 80 km away from productive feeding areas and remained there for 1-2 days before returning (17). Some of the strongest reactions occur when the sounds are similar to those made by predators. Harbor seals responded to playbacks of mammal-eating killer whales and unfamiliar fish-eating killer whales, but did not respond to familiar calls of fish-eating killer whales (18). (For more information on killer whale calls see group-specific vocalizations). This also suggests that animals can become accustomed to sounds that appear harmless by learning from previous experiences and stop responding to them or habituate.

Sometimes it is difficult to know whether observed changes in behavior are due to sound or to other causes. Manatees reduced their use of critical habitats when continually disturbed by boats (19). Similarly, gray whales left a breeding lagoon in Baja California during extensive industrial activity and returned to the lagoon when the shipping decreased (20). It is not known whether these disturbances were related to increased anthropogenic sound levels or to the physical presence of increased numbers of boats.

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