Acoustic monitoring of animal vocalizations is often used in conjunction with visual observations and surveys. Passive acoustic monitoring is a powerful tool compared to visual surveys because sound can travel great distances underwater. This not only allows for the detection of marine animals outside the visual range of the observers, but it can also provide information as to the location of the animals relative to the observation vessel. However, acoustic monitoring is only effective to study animals that are vocalizing. The absence of vocalizations does not necessarily mean that animals are not present.

Hydrophones, such as this array being towed from a ship, pick up sounds in the water and help locate the animals that are producing the sounds.

In addition to using towed listening arrays, scientists use a network of hydrophones on the seafloor operated by the U.S. Navy to record animal sounds and determine where they come from. For animals that regularly make sounds, this system can track the animals from a distance, measuring both their patterns of movement and patterns of sound production (See How is sound used to study marine mammal distribution?).

Acoustic monitoring is also used to determine whether or not a sound source is affecting the behavior of marine animals. For example, dolphins and beluga whales have been found to shift the frequency of their clicks to avoid noise in the normal frequency range of their echolocation (1). This indicates that noise may be affecting the animals by reducing the efficiency of their echolocation. However, the animals are able to compensate for this noise by changing the structure of their echolocation clicks. Acoustic monitoring has also been used to show that the average length of the humpback whale song increased during and following SURTASS-LFA sonar transmissions, although much of the variation in song length was not related to the transmissions (2).

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