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Marine Mammal Acoustic Behavior
to select prey, it would need to use a high-frequency sonar signal and have an auditory system capable of detecting fre- quencies more than four times higher than those humans can hear. These remarkable capabilities of bats and toothed whales to produce and to hear signals of unusually high fre- quency are thought to have evolved in response to the need for their sonars to resolve small targets.
Some fish have a gas-filled swim bladder that is used to maintain buoyancy. This gas-filled bladder is an excellent sonar target, especially when ensonified with sound at the resonant frequency of the bladder. This is much lower in fre- quency than for a solid target of the same size, with swim bladders typically having resonance frequencies of a few ki- lohertz. At the higher frequencies of toothed whale clicks, each size and kind of fish reflect different patterns of sound, allowing echolocating whales to select prey.
The range at which an echolocator can detect a target de- pends on the intensity of the sonar signal, on how much sound energy is lost as it propagates from the echolocator to the target and back, and on how much of the sound energy hitting a target reflects back. Sound energy is absorbed by seawater molecules either by viscous or chemical interac- tions, especially at higher frequencies. For example, a sonar signal at 100 kHz loses between a quarter and a half of its energy as it passes through 100 m of seawater. Absorption thus limits the effective range of high-frequency echoloca- tion from tens to a few hundreds of meters.
Communication
Long-Range Propagation of the
Low-Frequency Calls of Fin Whales
In contrast to the absorption of high-frequency sound in seawater, low-frequency sound can travel over much longer ranges than we are used to for sound in air. Low-frequency sounds below about 100 Hz have negligible absorption even when traveling thousands of kilometers through the ocean. In addition, in the deep ocean, there is a special sound chan- nel that allows sound to spread with little loss over great ranges. When a sound is made in the open ocean, down- ward heading sound rays will refract back upward as they encounter water that is denser due to hydrostatic pressure. And upward heading sound rays refract downward as they near warmer surface waters in most of the ocean. These two phenomena cause sound energy to concentrate at depths where the sound speed is at a minimum, around 1,000 m in temperate and tropical seas, leading this to be called the deep sound channel.
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The physics of sound propagation in the deep sound channel was understood by the end of World War II. During the start of the Cold War in the 1950s, the US Navy developed an un- derwater listening system codenamed “Jezebel” to take ad- vantage of the deep sound channel to detect low-frequency sounds at great ranges (Nishimura, 1994; http://www.dosits. org/people/history/SOSUShistory/). As soon as the Navy was able to listen to low frequencies, Navy acousticians of- ten heard low-frequency sounds that they called the “Jeze monster.” This discovery of an unknown sound source in the ocean led marine bioacousticians on a search for the Jeze monster, which finally was identified as the 20-Hz calls of fin whales (Schevill et al., 1964; http://www.dosits.org/files/ dosits/fin7.mp3). This kind of detective work formed the ba- sis of the first generation of marine mammal bioacoustics during the 1950s and 1960s.
Payne and Webb (1971) used early data on the low-frequency calls of finback whales to report that if finback sounds spread evenly in all three dimensions, they would be detectable out to about 100 km but that spreading in the deep sound chan- nel would enable these calls to be detected at ranges of 1,000 km or more. Despite such a compelling prediction, it was not until after the end of the Cold War that biologists were able to demonstrate such long-range propagation of whale calls.
The problem was one that is common in ocean science: how to develop appropriate scales of measurement. During the 1960s and 1970s, marine mammal bioacousticians learned how to record the sounds of marine mammals from small vessels, where they could get close enough to link a call to an animal under visual observation. We knew that these whales swam thousands of kilometers, and Payne and Webb (1971) had hypothesized that their sounds could travel over simi- larly large ranges. But it was not until naval technologies for tracking underwater sounds were made available to whale biologists that investigators could actually test long-range detection of whale calls.
Fin and blue whales tend to disperse into deep oceanic areas where they produce these calls during the winter breeding season. Stafford et al. (1998) used Navy listening stations to locate blue whale calls far out to sea (http://www.dosits.org/ files/dosits/blue1.mp3). They then sent a naval air patrol to the site 400-600 km from their receiver locations. It dropped buoys that could record underwater sound and transmit the signal via radio, confirming the location of the calling whale. An observer was able to sight a large whale but was not able to confirm the species identification. Figure 2 shows a 1,700-