Different species of whales and dolphins produce different sounds, such as songs, moans, clicks, roars, sighs, and many other characteristic noises (See the Discovery of Sound in the Sea Audio Gallery). Each species is unique in its vocalizations. For instance, male humpback whales sing a long, patterned song on their winter breeding grounds. Fin whales produce loud, short calls that are termed "20-Hz pulses."

Scientists can listen for these sounds and track the different marine mammal species, and sometimes even individual animals, while they are producing sound. An individual blue whale was tracked for 43 days throughout the North Atlantic Ocean using the US Navy's underwater listening system.

A blue whale was tracked for 43 days in the north Atlantic Ocean using the US Navy's underwater listening system. Map courtesy of US Naval Research Laboratory.

Many researchers use hydrophone arrays to determine the location of a sound producing whale. A hydrophone array consists of at least three hydrophones deployed at known locations. Sounds are received by the hydrophones at different times because the hydrophones are different distances away from the whale. This time difference is called the time-of-arrival difference. With the information from the hydrophones, the time-of-arrival differences can be turned into a distance and direction to the whale.

A hydrophone array can be towed behind a ship or placed on the seafloor. It contains at least three hydrophones that pick up sounds in the ocean. Sounds, such as whale calls, are received at different times because the hydrophones are different distances away from the sound source (i.e. whale).

Hydrophone arrays can either be towed behind a ship, or placed on the seafloor. One hydrophone array used by the U.S. Navy to track submarines in the North Atlantic and North Pacific oceans is called the Sound Surveillance System (SOSUS). SOSUS is a network of hydrophones mounted on the seafloor. Since the end of the Cold War, the Navy has allowed whale researchers to use SOSUS to track marine mammals and attempt to determine their distribution during times of the year or in places in the ocean where it is too difficult to tow a hydrophone array with a vessel. Scientists have used this, and other methods, to learn that whales, particularly baleen whales (mysticetes) that feed on plankton, migrate between low latitude, tropical waters in the winter, and high latitude, food-filled waters in the summer.

Recent applications of hydrophone arrays have demonstrated the feasibility of using such systems for locating and tracking pelagic species that produce sound. A combination of acoustic tracking and visual observation techniques have been used as a census tool to estimate relative abundances of blue, fin, and minke whales. Passive acoustic tracking methods (towed arrays and fixed horizontal arrays) have now been successfully applied to follow individual whales for many days, acoustically observe pods of whales, and describe species specific seasonal changes in distribution and relative abundance.

Another system of acoustic technology used in tracking marine mammals involve arrays of seafloor seismometers. Numerous seismic experiments are conducted each year in the deep oceans to study the nature of oceanic crust and to map the source of seismic signals associated with small earthquakes or volcanic activity. Whale calls of the type associated with blue and fin whales are often recorded on the arrays of seafloor seismometers and hydrophones used for these experiments. These systems provide the ability to track marine mammals in the deep ocean over a radius of 20 km. These tracking data can be used to determine swimming speed and apparent respiration rate, as measured by position change and by repeated pauses in the animal's vocalization sequence. Speed and respiration rate can be compared during times of relative quiet, during the passage of large ships, and during times of man-made acoustic transmissions to evaluate possible effects of low-frequency noise. Seafloor seismometer arrays have also allowed for the study of fin whale call sequences, which often are interactions between multiple whales located several kilometers apart.

Researchers are looking at the possibility of using sound to protect the endangered North Atlantic right whale. Aerial photo of mother & calf courtesy of New England Aquarium.

Researchers are looking at the possibility of using sound to protect the endangered North Atlantic right whale. The right whale lives in the waters of some of the busiest ports on the east coast of the U.S., and ship strikes are a severe threat. Currently, if large numbers of right whales are seen in certain areas, vessels are routed around those high concentration areas. Researchers are looking for better ways of identifying these high concentration areas. By deploying hydrophones in Cape Cod Bay, Massachusetts, scientists are comparing the number of calls they hear to the number of whales they count from planes flying overhead. The ultimate goal would be to deploy hydrophones that allow scientists to listen to the recordings in real-time and warn vessels away from high concentration areas. A second project to help reduce vessel/whale collisions is a sonar similar to a fish finder. The sonar is attached to the bow of a vessel and transmits signals. The sonar receives echoes that bounce off objects in the path of the vessel. This sonar hopes to reduce vessel/whale collisions by locating whales that are not visible when they are diving or feeding below the surface.