Different species of whales and dolphins produce different sounds, such as songs, moans, clicks, roars, sighs, and many other characteristic noises (See 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).

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.

References
• Clark, C.W. and Clapham, P.J. 2004. Acoustic monitoring on a humpback whale (Megaptera novaeangliae) feeding ground shows continual singing into late spring. Proceedings of the Royal Society of London B 271: 1051-1057.
• McDonald, M.A., Hildebrand, J.A. and Webb, S.C. 1995. Blue and fin whales observed on a seafloor array in the Northeast Pacific. Journal of the Acoustical Society of America 98(2): 712-721.
• Mellinger, D.K. and Clark, C.W. 2003. Blue whale (Balaenoptera musculus) sounds from the North Atlantic. Journal of the Acoustical Society of America 114: 1108-1119.
• Mellinger, D.K., Stafford, K.M. and Fox, C.G. 2004 Seasonal occurrence of sperm whale (Physeter macrocephalus) sounds in the Gulf of Alaska, 1999-2001. Marine Mammal Science 20: 48-62.
• Mellinger, D.K., Stafford, K.M., Moore, S.E., Munger, L. and Fox, C.G. 2004. Detection of North Pacific right whale (Eubalaena japonica) calls in the Gulf of Alaska. Marine Mammal Science 20: 872-879.
• Moore, S.E., Stafford, K.M., Dahlheim, M.E., Fox, C.G., Braham, H.W., Polovina, J.J., and Bain, D.E. 1998. Seasonal variation in reception of fin whale calls at five geographic areas in the North Pacific. Marine Mammal Science 14(3): 617-627.
• Nieukirk, S.L., Stafford, K.M., Mellinger, D.K., and Fox, C.G. 2004. Low-frequency whale and seismic airgun sounds recorded in the mid-Atlantic Ocean. Journal of the Acoustical Society of America 115(4): 1832-1843.
• Stafford, K.M., Nieukirk, S.L. and Fox, C.G. 1999. Low-frequency whale sounds recorded on hydrophones moored in the eastern tropical Pacific. Journal of the Acoustical Society of America 106: 3687-3698.
• Stafford, K.M., Nieukirk, S.L. and Fox, C.G. 2001. Geographic and seasonal variation of blue whale calls in the North Pacific. Journal of Cetacean Research and Management 3(1): 65-76.
• Stafford, K.M., Bohnenstiel, D., Tolstoy, M., Chapp, E., Mellinger, D.K. and Moore, S.E. Antarctic-type blue whale calls recorded at low latitudes in two oceans. 2004. Deep-Sea Research I 51(10): 1337-1346.
• Stafford, K.M., Moore, S.E., and Fox, C.G. 2005. Diel variation in blue whale calls recorded in the Eastern Tropical Pacific. Animal Behaviour 69(4): 951-958.
• Watkins, W.A., Daher, M.A., Reppucci, G.M., George, J.E., Martin, D.L., DiMarzio, N.A. and Gannon, D.P. 2000. Seasonality and distribution of whale calls in the North Pacific. Oceanography 13: 62-67.
• Watkins, W.A., Daher, J.E. George and D. Rodriguez. 2004. Twelve years of tracking 52-Hz whale calls from a unique source in the North Pacific. Deep-Sea Research I 51:1889-1901.

Additional Resources
• Moore, S.E., Watkins, W.A., Daher, M.A., Davies, J.R. and Dahlheim, M.E. 2002. Blue Whale Habitat Associations in the Northwest Pacific: analysis of remotely-sensed data using a Geographic Information System. Oceanography 15 (3): 20-25.