Sonar and Its Effects on Marine Life
Carrie Hunt
Hearing is a marine mammal's most important sense. Noise that blocks their hearing can threaten their ability to function and survive because they use their hearing when they migrate, to locate one another over great distances, to find food, and to care for their young. The measurement of sound in water and sound in air are both based on a "decibel" scale, even though there is an offset of 63 dB between the two scales.
The Navy has been developing a Low-Frequency Active Sonar (LFAS) in order to detect "quiet" nuclear and diesel electric submarines that are other wise undetectable. LFAS consists of very loud, very low frequency sounds designed to carry long distances underwater. Each loudspeaker in the system can generate 215 decibels of sound. A hundred miles away from the ship the sound level would be 150 to 160 decibels, still loud enough to cause permanent hearing damage in humans and many marine animals. To test the system, the Navy targeted a series of sonic "pulses" of increasingly higher decibel levels at whales from progressively closer distances until the whales showed signs of distress or changes in behavior. The US Navy claims that LFAS is safe for marine mammals up to exposure levels of 180 dB. But half of the marine mammals exposed to 165 dB may experience damaging effects. Whales try to avoid sound levels starting at 115 decibels.
The LFAS was tested in Hawaii in 1998 on humpback whales. Researchers observed several lone whale calves in the area during this period. The sonar tests are believed to cause disorientation so mothers and calves become separated and then cannot find each other. Strandings are also a major effect of the tests. Four species of whales and dolphins were stranded on beaches in the Bahamas in 2000, after the Navy used active sonar in the area. The Navy denied that the sonar was to blame, but hemorrhages where found around the eyes and ears, indicating severe acoustic trauma. Also, the area’s population of beaked whales has disappeared, leading researchers to conclude that they either abandoned their habitat or died at sea.
An alternative to active sonar is passive sonar. Passive sonar is undetectable. These systems are either stationary or dropped into locations and have data links to military control networks. There are other non-acoustic sensors that can also be used, such as a magnetic sensors and satellites. Magnetic sensors can silently detect temporal magnetic anomalies such as submarine hulls, and satellite-imaging analysis reveals submarine trails and water disturbances, but both have limited ranges.
Resources:
http://www.cnn.com/EARTH/9804/16/whales.sonar/index.html
http://www.oceanmammalinst.com/marinemammals-lfas-fact-sheet.htm
http://www.oceanmammalinst.org/safealternatives-to-lfas.htm
http://www.nrdc.org/wildlife/marine/nlfa.asp