Sea notes: navy ships send sounds into the sea to hone in on underwater objects. Could the noise be harming ocean life?
Some environmental groups blame the whales' unusual behavior on nearby U.S. Navy ships. Around the time the whales approached the beach, the ships were using active sonar. This tool uses sound waves, or vibrating energy waves, to detect underwater objects (see diagram, p. 17).
The U.S. Navy says that sonar was not responsible. And scientists agree that there isn't enough evidence to link the whales' behavior to the ships' actions. "We can't rule out a relationship, but we don't know for certain what caused that event," says Brandon Southall, a biologist at the National Oceanic and Atmospheric Administration Fisheries Acoustics Program. But over the last 50 years, similar incidents--including many whale deaths--have occurred in areas where active sonar was being used.
WHALES AT RISK
Scientists haven't normally linked the strandings of melon-headed whales, Peponocephala electra (PEP-on-oh-SEF-uh-la el-EK-tra), to sonar. The whales usually associated with sonar are beaked whales, particularly Cuvier's beaked whales, Ziphius cavirostris (ZIH-fee-us CAV-uh-RAHS-tris). Unlike many whale species, Cuvier's beaked whales mostly stick to deep water and almost never strand on their own. But they have been found beached near areas where sonar had been used. Since 1960, there have been as many as 25 beaked-whale strandings that may have been related to military sonar, says Darlene Ketten, a biologist at Woods Hole Oceanographic Institution.
Scientists aren't sure why simple sound waves might cause whales to swim for shore. The type of sonar that some researchers have linked to strandings is called mid-frequency (number of vibrations per second) sonar. Next to the source, sound levels can reach over 200 decibels (measurement of loudness)--similar to levels next to a jet engine.
Some researchers think these loud sounds may cause the whales to panic. Trying to escape the noise, the whales may change their diving patterns, or swim toward the beach and become stuck onshore.
Sonar's effect on whales isn't yet understood. Still, "Navy guidelines require precautionary measures to avoid active-sonar use in close proximity to marine mammals," says Frank Stone, Living Marine Resources Manager for the Chief of Naval Operations Environmental Readiness Division. Example: Sonar is turned off when whales are spotted nearby.
Meanwhile, scientists are trying to learn more about a possible link. "There is enough evidence to suggest a relationship between certain kinds of military sonar and some marine mammal strandings," says Southall. "At least to the point that it justifies us asking more questions."
For more about natural and artificial ocean sounds, including downloadable audio files, check out: www.dosits.org/dosits.htm
HOW DOES ACTIVE SONAR WORK?
Navy ships from around the world use sound waves to check water depths and pinpoint the location of objects underwater.
(1) SEA SPEAKER
On most ships that use mid-frequency soner, a device fitted to the ship emits sound waves between 1,000 and 10,000 hertz (measurement of frequency). The sound is typically sent out twice per minute in short pulses.
(2) LISTENING IN
Scientists think sonar noise--particularly when created by multiple ships in the same area--may frighten whales. The animals may try to free to safety and instead end up onshore, where--exposed to the sun--they can die.
When a sound wave collides with an object like a submarine, the wave reflects, or bounces back. The reflected sound wave, called an echo, has a weaker intensity (energy a wave carries through an area per second) than the original wave. That's because the water and object that was hit absorb and scatter some of the wave's energy.
(4) MESSAGE RECEIVED
A receiver inside the sonar device detects echoes that reach the ship. A tool measures the time it took for the sound waves to make the round trip. It uses that information to calculate the distance to the underwater object.
SONAR THREAT? In addition to the Hawaiian event, scientists are still investigating other occasions when large numbers of whales died while sonar was being used nearby: 12 whales died in the Greek Islands in 1996; 6 died in the Bahamas in 2000; and 9 beaked whales died in the Canary Islands in 2002.
DID YOU KNOW?
* The sounds made by mid-frequency sonar are within the hearing range of humans as well as whales. Since humans don't usually dive near cruising Navy ships, people don't normally hear the noise.
* In the 1970s, Navy ships were able to use passive sonar to detect, most submarines. That means the ships would tow a hydrophone (a device that detects sounds underwater) and listen for submarine-made sounds, such as motor noise. By the 1990s, many submarines were running on ultra-quiet battery power. This made passive sonar ineffective.
* Research other types of artificial sea-sounds, and the possible effects on marine life. Discuss: Should these noises be limited?
ART: Create a poster that alerts the public on what they should do if they find a marine mammal stranded on the beach. For help, see: http://nmml.afsc.noaa.gov/ education/cetaceans/cetaceastrand.htm
* Grolier search term: whales
* For information on underwater sounds, visit: http://oceanlink.island.net/oinfo/acoustics/acoustics.html
CHECK FOR UNDERSTANDING
DIRECTIONS: Fill in the blanks to complete the following sentences.
1. Navy ships sometimes use -- a tool that uses sound waves, or -- to detect underwater objects.
2. Some scientists have linked -- sonar to whale stranding. Next to the source, the sonar sound levels can reach over --, similar to levels next to a -- engine.
3. When a sound wave collides with a submarine, the wave reflects, or --. The reflected sound wave is called an --.
4. The reflected wave has a weaker -- or energy a wave carries through an area per -- than the original wave.
1. active sonar, vibrating energy waves
2. mid-frequency, 200 decibels, jet
3. bounces back, echo
4. intensity, second
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|Title Annotation:||Physical Sound|
|Date:||Dec 6, 2004|
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