The Dark and Noisy Ocean Depths
SAN DIEGO, California, August 21, 2006 (ENS) - The expanding global shipping trade is making the underwater world a noisy environment, with unknown effects on marine life, according to a new study of California offshore waters published Friday. The study shows a tenfold increase in underwater ocean noise off the coast of Southern California as compared with the same area in the 1960s.
John Hildebrand and Sean Wiggins of Scripps Oceanography, working with Mark McDonald of WhaleAcoustics in Bellvue, Colorado, accessed acoustic data recorded in 1964-1966 through declassified U.S. Navy documents and compared them against acoustic recordings made in 2003-2004 in the same area.
The results showed that noise levels in 2003-2004 were 10 to 12 decibels higher than in 1964-1966, an average noise increase rate of three decibels per decade.
"We've demonstrated that the ocean is a lot noisier now than it was 40 years ago. The noise is more powerful by a factor of 10," said Hildebrand, a professor of oceanography in the Marine Physical Laboratory at the Scripps Institution of Oceanography at University of California San Diego.
The scientists say the noise detected off Southern California originates from ships traveling across the entire North Pacific Ocean. But their area of study is more heavily used by the U.S. military than other North Pacific areas.
Owned by the U.S. Navy, San Nicolas Island, 160 miles west of San Diego, is the most northwesterly of the southern group of Channel Islands. The island is described by the U.S. Army on one of its websites as "the cornerstone" of the Navy's NAVAIR Weapons Division Sea Range capabilities ... "ideal for conducting test and training exercises."
Missiles are launched from the San Nicolas Island Vandal Launch Pad complex; and tri-service and theater warfare exercises as well as classified operations are conducted there, the Army website says.
San Nicolas Island Navy airfield supports C-5's, with a 10,000 foot runway and two hangers. A barge landing area handles cargo too large or bulky for aircraft.
Hildebrand says the noisier underwater environment appears to be due to the increase in the global shipping trade, the number of ships traveling the world's oceans, and the higher speeds and propulsion power for individual ships.
According to Lloyd's Register figures quoted in the study, the world's commercial fleet more than doubled in the past 38 years, from 41,865 vessels in 1965 to 89,899 vessels in 2003.
"If we've doubled the number of ships and we've documented 10 times more noise, then the noise increase is due to both more ships and noisier individual ships than in the '60s. And that may be because the ships are now bigger, faster and have more propulsion power," Hildebrand said.
Several years ago, while searching for information about noise levels off Southern California, Hildebrand obtained declassified documents that described a U.S. Navy sound surveillance system that used cabled hydrophones to measure ambient ocean noise in the 1960s. A detailed analysis of the recordings was reported in 1968.
Hildebrand's group sought to obtain similar readings in the same location using advanced listening devices called acoustic recording packages, ARPs, developed in Hildebrand's laboratory.
Hildebrand and members of his group deploy ARPs at various locations around the world to obtain and analyze acoustic signals emitted by whales, dolphins and other marine animals.
The study's authors say the increase in noise documented off San Nicolas Island may represent the entire northeast Pacific Ocean.
Whales and dolphins use sound signals to hunt, mate and communicate with one another.
Sperm whales, for instance, use echolocation consistently to track down their prey at depth, according to a 2005 a study by scientists from Woods Hole Oceanographic Institution and the University of St. Andrews.
Working in the Atlantic, the Gulf of Mexico and the Ligurian Sea, scientists tracked the world's largest deep-diving toothed whale, the sperm whale, which feeds mainly on squid.
First, the researchers attached acoustic recording tags to the dorsal surface of sperm whales with suction cups. The whales were then tracked acoustically with a towed hydrophone array.
As the whales descended, the tags recorded a regular series of "clicks."
When the whales reached the bottom of their dives, these clicks become more frequent, merging to form "buzzes" of sound when they were approaching food such as squid.
Until now little has been known about the timing of prey detection and capture during dives. Dr. Stephanie Watwood and colleagues found that sperm whales produced buzzes on every deep dive they made, in all three locations, suggesting that they are successful at locating prey in the dark ocean depths by acoustic means.
"The impact of the increased noise on marine animals is unknown," said Hildebrand. "If impacts are shown to exist, what can be done to protect marine animals?"
Hildebrand suggests that shipping lanes might be moved away from concentrations of marine animals. "The impact of ocean noise pollution may be minimized by diminishing the noise source or by separating the noise from things that are sensitive to it," he said.
The Scripps research was supported by the U.S. Navy and the Office of Naval Research. The study is published in the August issue of the "Journal of the Acoustical Society of America."
A 2003 report by the National Research Council Committee on Potential Impacts of Ambient Noise in the Ocean on Marine Mammals recommended that a federal agency should be mandated to investigate and monitor marine noise and the possible long-term effects on marine life. The agency would serve as a sponsor for research on ocean noise, the effects of noise on marine mammals, and long-term trends in ocean noise.
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