Science Tribune - Article - April 1998
James Rennell Division for Ocean Circulation, Southampton Oceanography Centre, Southampton SO14 3ZH, UK
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wrong when he christened the bottom of the sea the "silent world". While some ocean waters are indeed as
It appears that the late Jacques Cousteau got it quiet as a Scottish mountain top at midnight, others are as raucous as "underwater fiestas", according to researchers based at the New England Aquarium, the Massachusetts Institute of Technology and Cornell University in the United States (1). Noise polluters include chattering shrimp, commercial shipping and screeching icebergs in polar waters. "Whaling areas in the North Atlantic have about the same noise level as a busy New York intersection, or more than 100 decibels," reports Arthur Baggeroer, an ocean engineering professor at MIT.
Human activity at sea has undoubtedly generated additional noise and the best place to avoid noisy underwater neighbours is in the South Pacific, far from any shipping lanes. But are marine animals such as whales and dolphins being adversely affected by man-made noise? As one marine biologist succinctly puts it, "A deaf whale is a dead whale." "What we now need to find out is if low-frequency sounds made by man can hamper communication between animals," says Baggeroer. "We need to know whether the diffusion of sound can affect mating or even migration."
But apart from shipping, where does underwater man-made noise come from?
Ocean researchers are deliberately setting off controlled underwater explosions in a clever technique designed to monitor global warming. A group at Scripps Institute of Oceanography in California is engaged in a long-term experiment known as Acoustic Thermometry of Ocean Climate (ATOC) (2). According to a group spokesman, "the basic idea of ATOC is simple. Sound travels faster in warm water than in cold water. The travel time of a sound signal from a source near California to a receiver near Alaska, for example, will decrease if the intervening ocean warms up, and will increase if the ocean cools down."
The experiment exploits a natural "sound channel", an acoustic wave guide deep within the ocean that carries sounds over very long distances. Previous experiments have shown the feasibility of measuring ocean temperature by transmitting signals between sources and receivers separated by up to 2,000 km. The group hopes to demonstrate that such an "acoustic thermometer" can be used to monitor temperature over entire ocean basins such as the North Pacific. According to ATOC researchers, such measurements "will provide important information for studying global climate questions, particularly global warming due to the 'greenhouse effect'. "
But the group has been attacked by some environmentalists who have been calling for more intensive monitoring of marine life or even a complete halt to the experiments. The attacks intensified after three dead humpback whales were discovered near one ATOC sound source off the coast of California in 1996. A federal investigation found no evidence for a link between the whale deaths and the acoustic experiment but, according to Environmental Defense Fund marine ecologist Rodney Fujita, "it was grossly irresponsible of Scripps to turn on the source without having the monitoring in place."
Scripps Institute of Oceanography counter that the acoustic experiments go hand in hand with a marine mammal research program which "is designed to provide information on hearing capabilities of marine mammals and sea turtles, response of marine mammals and other marine organisms to man-made sounds - and to provide information needed to direct policies for long-term protection and conservation of marine species. " On the acoustic thermometry experiments themselves, the Institute explains that "whales are routinely subjected to far more intrusive man-made noises such as cargo ships and seismic air guns."
Indeed, environmental groups - as well as fishermen - have expressed concern that high sound levels involved in undersea oil and gas exploration may adversely affect marine animals. During offshore exploration, powerful underwater air guns are deployed from seismic survey vessels. The sound released from the underwater air guns penetrates the ocean floor. Some of this sound energy is reflected at the interfaces between different geological formations and travels back up to the sea surface. The survey vessel tows long streamers of microphones which record the sound reflections and - after sophisticated computer processing of the data - a picture of what is beneath the sea floor can be built up.
Rob McCauley, a researcher at Curtin University of Technology in Western Australia, is currently involved in a three-year investigation of the environmental effects of such surveys on marine life (3). The experiment will monitor the reactions of humpback whales, dugongs and sea turtles to controlled air gun releases. "All animals closest to the air gun will be monitored directly by underwater video. The noises will be stopped as soon as the [animals] start to react or show avoidance."
Early results indicate no "gross displacement of migrating humpback whales in the study area." Although some whales seem to prefer keeping out of the way of the seismic vessel used in the experiment, others actually approached as though curious. Proof, if any was needed, that whales have different personalities, just as humans do.
The question is, now that we are encroaching ever more on their marine territory, can we peacefully co-exist on the same watery planet - or shall we end up drowning out their pleas for a bit of peace and quiet?
2. The Acoustic Thermometry Ocean Climate group has its Web site at: http://atocdb.ucsd.edu