Ocean Noise Pollution Could Be Affecting Whales And Dolphins More Than We Thought
Commercially available sonar systems are designed to blast sound waves outside the hearing range of whales and dolphins. But a new study suggests some sounds resonate low enough that whales can hear them and may affect the animals' behavior.
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Published last month in PLOS ONE, the new paper by Pacific Northwest National Laboratory scientists builds on growing evidence that noise pollution in the ocean is literally driving whales crazy. Beaked whales, which are known for their deep diving abilities, are perhaps the most sensitive to sound. When they hear loud noises, they dive too deep and resurface too quickly, and they've been known to die from the bends. Other studies have blamed military sonar and mapping sonar by oil companies for mass strandings. One study showed that whales along the Eastern Seaboard of the United States suffer from chronic stress from the constant rumble of boat noise. Noise may also inhibit their ability to find food, leading to lower birth rates among already endangered species.
It's unclear how these commercial sonar systems are affecting (or not affecting) whales and dolphins — because until now, nobody knew they could hear them. Biologists don't believe that any marine animals can hear frequencies at or above 200 kHz. Consequently, the sonar systems used on most boats for navigation and by fishermen to locate fish have not been included in environmental impact studies.
Recently, the Energy Department asked Pacific Northwest National Laboratory to investigate the killer whale population in Washington's Puget Sound. These whales are endangered and may be further threatened if construction on a tidal turbine project there is allowed to go ahead. The scientists wanted to know whether 200 kHz really was such a safe frequency.
To test it, they sent commercial sonar signals into the water and listened for them with sensors. They discovered that even though the loudest volumes were at or about 200 kHz, quieter incidental sound was escaping at much lower frequencies thought to be audible to killer whales. "These signals are quiet, but they are audible to the animals, and they would be relatively novel since marine mammals don't encounter many sounds in this range," said study co-author Brandon Southall in a statement.
The scientists reported that they spoke with experts at The Whale Museum in Friday Harbor, Washington, who said they witnessed killer whales behaving peculiarly in the presense of commercial sonar. The noises would be too quiet to cause any direct injury to the whales' tissue. But the sounds could diminish the whales' ability to communicate with each other and navigate their habitat.
"These sounds have the potential to affect animal behavior, even though the main frequency is above what they primarily hear," Southall said. "It may be that environmental assessments should include the effects of these systems. This may not be a major issue, but it deserves further exploration."
Above photo courtesy of Shutterstock
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