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In recent years, the U.S. Navy has come under fire because of training exercises involving sonar that whale-lovers charge is deafening marine mammals and, in some cases, leading to their deaths by disrupting their communications and sending them astray. New research suggests that sonar does cause hearing loss, but only when it's extremely loud and extremely close.

Anecdotal evidence abounds of links between sonar training and beachings. For instance, a pod of whales apparently lost their way and washed ashore in the Puget Sound, Wash., in the summer of 2005 following a naval training. But until now, no one had tested the actual impact of the sub "pings" on marine mammals.

Marine biologist Aran Mooney of the Woods Hole Oceanographic Institution and his colleagues report in Biology Letters that they exposed a bottlenose dolphin with an  electro-encephalogram (EEG) strapped on his head to a tape of the sonar sounds from that same Puget Sound exercise—15 sonar "pings" over two minutes—and measured his reaction.

Their findings: they could temporarily deafen Boris the dolphin if the sounds were top-volume (203 decibels or more) and unleashed relatively close to his open Pacific Oean pen off the coast of Hawaii.

"We had to expose him to very loud sounds repeatedly," Mooney says, noting that Boris's hearing returned within 20 minutes or so. "The animal would have to be relatively close to the sonar source, the equivalent of 40 meters [131 feet] from the Navy ship."

And it's not clear exactly how sonar would lead dolphins and whales to beach themselves, even if they lose their way because of temporary hearing loss, Mooney says. "Even if we know how they react to sound, it doesn't give us a good idea why they end up on the beach," he says. "We may never really know that answer."

Such sonar-induced strandings may occur in relatively rare special acoustic circumstances, such as when the ocean's underwater topography allows for strong echoing or water conditions prevail that don't let the sound diminish over distance as it would normally. "No one knows what beaked whales hear," Mooney says. "Maybe they're exceptionally sensitive to sounds."

Other research has shown that some whales' hearing range has diminished from 1,000 miles (1600 km) in 1940 to just 100 miles (160 km) today,  due to increasing noise pollution in the ocean. And the U.S. Navy in its own environmental assessments has admitted that its sonar use may permanently damage whales.

The new experimental data will allow those deciding the appropriate noise level under the sea, such as the National Marine Fisheries Service, to better understand what might help avoid any such situations. "It shows us that these sounds do have to be relatively loud and the animals close and that leaves a lot of room to mitigate the situation," Mooney says. "It should be relatively easy to avoid problems."

Image: A. Mooney

9 Comments

1. 1. DrMary 09:49 PM 4/8/09

   I was enjoying reading this interesting article from what I thought was a reputable scientific source, until I came a across an advertisement linked within the text! Is that really necessary Scientific American? I thought you were better than that.

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2. 2. Happy Phil 09:59 PM 4/8/09

   Pay no attention to the man behind the curtain. Something happens during naval training exercises that causes deadly harm to pods of whales and other ocean beings. Just because a solitary test dolphin did not beach itself, does not rule out miscommunication among leaders of a pod of traveling whales.

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3. 3. alflanagan in reply to Happy Phil 10:37 PM 4/8/09

   @Happy Phil: "Something happens during naval training exercises that causes deadly harm to pods of whales and other ocean beings."
   
   And your direct evidence of this is...?

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4. 4. Happy Phil in reply to alflanagan 02:01 AM 4/9/09

   It's in the second paragraph of this article;
   
   "For instance, a pod of whales apparently lost their way and washed ashore in the Puget Sound, Wash., in the summer of 2005 following a naval training. But until now, no one had tested the actual impact of the sub "pings" on marine mammals."
   

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5. 5. MrGneissGuy 02:13 PM 4/9/09

   @ Happy Phil : correlation does not imply causation. It is curious that these two events occurred in tandem, yet it could be just a coincidence.
   To borrow from a popular example: the rate of pirate decline correlates perfectly with an increase in global temperatures. Obviously these two are not related
   I'm glad scientists are finally beginning to investigate this phenomenon. Whatever the cause, I hope we can pinpoint it soon.

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6. 6. EdoReyes 07:47 PM 4/10/09

   Please, remember that Aristoteles regstered dolphins beaching in Greece some time ago. EdoReyes

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7. 7. Happy Phil 09:41 PM 4/10/09

   I think that because 1 test dolphin didn't beach itself after being subjected to what was assumed to be the disturbing sound, it's not enough evidence to give the navy training exercises a green light.
   
   It is true that historically, there has been beaching, but not with the
   frequency that is occurring now. We should attempt to suspend as much noisy activity in our oceans for a year or two and observe the effect.

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8. 8. Acoustic Ecology Institute 10:34 AM 4/17/09

   My goodness, a surprising lack of accuracy for Sci Am. While the Puget Sound incident remains one of the most compelling incidents in which a group of whales were directly observed during sonar exposure (and there was clear agitation), they did not beach. More importantly, this study, while showing that it takes a lot of noise to cause a TTS in a (particular, captive) whale, does not remove the (occasional) link between sonar and beachings. Few researchers in agency, academic, naval, or NGO communities now pin the cause for these incidents on direct physiological damage to auditory systems; behavioral responses are the more likely mechanism. Several possibilities remain under investigation. These issues are covered in more depth by the Acoustic Ecology Institute at http:AEInews.org

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9. 9. KWPenland 09:20 PM 5/31/09

   Author Biello does not define "ping," which in US Navy parlance can be up to 100 seconds in length at 240dB+ (low-frequency sonar) and can travel hundreds of miles at TTS levels. Mid- and high-frequency sonars are usually much shorter in length but delivered in rapid succession and travel much shorter distances. All acute sounds, whether anthropogenic or natural, can cause disturbance and behavioral disruption in marine mammals if not outright injury or death, but sonars in particular, because of their duration and intensity, have been well-documented in numerous multi-species strandings of cetaceans around the globe.

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