Navy funds ENG study of sonar’s effect on whale strandings
By Brian Fitzgerald
With a $789,000 grant from the U.S. Department of Defense, two BU engineering professors are studying the auditory function of the world’s largest animals. Their aim is not just to save the whales’ hearing. They are trying to save their lives.
Man-made noise in the ocean, from supertanker propellers to sonar, may
cause acoustic trauma, which many scientists say leads to mass strandings.
When the sound levels are high enough to cause extreme discomfort, they
believe, some whales respond by panicking. Last year, for example, 14
of the marine mammals beached and 8 of them died in the Canary Islands.
In 2000, the same thing happened in the northern Bahamas, with 17 strandings
and 7 deaths. Both events coincided with naval exercises.
In one sense, the cause of the Canaries and Bahamas strandings is no
big mystery. The government of the Canaries pointed to Spanish naval sonar
activity in the area, and the U.S. Navy admitted that sonar from its warships
in the Bahamas contributed to the whale deaths there. What needs to be
explained, however, is how certain types of sonar affect cetaceans, especially
beaked whales, and why some sonar operations cause strandings, although
most do not.
It could partially ‘blind’ them. I compare the effect to
someone walking down a dark hall while a strobe light is flashing. It
may disorient them.”
Beaked whales, so named because of their long, narrow snout, seem to be particularly vulnerable to the standard midfrequency sonar used by NATO countries to look for enemy submarines. “The sonar may also contribute to lower populations of the species because it might be altering their feeding habits and reproductive success,” says Mountain.
“Normally, when we talk about acoustic trauma, we are concerned with dysfunction of the sensory cells within the inner ear,” he says. This function can be temporary, an acute condition called temporary threshold shift (TTS), or permanent, a chronic condition called permanent threshold shift (PTS). “TTS is what most of us have experienced after exposure to loud music or working with a chainsaw without hearing protection,” he says. “TTS isn’t something that can easily be identified without behavioral or electrophysiological testing, and these techniques aren’t very practical for stranded or dead animals. PTS can be identified anatomically, but only if the tissue is very well preserved, a condition that is not usually met with stranded whales.”
Also, there is still much to learn about beaked whales, the least known cetacean, and how sonar affects them. “They have no external hearing parts, so we don’t know how sounds get into the inner ear,” says Mountain. Complicating matters is the fact that beaked whales are reclusive and “usually remain far offshore -- very few people have seen them.” However, because the Bahamas stranding was reported quickly, two of the six beaked whales were in good shape for a necropsy, or animal autopsy.
Mountain and Hubbard work with Darlene Ketten, an auditory specialist at the Woods Hole Oceanographic Institution who helped conduct the necropsies and anatomical studies on the whales in the Bahamas, as well as other suspicious cases. The BU professors, with their students and laboratory staff, are making mechanical measurements and creating computer models of the whales’ hearing systems. Their research will help predict the whales’ hearing sensitivity to sonar and provide information for determining which combination of factors is most likely to cause another mass stranding. “The Marine Mammal Protection Act states that these animals can’t be harassed or injured,” says Mountain, “so the Navy funds this type of project, along with studies on the migratory habits of the whales, to avoid injuring the animals when they plan major exercises.”
The Navy will review the research and implement measures to ensure fewer adverse effects on beaked whales when it uses tactical midrange sonar. “Marine mammal strandings are not unusual and the overwhelming majority of these strandings are not correlated with any known human activity,” says Mountain. “The beaked whale strandings have attracted attention because these species are much less likely to strand than other species. In the case of the mass beaked whale strandings, as many as a third of them may be associated with naval activity. To me, the mystery is why beaked whales seem to be responding badly to conventional midfrequency naval sonars, while other species don’t appear to be showing this behavior.”
Mountain, who has been teaching at BU since 1979, has always been interested
in research that combines engineering and neuroscience. He says that the
Navy, “in the interest of national security, certainly won’t
stop using these types of sonar during wartime -- it’s their bread
and butter. But in peacetime, when it’s used during training exercises,
they have some leeway.”