The mysterious mass stranding of 16 whales in the Bahamas in March 2000 was caused by U.S. Navy tests in which intense underwater sounds were generated for 16 hours, according to a newly released government report compiled by civilian and military scientists.
The report's conclusions mark the first time that underwater noise other than from an explosion has been shown to cause fatal trauma in marine mammals. The military's acknowledgment of responsibility also marks a sharp departure from earlier statements by the Navy, which had denied responsibility for the Bahamian beachings and other mass strandings of marine mammals that coincided with sonar exercises.
Experts said the study -- which relied on an elaborate airlift of frozen whale heads from the Bahamas to a Harvard Medical School X-ray facility -- places the Navy on notice that it will have to balance more carefully its need to conduct underwater sonar tests against the need to protect marine mammals. The report, approved by Navy Secretary Gordon R. England, concludes that the Navy should "put into place mitigation measures that will protect animals to the maximum extent practical" during peacetime training and research efforts.
But the report also allows for the suspension of such protections in the interest of "national security," a broad exemption that has yet to be defined in practice. And it does not answer the contentious question of whether marine wildlife may also be imperiled by a different kind of sonar test proposed by the Navy, one that would involve much lower-frequency sound waves in the ocean.
The latest report, a joint project of the Navy and the National Marine Fisheries Service, grew out of the beaching of 16 whales and a spotted dolphin on Bahamian shores over 36 hours starting March 15, 2000. Seven of the animals -- five Cuvier's beaked whales, one Blainville's beaked whale and the dolphin -- died. Ten other whales were pushed back to sea, and their fates are unknown. The international group that lists threatened and endangered species classifies the beaked whales as being too poorly understood to know whether they are endangered.
The strandings coincided with a nearby Navy exercise meant to improve coordination among ships sailing through enemy-infested channels. The test involved middle-frequency (about 3,000 to 7,000 cycles per second) sonar studies in which underwater noises of about 230 decibels were generated. For comparison purposes, engineers have calculated that if Luciano Pavarotti could sing underwater, his voice would register at about 173 decibels. Tissue damage in sea animals is known to occur at about 10 times that sound intensity, or 180 decibels (the decibel scale increases logarithmically), and a 230-decibel sonar sound is about 100,000 times louder than that.
The cause of death in the Bahamian strandings may have remained unsettled had it not been for Ken Balcomb, who with his wife, Diane Claridge, runs the Bahamas Marine Mammal Survey on the Bahamian island of Abaco.
"The first whales stranded right in front of our research station," Balcomb said last week. He and others worked furiously to push surviving animals back to sea. But he also knew that studies of the cause of death in previous strandings had been inconclusive because of a lack of preserved tissues -- in particular intact whale heads, which can allow careful study of the inner ears and other pressure-sensitive organs.
So when Balcomb and his colleagues heard about two whales that were already dead on the beach, they did not waste any time cutting off the animals' heads.
"We went to the local restaurant and persuaded them to put them in the freezer," he said -- a big request, as each head was about four feet long and weighed a couple of hundred pounds.
National Marine Fisheries Service scientists flew out to study the beached carcasses. But to obtain a more definitive diagnosis, arrangements were made with Darlene Ketten, a whale hearing specialist with Harvard's department of otology and laryngology and the Woods Hole Oceanographic Institute to perform three-dimensional CT scan studies of the frozen heads.
Balcomb got the whale heads to Miami on a chartered plane and booked himself a seat on a flight to Boston. The mission was briefly jeopardized when the airline balked at the 600 pounds of thawing, excess baggage, Balcomb said. But a sympathetic skycap gave providential advice: Upgrade to first class. Balcomb did, smoothing the way for the heads' final leg of travel.
"We got there at 11 p.m. and we did scans all night," Balcomb recalled. "By 3 a.m., the damage was evident."
The X-ray studies showed bleeding around the inner ears, along with trauma to the auditory system and parts of the brain and throat sensitive to intense pressures. In one animal, the ligament that holds an eardrum-like membrane taut had ruptured, evidence of having been exposed to a powerful physical force. Other studies found that all but one of the animals had been healthy (the dolphin was diseased, and its demise has not been linked to the Navy), and the report ruled out other causes of injury, such as physical strikes by ships or underwater seismic events.
"There's no question that these tactical mid-range sonars were the sound source that caused the trauma," said Roger Gentry, who heads the acoustical research team for the National Marine Fisheries Service, an agency of the National Oceanic and Atmospheric Administration.
It remains unclear whether the whales were fatally injured by the sounds themselves or whether the sound-related injuries disoriented the animals, sending them ashore, where they overheated and drowned, said David Cottingham, NMFS's deputy director of protected resources.
Also uncertain is the fate of the 10 whales pushed back to sea. Balcomb, who has maintained photographic records of all whales in the region for the past 10 years, said he has not seen those 10 or any other beaked whales since the week of the strandings, leading him to believe that at least some died.
Navy spokesman Patrick McNally said the Navy believes that the injuries were caused by the unique characteristics of Bahamian underwater topography and other factors, and that similar tests may still be appropriate in other waters. Meanwhile, the Navy is instituting new policies to prevent such injuries, he said, and will increase funding of marine mammal research to $9 million in the coming year.
The Navy is expected to get federal permission to conduct tests of a low-frequency (100 to 500 cycles per second) sonar system early next year -- permission that environmental groups have promised to fight.
© 2001 The Washington Post Company