Navy sonar may have caused a mass stranding of whales this spring in the Bahamas. The National Marine Fisheries Service (NMFS) and the U.S. Navy are jointly investigating that possibility after 14 beaked whales beached at several different locations in the northern Bahamian islands within several hours. The whales stranded in a south-to-north pattern as navy ships using tactical sonar passed over a deep underwater canyon beneath the New Providence channel.
NMFS asked marine biologist Darlene Ketten of the Woods Hole Oceanographic Institute and Harvard Medical School, where she is assistant professor of otology and laryngology, to assist in the investigation. Ketten is an expert on whale auditory systems and underwater acoustic trauma in particular, including impulse and blast effects. What she and other scientists found in studying the six whales that died from the stranding (the rest were successfully pushed back into the sea) was that all had hemorrhages in or around the ears. Says Ketten, "The traumas we found were to the auditory system and to some brain and throat regions that are commonly injured by intense pressures.
"We know from the necropsies that the animals were otherwise healthy," Ketten continues. "We can rule out some causes found in humans with similar symptoms, such as birth trauma, spontaneous hemorrhage, and diseases like hemophilia" as causes, she says. "That leaves us with an intense pressure event, which could be an explosion or some type of underwater sound or impulse." NMFS and the navy are in the process of analyzing their acoustic records from the period in question to see what they might show.
"Everyone agrees that it is unlikely that a sonar pulse from just one sonar on one ship could do this," Ketten says. "Otherwise, we'd have mass strandings of beaked whales all over the place." That deepens the mystery of what happened in this particular case. One theory is that the unusual shape of the sea bottom, a steep-walled underwater canyon, could have created an intense "sound duct" that might have traumatized the whales.
The Bahamian strandings are not the first to occur during naval exercises. In 1996, a mass stranding of beaked whales in Greece coincided with NATO maneuvers there. And earlier this year, four beaked whales beached in the Madeira Islands while NATO ships were in the area.
Although certain whale species, such as pilot whales, have been known to "mass strand" for centuries, mass beaked whale strandings are rare, but appear to have become more common recently. Could there be something about these whales that makes them particularly vulnerable to certain sonic events? "Beaked whales are toothed, and are likely to use high-frequency sounds to echo-locate their prey," says Ketten. Hunting squid and fish in deep ocean trenches, they tend to travel alone or in small groups. "We see them very, very rarely," Ketten reports, but scientists know that they make fast, extraordinarily deep dives regularly. Says Ketten, "Eighteen hundred meters is a standard dive for one of these guys, and they appear to be able to do that within an hour, in some cases half an hour." They do this without suffering barotrauma--injuries related to rapid changes in atmospheric pressure. Most whales typically dive no deeper than 500 meters.
Ketten says beaked whales, like other whales, have special structures surrounding the middle ear that protect them from underwater pressure changes. But beaked whales also have unusually large air passages leading to the middle ear as well as a comparativly well-developed and sensitive vestibular system--the membranous labyrinth of the inner ear that controls the sense of balance--a trait they share with land mammals. These two features may make beaked whales particularly vulnerable to sonic trauma and may have played a role in the strandings, she says.
Ketten is now in the process of decalcifying the bone around the soft tissues of the inner ear of one of the dead whales in the hope of defining the acoustic parameters of the event that caused the hemorrhages. "In an ideal case," she says, "you can sometimes get information about the general spectral characteristics of the event, such as whether it was impulse noises or an explosion."
The hemorrhages didn't kill the whales, Ketten emphasizes. Instead, they would have caused "extreme discomfort" that might have led to panic and hence the strandings. Because the decalcification process takes nine months, the final report of the joint NMFS/navy investigation will not be available until early in 2001.
~Jonathan Shaw
