The U.S. Navy has agreed to limit the use of its latest sonar system, bowing to environmentalist concerns that the technology causes whales to beach themselves. How might underwater sound waves drive cetaceans ashore?
The hottest theory is that the high-intensity signals lead indirectly to decompression sickness, more commonly known as the bends. The sonar system in question, called SURTASSLFA, uses low-frequency waves, which can travel great distances underwater without losing strength. The Navy argues that SURTASS LFA is vital to detecting quiet diesel submarines at great distances and thus provides surface ships ample time to prepare for a military encounter.
Environmentalists contend that the sonar signals are frighteningly loud, especially for sharp-eared whales and porpoises. They claim that sound bursts near a SURTASS LFA-equipped ship can register at upward of 215 decibels and persist at around 160 dbs hundreds of miles from the source. Even the most thunderous rock concert tops out somewhere around 150 dbs, while the pain threshold is 125 dbs. (The Navy points out that such figures are misleading, since decibel readings underwater are skewed by the use of a lower reference point; the Earth Island Institute counters with another intrepretation here.)
Tortured by the intensity of these signals, the argument goes, the whales quickly flee, rising to the surface at a rapid rate. This is where the problems might begin, since marine mammals, like scuba divers, are believed to accumulate nitrogen in their blood when deep underwater. The key to ridding one's body of this gas is to exhale while surfacing; however, befuddled by the sonar din, the whales may neglect to do so. Instead of dissipating, the nitrogen gas forms bubbles that can erupt and cause serious damage to vital organs, rending apart tissue and blood vessels. Disoriented and severely injured, the whales wind up on beaches, where they soon die.
The decompression-sickness theory is controversial, and the Navy continues to assert that SURTASS LFA does not threaten marine mammals. Many marine biologists dispute the idea that whales actually absorb nitrogen during their dives, and others believe that the mammals have developed mechanisms to combat the illness. But a study published in last week's issue of Nature has strengthened the environmentalists' case. The European researchers analyzed the carcasses of 10 whales that beached themselves in the Canary Islands last year, at the same time as a NATO naval exercise was being conducted in the area. They found that the whales suffered from gas-bubble lesions consistent with decompression sickness; they also noted that the naval vessels were using mid-frequency sonar, a shorter-range version of SURTASS LFA.