High and dry: the US Navy's use of high-intensity sonar left whales on the beach in the Bahamas. Credit: ANTONIO MIGNUCCI, CARIBBEAN MARINE MAMMAL LAB., UNIV. METROPOLITANA

The US Navy has admitted that its use of a high-intensity sonar system caused a rash of whale strandings and deaths in March 2000.

Sixteen beaked and minke whales were found stranded on beaches in the Bahamas shortly after US Navy ships using the high-intensity sonar had passed by. Six are known to have died, and the rest were pushed back into the sea. But a fall in sightings of beaked whales has led researchers working in the area to believe that many more may have died. Autopsies on the animals revealed bleeding around the whales' inner ears and in one instance in the brain.

The Navy and the National Marine Fisheries Service (NMFS) responded to the incident by launching a series of investigations. An interim synopsis of the reports, released on 20 December 2001, concludes that the bleeding was caused by sound waves produced by the high-intensity sonar.

The synopsis was welcomed by environmental groups, which claim that high-intensity sonar may have caused other strandings. Autopsy evidence in previous cases could not identify the cause of the stranding. “This is the first time the Navy has really acknowledged responsibility for anything like this,” says Andrew Wetzler of the National Resources Defense Council, a New York-based environmental group.

But the report says that the high-intensity sonar may not pose a widespread threat to marine life. Such systems are commonly used, and the synopsis says the strandings were the result of unique local conditions. The sound waves were trapped in a layer of warm water, preventing their dissipation, and the whales could not escape because they were feeding in underwater canyons.

But according to Roger Gentry, coordinator of the NMFS acoustics team, the possibility that other conditions might cause similar problems cannot be ruled out, as it is not understood how the sound waves caused the bleeding. Several US research groups are looking into this issue.

Although he welcomes the report, Wetzler claims that the Navy is playing down the importance of its conclusions by focusing on unique characteristics of the event. “We really believe that this is an important piece of evidence that calls for all parties to re-examine all use of high-intensity sonar around the globe,” he says.

The report comes at a sensitive time. The US Navy is seeking approval for a new high-intensity sonar system (see Nature 410, 505; 2001). The system, which operates at lower frequencies than that responsible for the Bahamas strandings, is needed to detect new, quieter submarines, the Navy says. The NMFS will have to approve use of the system under the Marine Mammal Protection Act, and Gentry says a decision should be made within a few months. But he says the report is unlikely to significantly affect the approval process.