Tokyo ? The U.S. Navy’s deployment of active sonar to detect submarine activity is believed to have been responsible for at least six incidents of mass death and unusual behavior among pods of whales in the past 10 years, according to a recent U.S. Congressional Research Service report.

In one of the most serious incidents, 150 to 200 melon-headed whales were observed milling in Hanalei Bay off Hawaii’s Kauai Island during a Rim of the Pacific Exercise on July 3, 2004, after midfrequency sonar was used, the CRS report said.

Known as RIMPAC, the naval exercise included the participation of Japan and other U.S. allies in Asia and the Pacific.

The CRS report also listed five other incidents in which smaller whales, such as goose-beaked whales, harbor porpoises and killer whales, were found beached and dead in groups of a few to nearly 20. Many of the dead mammals had damaged hearing organs, and all five incidents coincided with U.S. naval exercises in the areas, the report said.

The potential impact of active military sonar on marine mammals, whose hearing is critical for their survival, has long been a concern. Even the deployment of low-frequency active sonar is said to cause a roaring sound comparable to that of a twin-engine jet fighter, while the midfrequency sound is believed to equal that of a rocket. Experts have warned that the sound could critically damage the mammals’ hearing organs.

The CRS report comes amid a growing number of reports of whales colliding with ships. In the latest incident, more than 100 people were injured April 9 when a hydrofoil collided with an object, possibly a whale, off Japan’s Cape Sata.

Although the U.S. Navy has limited the deployment of active sonar in most oceans out of environmental concern since 2003, its use has increased in the seas surrounding Japan as U.S. forces are intensifying surveillance of China’s military activities.