Environmentalists are worried a U.S. warship may have frightened a pod of orcas swimming off southern Vancouver Island.

Orca whales and porpoises use their sensitive acoustics system to communicate, hunt and navigate.

On Monday, a navy destroyer blasted sound waves into the water while en route to the Nanoose test range near Nanaimo. Whale watchers say the sound was extremely loud and the whales appeared to be stressed.

"They heard the sound," said biologist Anna Hall of Prince of Wales Whale Watchers. "Whether or not it bothered them, we'll never know, and hopefully it didn't have any long-term effects on them."

When the sonar was used, Hall saw the whales all squashed together against the shoreline. She said it's a good sign the whales were all back in the area the next day.

Whale researcher Ken Balcolmb lives near a field station in the Bahamas. In March 2000, he noticed beaked whales were coming ashore with sustained damage to their sinuses, ears and eyes injuries consistent with exposure to loud sounds.

The worst-case scenario is that sonar causes physical damage; at best, it annoys the whales, scientists say.

Canada's navy won't use sonar when whales are around. For security reasons, the U.S. navy won't reveal what kind of sonar it used. Environmentalists say the U.S. navy should follow the same rules as other boaters.

"They're not going to find Osama bin Laden or weapons of mass destruction in the Strait of Georgia," said Peter Ronald of the Georgia Strait Alliance.

The Whale Watch Operators Association has filed a complaint with Canada's Department of Fisheries and Oceans, which is investigating the incident.

Larry Paike, district supervisor for DFO, said the U.S. could face charges for disturbing or molesting marine mammals.

The U.S. navy is already in an American court defending a new sonar that critics say threatens marine life.