Environmental groups sued the federal government yesterday to prevent the Navy from using active sonar during maritime exercises off Hawai'i next month, saying the sound could harm whales and other marine mammals.

The Natural Resources Defense Council asked a federal court in Los Angeles to issue a temporary restraining order unless the Navy takes "effective measures" to protect marine life when it uses high-intensity, mid-frequency active sonar to hunt submarines in the drills.

"It's not a choice between environmental protection and military readiness," said Joel Reynolds, senior attorney for the council. "The military can train but it has to do so consistent with the law and the law requires protection of the marine environment."

Navy lawyers haven't had a chance to look at the lawsuit and couldn't comment, said Jon Yoshishige, a spokesman for the U.S. Pacific Fleet.

Connie Barclay, a spokeswoman for the National Oceanic and Atmospheric Administration, or NOAA, declined to comment on the suit, but said the agency was confident the Navy's efforts would protect dolphins and whales.

In April, NOAA said Navy use of sonar during maritime exercises off Hawai'i in 2004 may have contributed to the mass stranding of more than 150 whales in Kaua'i's Hanalei Bay. The report, however, did not say definitively that the sonar caused the whales to gather in the bay.

NOAA on Tuesday granted the Navy a permit to use sonar during war games this time involving over 40 ships in Hawai'i waters. The exercises are scheduled to start next week and last through late July.

The Navy, in turn, agreed to several measures to limit the impact its sonar might have on whales and dolphins.

Those steps include not using active sonar in coastal waters — except when the sailors train in channels between Maui and the Big Island and between Kaua'i and Ni'ihau.

The Navy also plans to lower the sonar's power when marine mammals are nearby.

But the environmental groups say these steps don't go far enough.

Reynolds said the Navy should put greater distance between its sonar-emitting ships and marine mammals. He added the Navy should have more observers on the lookout for whales and dolphins.

The Navy says its sailors need to practice tracking and fighting enemy submarines to ensure national security.

The Pacific Fleet has made anti-submarine warfare a top priority as more countries, including North Korea, Iran and China, have been acquiring quiet diesel-electric submarines that are increasingly difficult to track.

Environmentalists say sonar systems endanger marine mammals and fish. They cite a 2000 incident in the Bahamas, when at least 16 whales and two dolphins beached themselves.

Eight whales died and scientists found hemorrhaging around the cetaceans' brains and ear bones, which could have been caused by exposure to loud noise.

Joining the Natural Resources Defense Council in the suit are the International Fund for Animal Welfare, Cetacean Society International and Ocean Futures Society.