LOS ANGELES — A federal judge in Los Angeles banned the U.S. Navy from using high-powered sonar in nearly a dozen upcoming training exercises off Southern California, ruling yesterday that its use could "cause irreparable harm to the environment."

U.S. District Judge Florence-Marie Cooper issued the preliminary injunction after rejecting the Navy's request to dismiss a lawsuit brought by the Natural Resources Defense Council.

The Navy said it plans to appeal.

The court case is another setback for the Navy and its use of mid-frequency sonar, which has increasingly been challenged by environmental groups.

The latest court ruling does not affect Navy anti-submarine warfare exercises off Hawai'i, but several environmental organizations in May sued in federal court to block the Navy from planned exercises using high-volume sonar in Hawaiian waters.

Navy ships use mid-frequency active sonar to detect silent diesel-electric submarines. The Navy said it has already conducted at least two such training exercises in Hawai'i waters this year, without any adverse effects on marine life.

The Defense Department in January exempted the Navy and its use of sonar from the federal Marine Mammal Protection Act for two years, prompting an outcry from environmentalists, who maintain the underwater sound harms whales.

Last summer, a six-month exemption granted during biennial Rim of the Pacific, or Rimpac, naval exercises off Hawai'i led to a legal challenge, and a federal judge briefly prohibited midfrequency sonar use during the war games.

WILDLIFE VS. SECURITY

The California lawsuit, along with a similar one filed by the California Coastal Commission, argues for broader safeguards to protect marine mammals from blasts of mid-frequency sonar that have been linked elsewhere to mass die-offs of whales and panicked behavior.

The Navy said even a temporary ban would disrupt training of sailors before they are sent overseas. The Navy uses the sonar to detect potentially hostile vessels, including quiet diesel submarines, which one captain likened to "the most lethal enemy known" on the high seas.

"It's akin to sending a hunter into the woods after one of the most lethal preys known, but sending him in partly deaf and blind," said Navy Capt. Neil May, assistant chief of staff for Third Fleet Training and Readiness.

More than 180 foreign diesel subs operate in the Pacific, the Navy said.

Navy officials said midfrequency sonar was crucial to national security and that they are "deeply concerned" about the California court ruling. Sonar training is essential for ships to complete before they deploy to to the western Pacific and Middle East, the Navy said.

The service has conducted similar exercises for 70 years off California and has used similar active sonar technology for 40 years, it said.

"In all those years, not a single stranding or injury of a marine mammal has been associated with the Navy's use of (midfrequency active sonar) in the Southern California Operating Area," said Vice Adm. Samuel Locklear, the San Diego-based commander of the U.S. Third Fleet.

SIDING WITH WILDLIFE

Cooper, the federal court judge, said it was never easy to balance the interests of wildlife with those of national security. But in this case, she said, the potential harm to whales and other marine life outweighs the harm to the Navy from forsaking sonar use off Southern California for a limited period.

The ban will remain in effect only until the lawsuit is resolved. The lawsuit, according to environmental lawyers and the California Coast Commission, could be settled quickly if the Navy agreed to more sweeping precautions, such as shutting off the sonar during foggy conditions or at night when deck watchmen cannot spot whales that venture too close.

Joel Reynolds, a lawyer with the Natural Resources Defense Council, praised the decision.

"This ruling means that Southern California waters — some of the richest in the United States when it comes to marine mammals — will be protected from the use of mid-frequency sonar while this case is pending," Reynolds said.

ARGUMENT FOR HAWAI'I

The Navy two weeks ago released an environmental study that it hopes will lead to a blanket authorization for sonar training around Hawai'i and do away with the need for case-by-case applications for permits from a federal regulatory agency that monitors harm to whales.

The ambitious study calls for increased training and testing over 2.3 million square nautical miles around Hawai'i.

The environmental impact statement — now in draft form at about 1,700 pages — lays the groundwork for what the Navy hopes will be a more efficient way of complying with the Marine Mammal Protection Act in the use of active, or "pinging," sonar.

The examination is part of a series of environmental reports to be produced on training plans around Hawai'i, near Guam and the Mariana Islands, and at East Coast and West Coast sea ranges.

Advertiser military writer William Cole contributed to this report.