But much is still unknown about how sonar affects whales and other marine mammals. For example, the sound can hurt some species while not affecting others, and experts don't fully understand why.

In an argument that has been going on for years, the Navy has continually said that the exercises are vital for training and that it works to minimizes the risk to marine life.

A statement from the Defense Department said that the new exemption covers the use of mid-frequency active sonar in a series of exercises scheduled to take place off California through January 2009 and that the Navy already applies 29 measures to mitigate the effects.

In a separate development, the Pentagon statement said, Navy Secretary Donald Winter signed a memo Tuesday agreeing to greater public participation and better reporting on the issue while officials complete an environmental impact study for Southern California.

Use of sonar "is part of critical, integrated training that must be done in the Navy's operating area off the coast of San Diego to take advantage" of features there related to water depth, as well as extensive ranges, airfields and other infrastructure needed for training, the Pentagon statement said.

About half the Navy's fleet will receive "its most critical, graduate level training" there before it deploys its forces around the world, it said.

Chief of Naval Operations Adm. Gary Roughead said that exercises with sonar train sailors to detect quiet submarines that might threaten its ships.

"We cannot in good conscience send American men and women into potential trouble spots without adequate training to defend themselves," Roughead said.

"The Southern California operating area provides unique training opportunities that are vital to preparing our forces, and the planned exercises cannot be postponed without impacting national security," he said in the Pentagon statement.

On the Net: Navy: www.whalesandsonar.navy.mil

Southern California Range Complex Environmental Impact Statement: www.socalrangecomplexeis.com/