The National Oceanic and Atmospheric Administration said yesterday that research and stringent monitoring indicate marine mammals are unlikely to be injured when the Navy uses low-frequency active sonar under normal operating conditions.

The agency added that overall, the sonar would have a "negligible impact" on marine mammals species and stocks.

The rules require sailors to shut down their sonar when marine mammals are nearby and adopt other measures to protect the animals.

Scientists say loud sonar can damage marine mammal brains and ears. Sonar may also mask the echoes some whales and dolphins listen for when they use their own natural sonar to locate food.

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.

The Navy says it takes steps to ensure its sonar doesn't harm whales and other marine mammals.

Mark Matsunaga, a U.S. Pacific Fleet spokesman, said the Navy is happy with the NOAA decision.

"We had requested the extension and we're pleased. We're happy," Matsunaga said.

NOAA's proposal for the five-year rule extension must go up for public comment before it is implemented. The current rules expire Aug. 16.

Low-frequency active sonar is operationally available on only two ships the Navy uses. Those vessels are in the Western Pacific and they haven't been allowed to use such sonar near the Hawaiian Islands.

The low-frequency active sonar is a newer technology than mid-frequency active sonar, which the Navy has installed on many of its submarines and other vessels to detect submerged enemy ships.

In both cases, sailors pump sound into the water and listen for any objects the sound waves bounce off of.

Low-frequency active sonar, however, travels greater distances than the other variety.

For the Navy, it has the benefit of allowing sailors to detect submarines even when they are far away.

Environmentalists say the low-frequency version poses a greater danger to marine mammals because it sends sound over longer distances.

The Natural Resources Defense Council, which sued to challenge the low-frequency rules when they were first established five years ago, said it was disappointed with NOAA's move.

"This particular system generates noise pollution over a staggering geographic scale," said Joel Reynolds, a council attorney.

He said the agency's rules are flawed in part because they only obligated the Navy to turn off its low-frequency active sonar when marine mammals or sea turtles were detected within about 1.1 nautical mile from the sound source.

But Reynolds said low-frequency active sonar could have a harmful effect over much greater areas.

Reynolds added that the five-year extension would allow the Navy to practice with low-frequency active sonar over a broader geographic area.

The Navy now only uses the low-frequency sonar in the western Pacific under an agreement with the council reached several years ago in response to a lawsuit, Reynolds said. The rule extension doesn't impose that geographic limit, he said.

The council plans to sue again if NOAA finalizes the rule extension, Reynolds said.

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On the Web:

National Oceanic and Atmospheric Administration: http://www.noaa.gov/

Navy's low-frequency active sonar site: http://www.surtass-lfa-eis.com/