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In a boat off the central California coast, scientists huddle around a computer screen sprinkled with slow-moving white dots, each representing a migrating whale detected with sonar.

The researchers are testing an experimental sonar system, designed to detect any Pacific gray whales within a 1.5-kilometre radius using high-frequency sound waves that are believed to work above their normal hearing range.

Researchers at Scientific Solutions Inc., the New Hampshire firm that developed the system, say the sonar appears to work, detecting marine mammals more reliably than other methods without causing the whales to break away from their migratory path or otherwise show signs of injury.

Still, some environmentalists worry that the sonar's impact on whales is not fully understood — that despite the findings of an environmental assessment that enabled the testing to proceed, the sonar could distress the whales, drive them from their habitat or separate migrating calves from their mothers.

“There's no way to know what the long-term effects on the whales will be,” said Robin Mankey of San Francisco-based Sea Sanctuary, one of five environmental groups whose request for an injunction was denied by a federal judge. “There's no way to know if they're washing up dead on the beach or sinking in the ocean.”

Supporters say a reliable high-frequency sonar could help protect whales from a variety of ocean hazards: long-range military sonar; collisions with ships; underwater demolitions; U.S. Navy battle simulations involving live explosives; and seismic mapping by oil and gas companies.

“Nobody wants to go out and kill whales,” said Bob Gisiner, who manages the marine mammal program of the Defense Department's Office of Naval Research, which has funded most of the $2-million project. “I don't understand how any group that's interested in the conservation of marine mammals would not be interested in seeing this sonar developed.”

The navy has been criticized in recent years for its low- and mid-frequency sonars, which can travel long distances to detect enemy submarines. These sonars have been blamed for injuring or killing whales, whose hearing can be severely damaged by the sound.

The navy's role has fed a darker fear for environmentalists — that if it proves successful, the new sonar will make it easier for the military to declare an area of the deep sea to be relatively free of protected species, and thus open to more destructive activities.

“This sonar will be used as an excuse to engage in activities harmful to whales,” said Lanny Sinkin, an attorney for the environmental groups. “It helps them escape responsibility for disrupting the normal activities of whales, by saying they're not injuring or killing them.”

The sonar's backers say they share the same goal of protecting difficult-to-locate whales and other marine mammals that could be unintentionally injured or killed by human activities.

The sonar's backers say it is an advantage over a method known as passive sonar, which can hear underwater sounds without emitting sound waves. But that works only when whales are vocalizing, and it cannot accurately determine their location, only their presence relatively nearby.

“We need to do a better job of safely detecting and tracking marine mammals to better protect them from man's seafaring activities,” said Peter Stein, Scientific Solutions' president.

Just after sunrise on a recent January morning, Stein's researchers anchored their 51-metre research vessel and lowered an 815-kilogram, barrel-sized device, the key to their “Integrated Marine Mammal Mapping and Protection System,” into the rocky Pacific waters.

It sends out high-frequency sounds into the water every several seconds. The sound waves hit objects in their path and bounce back to the transducer, which transmits the data to computers that calculate the objects' size and location.

The tests were delayed for a year after a lawsuit prompted an environmental assessment, which found that the research would not significantly impact marine life. A five-year permit from the National Marine Fisheries Service was upheld last month, allowing the tests to proceed.

Tests are being conducted over 20 days in the middle of the gray whales' migratory path from Alaska to Mexico. The whales were hunted nearly to extinction until they were protected by the International Whaling Commission in 1947. Their numbers have since rebounded.