A spotter bangs three times on the boat's cabin roof, signaling the captain to cut the throttle _ now. In the foggy gray of Cape Cod Bay, the reason for the abrupt stop soon becomes apparent: The research vessel is surrounded by rare North Atlantic right whales, their glossy black heads bobbing just above the surface as they feed on plankton slicks.

Ship strikes are the top human-related cause of death for these mammals, which are in danger even from this vessel, a slow-moving research boat called the Shearwater. But new technology could soon help safeguard the whales by using sound, not sight, to track the creatures' movements.

"We're listening to their chatter," whale expert Christopher Clark said aboard the Shearwater, referring to the grunts and groans whales use to communicate. "They can't keep their mouths shut."

In the past, tracking whales often depended on inefficient aerial surveys, which were limited by weather and how often the whales surfaced.

Now researchers listen for the whales using 13 underwater microphones attached to buoys off the coast of New England. Eventually, scientists hope to follow their movements closely enough so boats can slow down and post lookouts.

"The slower the ships go, the lower the risk of killing a whale with a ship," said Clark, director of the bioacoustics research program at the Cornell Lab of Ornithology, and the project's lead scientist.

Kathy Metcalf of the Chamber of Shipping of America said shippers would welcome a listening system because they are currently being asked to reduce their speed despite uncertainties about where the whales actually are.

"We've been saying all along that if we can get real-time information, we want to avoid them," Metcalf said.

The right whale was hunted nearly to extinction in the late 18th century, and the death of even one in the estimated population of 350 to 400 is a setback. Since 1986, at least 32 right whales have been killed by ships.

The slow-moving whale is oblivious to its surroundings while feeding and is frequently at risk while migrating up and down the East Coast through busy shipping lanes and waters laced with fishing gear in which it can get tangled.

Clark got the idea after a chat in 2001 with fellow whale researcher Moira Brown, who wondered if they could record the whales in Cape Cod Bay and then match the sounds with what scientists were seeing.

Clark was shocked to hear the tape loaded with calls even when no one knew whales were present. He started recording more frequently in larger areas and discovered the whales were always around, even when the planes spotted nothing.

Clark believes whales use the calls, similar to a grunting "moo" or a high-pitched "whoo," to communicate who they are, where they are and where to find food. Sound moves much more efficiently in water than air, and the animals can easily talk over several miles on a calm day.

Engineers at the Woods Hole Oceanographic Institution developed a thumb-sized underwater microphone attached to a buoy to listen for the whales.

Ten of the 13 buoys installed so far are in the shipping lane that runs to Boston through Stellwagen Bank, an underwater plateau at the mouth of Massachusetts Bay. Each can detect a whale within a five-mile radius.

Computers on the buoys separate the whale calls from other ocean noise, collect that data and periodically transmit the information to the Cornell lab, where researchers can contact navigators or call Clark's cell phone with their findings.

"I get this little beep all the time, that says, 'You've got whales,'" he said.

When the system is complete, it will send the whales' whereabouts by satellite to Cornell. From there, the information can be sent to a radio transmitter and broadcast to vessels.

Ships operated by Excelerate Energy and Neptune LNG, both shippers of natural gas, must brake to 10 knots in whale areas under the terms of their federal license. Other ship slowdowns are voluntary.

A proposed regulation under review by the White House would require all ships to slow to 10 knots if whales are in the area. Clark thinks that is a reasonable limit, but shippers object to the mandate for safety and economic reasons.

Container ship lines, they say, operate on tight schedules, so slowing down means adding time to a trip and risking higher costs and lost customers.

Metcalf, of the Chamber of Shipping, said reducing speeds to 10 knots can also reduce maneuverability. Her group is pushing for a provision to allow ships to increase speeds in whale areas if needed to safely navigate.

"There's no doubt that anybody on a ship, given sufficient room to do it, would take all the avoidance behaviors in the world" to prevent a strike, she said.

On the Shearwater's recent trip, whales could be seen surfacing amid whitecaps as a team sampled the reddish gooey plankton so researchers could study the whale's food source. Knowing more about the whales' feeding habits could eventually help scientists forecast where the animals appear.

Meanwhile, Clark retrieved a malfunctioning listening buoy and repaired it.

Everything on the boat stopped when a female right whale who had apparently sent out a mating call rolled on her back and waved her flank as several suitors rushed to accept the invitation.

It was a hopeful sign, but Charles Mayo of Provincetown's Center for Coastal Studies cautioned that the species remains on the edge of extinction, despite the sightings of as many as 80 right whales around Cape Cod Bay in the past month.

He wonders how many concessions can be pulled out of the shipping companies or the fishing industry, which is struggling to survive.

"So we slow vessels down, have we done well? Well, we've done as well as we could," Mayo said. "But will that make the difference? Boy, we don't know. It's tough. It's very tough."

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Right Whale Listening Network: http://listenforwhales.org/