KAILUA, KONA — Robin Baird's research team gazes for hours into the horizon, searching for rarely seen beaked whales.

The small, grey animal has been at the center of the dispute over the Navy's use of sonar ever since several washed ashore bleeding around their brains and ears during Navy exercises in the Bahamas seven years ago.

Scientists, environmentalists, and the Pentagon have been scrambling to solve the mystery of their deaths ever since.

"They appear to be the most susceptible group of cetaceans to impacts from Navy sonars," said Baird, an Olympia, Wash.-based marine biologist whose team recently spent three weeks off the Big Island studying whales.

The studies come at a critical time for the Navy and the whales.

Training sailors to use sonar has become one of the Navy's top priorities as more nations, including China, have acquired quiet, hard-to-detect diesel submarines. In many cases, the only way the U.S. Navy can find these stealthy ships is by pumping sound through the water with mid-frequency active sonar and listening for an echo.

Galvanized by the strandings, environmentalists are filing lawsuits challenging the Navy's plans to exercise with sonar because they claim the drills will harm whales.

Beaked whales rarely show themselves before humans and are among the least understood marine mammals.

To learn more, Baird's research team ventured off the Kona coast of the Big Island to attach time-depth recorders and satellite tags to beaked whales to monitor the animals' diving patterns and movements around the Islands.

The researchers also took photos of each whale and dolphin they saw to add to a catalog tracking the various cetacean species inhabiting the islands.

Beaked whale adults stretch an average 18 feet — about half the length of a typical humpback whale, the whale most commonly seen in Hawaiian waters. They weigh about 2.7 tons, only about one-tenth or less as much as the humpback.

Making themselves even harder to spot, beaked whales spend much of their time below 2,200 feet, where they forage for squid.

They dive deeper than almost any marine mammal: one of the beaked whales Baird tagged in previous years descended more than 4,900 feet.

Many scientists suspect beaked whales' unique ability to swim at great depths for long periods makes the species more vulnerable to the effects of sonar.

One theory says the sonar's loud noise startles the whales, prompting them to surface unusually rapidly. This gives them gas bubble lesions in organs including the liver, a phenomenon similar to the bends in humans. Still, this hypothesis hasn't been verified.

"The question is, why would it have a different response from other species? Or why would a behavioral response affect them more?" said Baird, a research biologist with Cascadia Research Collective.

Baird, based on four years of field work, estimates there are only 55 Cuvier's and 140 Blainville's beaked whales around the Big Island. His team spotted beaked whales only twice in 17 days on the water off the Kona coast this year. In contrast, it saw pods of pilot whales a dozen times.

But experts don't know enough about beaked whales to say whether their numbers in Hawai'i are growing or shrinking, making it impossible to measure sonar's effect on the population as a whole, Baird said.

For now, much research is focused on beaked whales because they've stranded more frequently than any other marine mammal when and where the Navy has used sonar.

In the Bahamas, scientists this month are running controlled experiments testing how beaked whales and other marine mammals respond to different sounds. Eventually, the scientists will observe how the whales react to mid-frequency active sonar.

This study — near the area where six beaked whales died during Navy sonar training in 2000 — is the first to directly measure how sound affects deep diving marine mammals.

"We still know almost nothing about the reactions of marine mammals to underwater sound," Brandon Southall, the study's principal investigator, told reporters in a recent conference call. "Our field is very much in its infancy."

Differing interpretations of the limited studies available are fueling the prolonged debate between the Navy and environmentalists over the appropriate use of sonar.

The Navy doesn't want to deny its sailors the full spectrum of sonar training because of unproven theories. Environmentalists argue there's enough evidence to require the Navy act on the safe side and take more aggressive measures to protect whales.

Adm. Robert F. Willard, U.S. Pacific Fleet commander, said the Navy is willing to post lookouts on its ships to monitor for whales and limit sonar use when whales get too close.

But he said there's no scientific basis for more stringent measures demanded by some environmentalists, including designating entire areas as non-sonar zones.

"The frustration and challenge is that we are being asked to put mitigating procedures into place, or to not operate and restrict our freedom of operations, without any foundation whatsoever," Willard said in an interview.

But Joel Reynolds, a lawyer for the Natural Resources Defense Council, said science shows high intensity sonar can affect whale behavior.

"It is essential to use the technology in a precautionary fashion and implement common sense measures to reduce the risk of harm," said Reynolds, who leads a team suing the Navy over sonar in federal court.

Southall, who directs the ocean acoustics program at the National Oceanic and Atmospheric Administration, said the results of the Bahamas study should help regulators decide what type of sonar to allow.

The Navy is almost entirely funding the Bahamas study's cost, about $3 million this year. The experiment is still in its first phase and will likely take about three years to complete.