Seven rehabilitated rough-tooth dolphins released off the Florida Keys more than a week ago were located Tuesday between the southern Bahamas and eastern Cuba, marine officials said.

Data received from satellite transponders attached to two of the mammals tracked them to about 70 miles north of Guardalavaca, Cuba, said Randall Wells, director of the Mote Marine Laboratory's Center for Marine Mammal and Sea Turtle Research.

The Bahamian Coast Guard visually sighted all seven dolphins off the southern tip of Andros Island Saturday afternoon, said Robert Lingenfelser, president of the Marine Mammal Conservancy in Key Largo.

The mammals were among 68 that stranded March 2 on a mud flat near Marathon in the middle of the Florida Keys.

The dolphins are thought to have traveled about 500 miles together since their release May 3 off Key Largo, Lingenfelser said.

''We're very pleased with the progress of these animals,'' he said.

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When more than 70 dolphins beached themselves in the Middle Keys in March, leaving well over half their number dead, there was one immediate suspect: a U.S. Navy submarine conducting exercises off the Keys in the days before the marine mammals came to shore.

The USS Philadelphia used mid- and high-frequency sonar at least some of that time to navigate - a fact that piqued the interest of marine mammal experts and other scientists who believe that powerful sonar transmissions from Navy submarines and ships may harm dolphins and whales in ways not yet understood.

Nearly two months later, investigators from the National Oceanic and Atmospheric Administration are still conducting tests that may show whether sonar was responsible. And a few biologists who performed necropsies on some of the animals observed heart and lung problems that may not be indicative of harm from sonar. Sonar - "sound navigation and ranging'' - is an apparatus that transmits sound waves through water and uses echoes from undersea terrain or solid objects in the water to determine distance and movement.

"We haven't ruled anything out. We are in the process of compiling results,'' said Laura Engleby, a NOAA Fisheries biologist involved in the stranding response.

What remains are tantalizing questions - and another high-profile incident that has raised awareness about sonar technology and placed its patron, the Navy, on the defensive.

"I think many scientists are becoming increasingly concerned with this coincidence of stranding in time and space with the use of naval sonars,'' said Andrew J. Read, a Duke University professor considered a leading expert on the subject of sonar and dolphins. "This is one of those human activities that is rising to the top of our concerns.''

After surfacing Feb. 28, the nuclear-powered submarine used mid-frequency active sonar about 39 nautical miles from Marathon, Fla., to navigate for about 21 minutes. On three other days, Feb. 27, March 1 and March 2, when it was farther away, the sub used high-frequency sonar, the Navy said. The dolphins are believed to have begun beaching March 2.

Marine mammal strandings are fairly common, prompted by anything from algae blooms to a single, sick animal. Six weeks before the Keys stranding, about 35 whales of three species stranded off North Carolina. Though the Navy was conducting sonar exercises nearby, the cause of the deaths in that case has not yet been determined.

Marine mammals have their own highly developed "sonar'' systems of echo-location "clicks'' - used for everything from finding a mate to procuring a meal.

Humans hear sound frequencies up to 20 kilohertz, the highest pitch most babies can process. Frequencies above that are called ultrasonic. Some dolphins, whose inner ear is encased in a separate bone, have a range about five times higher.

Some scientists believe ships' sonar may disorient or scare marine mammals, causing them to surface too quickly and creating the equivalent of what divers know as the bends, nitrogen bubbles forming in tissue because of sudden decompression.

There are three types of sonar: low-frequency, which travels the farthest and is often used for search and surveillance; mid-frequency, commonly used in training; and high-frequency, which is the weakest. The exact ranges of military sonars, which emit individual "pings'' lasting a few seconds, are classified. The distance a sonar ping travels is shaped by water temperature, salinity, topography and the presence
of other objects in its path.

The more powerful, low-frequency sonar has drawn the most fire from groups such as the Natural Resources Defense Council, which sued the U.S. military in 2002 to limit its use.

"Sonar kills whales,'' said Michael Jasny, a senior policy consultant for the council. "What we've seen from recent events is that sonar seems to be responsible in the deaths of more marine mammal species than had earlier been supposed.''
Navy scientists - who argue that the science is still unfolding - don't agree.

"Until you know what the effects are, everything is speculation,'' said Frank Stone, head of marine mammal research and development for the chief of naval operations.

The Navy is the largest sponsor of studies on marine mammal responses to sonar, spending about $10 million a year on the topic.

The most conclusive link between the use of military sonar and injury to marine mammals was observed from a stranding of beaked whales in 2000 in the Bahamas. The Navy later acknowledged that sonar likely contributed to the stranding of the extremely shy species.

"We basically herded the mammals into a boxed canyon and they couldn't escape the sound of the sonar and so they beached themselves,'' said Rear Adm. Steven Tomaszeski, oceanographer of the Navy. "In our investigation of this, we did find out that in the case of the beaked whale, it does seem very susceptible to mid-frequency sonar.''

Military sonar has also been alleged as a factor in the strandings of porpoises off the coast of Washington state and of melon-headed whales off the coast of Hawaii, while other potentially sonar-related strandings have occurred in Greece and the Canary Islands.

While a NOAA report recently released on the Washington porpoise deaths left open the possibility that sonar played a part, the report was largely inconclusive, saying that decomposition of the animals made pinpointing the exact cause of death impossible.

Though sonar can produce signs of physical damage, like bleeding in and around acoustic tissue, some experts wonder whether sonar-induced behavioral changes might occur in some cases.

"Sound is the most important sense for these animals, and one of the big issues now that people are struggling with is whether the impacts that have been documented have been a direct result of the sound - basically damage to the tissues - or has the sound created a behavioral reaction that caused damage,'' said Trevor Spradlin, a NOAA biologist.

The U.S. government has fought to protect the military's ability to train with active sonar in the face of mounting international criticism from groups such as the International Whaling Commission, which last year issued a report linking sonar and whale deaths.

Among the military's main concerns is a new class of stealthy, easier-to-obtain diesel-electric submarines believed to be possessed by countries such as North Korea and Iran.

The Navy has commissioned and conducted a number of studies aimed at gauging the effect of particular levels of sound exposure, some of it from sonar, on the hearing of marine mammals. The research has shown that some whales experience what's called a "temporary threshold shift" in hearing, a change that fades within minutes, depending on how far away the animal is from a single sonar emission, said Mardi Hastings, program officer for marine mammal science and technology at the Office of Naval Research.

Still, "the problem with sonar is that there are usually multiple occurrences, that the animal may experience more than one ping,'' Hastings said, adding that tests now under way on dolphins in California should provide a more detailed understanding.

Another ongoing study involves tagging different animals with tracking devices that allow scientists to get a better idea of their behavior patterns. Duke's Read is also working on a Navy-funded project to synthesize statistics on marine mammal location and behavior into a model that's able to predict the animals' habitats at specific times.

Still, high costs, the secrecy surrounding the technology and difficulties testing federally protected animals in natural settings have turned basic research into a major challenge.

"The science is very much in its infancy,'' NOAA's Spradlin said.

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