PITY the poor hamletfish. Compared with whales, whose eerie aquatic arias can be found in record stores, hamletfish can't carry a tune in a bucket. In fact, their sounds register somewhere between a grunt and a politely suppressed burp.
But then the discovery that hamletfish make any intentional sound at all is fairly recent. And it's giving marine biologists a new tool for tracking conditions in sensitive coastal underwater ecosystems, says Phillip Lobel, chief scientist for environmental studies at the Pacific Equatorial Atoll Research Laboratory on Johnston Atoll, about 400 miles southwest of Hawaii.
Using relatively inexpensive but sensitive underwater microphones known as hydrophones and an 8-mm video camera, Lobel has been able to record fish sounds in the wild and link them directly to spawning behavior. The technique he and his colleagues have pioneered is one of a growing number of non-invasive approaches biologists are adopting to study species -- particularly threatened or endangered species.
''We've known for years that certain fish make sounds,'' largely by studying them in aquariums, Dr. Lobel says. But only with a few species have sounds been pegged to specific behaviors, particularly courtship.
Using the hydrophone-video camera package both in aquariums and in the wild, he hopes that researchers will begin to catalog sounds, as well as determine whether the sounds are behavior-specific and species-specific.
''This really is a frontier of science,'' Lobel says enthusiastically. He points out that there are 25,000 different species of fish, including 2,000 different kinds of fresh-water fish -- known as cichlids -- from South America, southern Asia, and Africa that are commonly kept as pets in home aquariums. With readily accessible subjects, relatively common and inexpensive technology, and a paucity of data, even serious amateurs ''can make legitimate discoveries,'' he says.
Lobel's approach grew out of his work on coastal marine environments at Johnston Atoll, which also is the site of a national wildlife refuge.
''The first problem in any environmental study is separating the anthropogenic changes from the natural changes,'' he says. Uninhabited until 1932, the atoll first served as a refueling station for the military and for transoceanic airline flights.
Despite its remote location, Johnston's problems mirror those of coastal areas everywhere -- problems ranging from chemical pollution to overfishing. It was a transshipment point for the herbicide Agent Orange during the Vietnam War, for example, and is the site of a huge incinerator to get rid of chemical weapons. One small area of the atoll hosts what may be the only remaining colony of a species of moray eel that crawls out on shore to catch crabs for a meal. Perhaps because it feeds on a crustacean that itself is a human delicacy, the eel has graced too many dinner tables for its own good.
Because of Johnston's intense use by the military, everything is well documented. ''That's what makes Johnston Atoll such a wonderful lab,'' Lobel says.
The scientist's interest in piscine mating behaviors stems from their value as environmental barometers. ''Long before a fish goes belly-up, it stops reproducing,'' he explains during an interview in his office at Boston University's Marine Biology Laboratory at Woods Hole.
''The majority of coastal marine fish produce plankton-like eggs and larvae,'' he says. Because these eggs and larvae float near the ocean surface, they are particularly vulnerable to any surface-borne contaminants. ''To assess risks, you have to know where the eggs and larvae are. Fish spawning is important to determining the impact of humans on fish populations.''
Yet traditional techniques for studying spawning patterns have major shortcomings. ''You can open a fish up,'' he says, but that doesn't give information over time and kills a member of a species researchers may be trying to save. Scientists also have dragged nets to collect eggs and larvae, but the method is time-consuming and costly. Or researchers can don scuba equipment and try to observe spawning first-hand, which can lead to odd hours and be disruptive to the fish under observation.
Lobel, who also spends time at marine labs in the Caribbean, first reported the mating-specific sounds in an article published in 1991. Using a Sony minicam in a waterproof housing and a hydrophone made from United States Navy sonobuoy components, he captured the mating sounds and activity for the striped parrotfish, damselfish, and hamletfish over a two-year period.
Because these are among the most widely studied tropical fish, correlating sounds with behavior was easier than it would have been with less-well-known species. The mating sounds of the parrotfish come as a rushing noise as a group flits toward the surface, deposits and fertilizes eggs, and plunges back to deeper water. The hamlet, which spawn in pairs, generate a pulsed sound. The recordings can be represented graphically to give the equivalent of a voice or sound print.
One of the challenges, Lobel says, is separating man-made noise from fish-made sounds. He demonstrates this by playing a videotape of fish mating: In addition to the fish sounds, the clanking of a mooring chain a kilometer away rings clear.
The distances sounds travel, the keen hearing of fish, and their vulnerability to predators when mating have led to sounds that are faint and short-lived. ''They need to be discreet,'' Lobel says. That discretion also made it difficult for researchers to hear and identify the sound until sufficiently sensitive technologies became available.
Although the detection technique is now about seven years old, ''we're still in a baseline-data gathering stage,'' Lobel says. With so many fish and so much else to do, little wonder that he encourages more people to join the effort.