BIMINI, the Bahamas -- The bulldozers moved slowly at first. Picking up speed, they pressed forward into a patch of dense mangrove trees that buckled and splintered like twigs. As the machines moved on, the pieces drifted out to sea.

Sitting in a small motorboat a few hundred yards offshore on a mid-July afternoon, Samuel H. Gruber -- a University of Miami professor who has devoted more than two decades to studying the lemon sharks that breed here -- plunged into despondency. The mangroves being ripped up to build a new resort provide food and protection that the sharks can't get in the open ocean, and Gruber fears the worst.

"At the end of my career, I get to document the destruction of the species I've been documenting for 20 years," he lamented as he watched the bulldozers. "Wonderful."

Gruber's sentiments have become increasingly common in recent years among a growing number of marine biologists, who find themselves studying species in danger of disappearing. For years, many scientists and regulators believed the oceans were so vast there was little risk of marine species dying out. Now, some suspect the world is on the cusp of what Ellen K. Pikitch, executive director of the Pew Institute for Ocean Science, calls "a gathering wave of ocean extinctions." Dozens of biologists believe the seas have reached a tipping point, with scores of species of ocean-dwelling fish, birds and mammals edging toward extinction. In the past 300 years, researchers have documented the global extinction of just 21 marine species -- and 16 have occurred since 1972.

Since the 1700s, another 112 species have died out in particular regions, and that trend, too, has accelerated since the mid-1960s: Nearly two dozen shark species are close to disappearing, according to the World Conservation Union, an international coalition of government and advocacy groups.

"It's been a slow-motion disaster," said Boris Worm, a professor at Canada's Dalhousie University, whose 2003 study that found that 90 percent of the top predator fish have vanished from the oceans. "It's silent and invisible. People don't imagine this. It hasn't captured our imagination, like the rain forest."

Many activists have focused on the plight of creatures such as the ivory-billed woodpecker and the grizzly bear, but relatively few have taken up the cause of marine species. Ocean dwellers are harder to track, and some produce so many offspring they can seem invulnerable. And, in the words of Ocean Conservancy shark fisheries expert Sonja Fordham, often "they're not very fuzzy."

Although a number of previous extinctions involved birds and marine mammals, it is the fate of many fish that worries experts. The large-scale industrialization of the fishing industry after World War II, a global boom in oceanfront development and a rise in global temperatures are all causing fish populations to plummet.

"Extinctions happen in the ocean; the fossil record shows that marine species have disappeared since life began in the sea," said Elliott A. Norse, who heads the Marine Conservation Biology Institute in Redmond, Wash. "The question is, are humans a major new force causing marine extinctions? The evidence, and projections scientists are making, suggest that the answer is yes."

Large-scale fishing accounts for more than half of the documented fish extinctions in recent years, Nicholas K. Dulvy, a scientist at Lowestoft Laboratory in England, wrote in 2003. Destruction of habitats in which fish spawn or feed is responsible for another third. Warmer ocean temperatures are another threat, as some fish struggle to adapt to hotter and saltier water that can attract new competitors.

But nothing has pushed marine life to the edge of extinction more than aggressive fishing. Aided by technology -- industrial trawlers and factory ships deploy radar and sonar to scour the seas with precision and drag nets the size of jumbo jets along the sea floor -- ocean fish catches tripled between 1950 and 1992.

In some cases, fishermen have intentionally exploited species until they died out, such as the New Zealand grayling fish and the Caribbean monk seal; other species have been accidental victims of long lines or nets intended for other catches. Over the past two decades, accidental bycatch alone accounted for an 89 percent decline in hammerhead sharks in the Northeast Atlantic.

Today, sharks, along with sturgeon and sciaenids (known as croakers or drums for the sounds they make undersea), are among the most imperiled of the species that spend most of their lives in the ocean.

Populations of sharks, skates and rays -- creatures known as elasmobranchs that evolved 400 million years ago and have skeletons of cartilage, not bone -- have difficulty rebounding because they mature slowly and produce few offspring. Shark-fin soup, an Asian delicacy that sells for more than $100 a bowl, has spurred intensified shark hunting in recent years.

Despite the sturgeon's fecundity, overfishing and habitat destruction have caused that population to dive as well. Beluga sturgeon, the source of black caviar, release 360,000 to 7 million eggs in a year, Pikitch noted, but they have declined 90 percent in the past 20 years. Just this month, scientists in Kazakhstan reported that they failed to find a single wild, reproducing beluga female, leaving them with no eggs for hatcheries.

Croakers' large swim bladders -- air-holding sacs that help them maintain buoyancy -- account for their imminent demise. Traditional Chinese medicine prizes the bladders, and the sound they make when pressed against vibrating muscles can reveal croakers' location to fishermen through sonar.

"They've been survivors on an evolutionary scale, but they've met their match, and it is us," said Pikitch, who writes about sharks and sturgeon in an upcoming book, "State of the Wild 2006."

Despite scientists' warnings, American and international authorities have been slow to protect marine species. The only U.S. saltwater fish to make the protected list is a ray, the smalltooth sawfish, which was added in 2003.

The National Oceanic and Atmospheric Administration's Fisheries Service is charged with protecting 61 threatened or endangered marine species. Director Bill Hogarth said his agency focuses on protecting vulnerable populations so they will not have to be listed.

"That's our job -- to make sure species don't wind up on the endangered species list," he said.

But conservationists said NOAA officials are reluctant to classify fish as endangered because doing so conflicts with the agency's mission of promoting commercial fishing.

Michael Hirshfield, chief scientist at the advocacy group Oceana, said he has repeatedly seen government officials provide shifting estimates of how many threatened or endangered sea turtles can acceptably die each year in eastern scallop fisheries.

"You never get an answer to the question how many turtles would have to be killed before you would say, 'That's not okay,' " he said.

On Bimini, 50 miles from the Florida coast, Gruber is trying unsuccessfully to stave off the golf resort that could bring 5,000 tourists a day. The island has just 1,600 residents but supports more than a dozen shark species.

Based on an 11-year survey starting in the mid-1990s, Gruber documented that between 2000 and 2001, during the heaviest dredging of the ocean floor for the resort's construction, the survival rate for lemon sharks fell 30 percent, and sharks in the dredging area had higher toxin levels. He has yet to assess the impact of the mangrove destruction, which began on a large scale this year.

The president of the Bimini Bay Resort and Casino, Rafael Reyes, said he understands the concern but questions Gruber's statistics and the idea that "sharks and development don't mix."

"We have a vested interest in making sure things remain as they are," Reyes said, adding that he is demolishing mangroves in a place that is "basically not a sensitive area. . . . I have to make sure the environment's pristine because my clients are fishermen."

But Gruber remains unconvinced.

"I believed when I started the ocean was so vast there was no way you could ever kill off the sharks or anything," he said. When it comes to being a fish, he said, "Now you can run, but you can't hide."

• Washington Post article