They are ghastly, if not ghostly images that will remain in the divers' minds for some time.

Those of the large hammerhead shark that ruled the reefs a day earlier, dangling lifeless from an invisible net, its slender body twisting in the current. . .

Of the large sea turtle that a day earlier was using its little legs to push through the blueness, using those same little legs in a vain attempt to free itself from the net wrapped tightly around its neck, squeezing its life away. . .

Of the giant manta that soared gracefully between the amber shafts of sunlight a day earlier, now stuck in a vertical position, its white underbelly pressed flat against a monofilament curtain, flapping its wing-like fins but going nowhere.

These, of course, were not the encounters the divers had hoped to have with the wildly wonderful inhabitants of Mexico's Revillagigedo Islands. But then they rarely share their paradise with poachers.

"To see those sharks wrapped up in those nets with their fins folded against their chests as if they're in some sort of deathly prayer is . . . just sickening," said Terry Maas, 55, a renowned free-diving spear fisherman from Ventura.

Maas, a blue-water hunter who at least can boast of being selective, was among several divers who for four days earlier this month witnessed commercial fishermen aboard seven Mexican vessels deploying and redeploying miles of gill-nets in the supposedly protected waters of San Benedicto Island, 220 miles south-southwest of Cabo San Lucas.

In all, the divers estimate, the netters killed 2,000-4,000 sharks, removed their fins--presumably for sale in Asian markets--and, in most cases, discarded their bodies.

San Benedicto is the northernmost of the four-island Revillagigedo archipelago, which was established as a "biosphere reserve" in 1994 because of its unique and highly vulnerable ecosystems.

The islands are home to such threatened species as the Socorro mockingbird, Townsend's shearwater and Socorro green parakeet. They boast a variety of native plant and animal species and have been referred to as "Mexico's little Galapagos."

The inshore waters are popular among free-divers and scuba divers, who enjoy the company of giant Pacific mantas, playful dolphins and curious turtles. The divers must also put up with several species of sharks, but the sharks are an attraction in their own right, Maas said, until they start "dogging us, which can be damn frightening."

Maas was one of five passengers aboard the Ambar III out of Cabo San Lucas. The Solmar V, also from Cabo San Lucas, was the other vessel arriving at San Benedicto on May 1, when the gill-netters had already gone to work, having "surrounded" the island with nets, working as close as 400 yards from shore.

By law, commercial fishing vessels are not allowed to even navigate within 12 miles of the islands. Sport and spear fishermen are allowed inside 12 miles, with special restrictions.

Mike McGettigan, owner of the Ambar III and founder of SeaWatch, a conservation organization, said in a report on his Web site, "In two days of diving at Benedicto after the gill-net boats left, we didn't see one live shark, whereas before we would have seen hundreds."

Photographs taken by the divers show nets wrapped around sharks being hauled in methodically by crews that generally ignored the divers, and decks of the rusty old vessels littered with shark fins. Also caught were several species of fish, turtles, and at least one giant Pacific manta. The killing of mantas can result in a $10,000 fine.

Mexican fisheries officials are investigating the incident, but as of Thursday had made no arrests. The seven vessels reportedly were still at sea, and there were unconfirmed rumors that one of them sank in rough weather.

Julia Carabias Lillo, Mexico's environment secretary, disputed the number of sharks killed, saying, "This figure [2,000-4,000] is not possible in that area."

Without offering specifics, she added that Mexico is taking steps to better enforce laws against fishing in Mexico's marine reserves.

"We know that there is a problem in the Revillagigedo of illegal fishing, but not in the amounts that have been reported, not at all," Carabias said.

A report by the Mexican government to be released next month will address the issue.

With the Revillagigedo Islands more than 350 miles off mainland Mexico and 220-plus miles from Cabo San Lucas, enforcement is difficult at best. There's a Mexican navy base on Socorro Island, the largest of the four and 40 miles from San Benedicto. Otherwise, the barren islands are uninhabited.

The navy's resources are limited and, in fact, the crews of sport fishing vessels from the United States, dive boats and private vessels from Cabo San Lucas are considered more effective watchdogs.

The last time a slaughter of this magnitude created such a stir, it was because two recreational sailors from the United States documented the fishermen's actions with a video camera.

The so-called Valentine's Day Massacre on Feb. 14, 1994 aired throughout Mexico and served as a catalyst in the movement to create a reserve and save the giant Pacific mantas, which diving and ecotourism industries say help generate at least $2 million annually.

The sailors, a man and woman who had been diving just the day before, were at San Benedicto when two Mexican fishing vessels arrived. Their crews used nets, baited hooks and harpoons to catch and kill anything they could.

Gill-nets and long-line gear were used to catch dozens of sharks, which were finned and discarded. The sharks were still alive, but without fins they couldn't swim and ultimately came to rest on the ocean floor.

The fishermen used harpoons and gaffs to kill and lift at least one giant manta, whereupon crew members in a skiff used axes to hack off its wings.

Other mantas were trapped in gill-nets and were too heavy to be hauled aboard, so the nets were simply cut loose with the mantas still in them. They settled over the reefs, trapping and killing other fish.

Two of the mantas the couple had been riding a day earlier were identified in the nets and eventually "torn to pieces." The woman's sobbing and cries, "Oh, God, no!" were picked up by the camera.

To be sure, the giant mantas are what make a trip to the islands truly magical. With wing spans of 20 feet, the docile plankton eaters often hover below boats, like alien craft waiting to take passengers on a trip through space.

Though responsible divers no longer ride on the backs of mantas, they do gently touch and rub the animals, which seem to appreciate the experience.

"When an animal with a 20-foot wing span swims by and stops and just lets you rub your hands against its body and looks you in eye, it's really just a humbling experience," said Leslie Lee, who owns a Cabo San Lucas dive shop with her husband, Jose Luis Sanchez. "And anyone who does this, soon develops a spiritual connection with these creatures."
At San Benedicto, one of the mantas' favorite hangouts is Boiler Rock, a pinnacle that rises to only a few feet beneath the surface, creating a boiling effect. The mantas often hold still in the current while "cleaner fish" such as jacks and angelfish pick parasites from their bodies.

"I think this is why they interact so well with humans," said Sanchez, who also is the U.S. representative for the Solmar hotel and Solmar V. "We show divers where to scratch their bellies and how to touch them without harming them. My theory is that they think of us as giant cleaning-station fish.

"They like our bubbles against their bellies. . . . And when we leave, they look at us, like, 'Why are you leaving? Please don't go.' "
They might have had other words for the gill-netters. Remarkably, the mantas were largely spared during the most recent raid--only one was seen trapped in the nets.

Sanchez was quick to point out that the islands' waters no longer resemble the macabre sort of graveyard created by the gill-netters, that Mother Nature has acted quickly in restoring the dazzling beauty the region is known for.

Reading from the log of the Solmar V, which was back at the islands a few days after the gill-netters had left, he said the 76-degree water with 80-foot visibility was teeming with life.

"The Boiler was very good with dolphins in the morning and numerous mantas, which were very friendly," Sanchez said.

Sharks were understandably scarce at San Benedicto, but were plentiful at nearby Roca Partida, the smallest of the four islands, and Socorro.

"The divers saw great numbers of sharks," Sanchez said. "Galapagos [sharks], duskies, silver-tip sharks and hammerheads. As in previous trips, the sharks put on a spectacular show by aggregating in groups of about 20 and coming in close to look over the divers--the divers were very happy."

If not a little bit frightened.

Times staff writer Esther Schrader, in Washington, contributed to this report.