The shark's mouth opened, wide and white as a porcelain dinner plate. Its luminous blue-green eyes glowed brightly in the sunlight.

A wiry, redheaded fisherman quickly wrapped a wet washcloth across the shark's big gray head - shielding its sensitive eyes from the sun - and shoved a hose full of sea water inside one of its large gills so it could breathe.

Briefly captive on a commercial fishing boat, the huge, bluntnosed sixgill shark returned to Colvos Passage with a quarter-sized orange identification tag wired to its dorsal fin. Once freed, the shark ducked beneath the gray surface, kicked twice and dove, invisible except for the white of its belly, which soon disappeared, too.

Mystery surrounds sixgills, Puget Sound's largest resident sharks at an average of 7 to 9 feet long. As predators at the top of their food chain, they are an important influence in the deep waters where they dwell.

Yet scientists and fish managers know little else about the habits of the Sound's sharks and their links to other sixgill populations, including those in parts of the world where overfishing has put them in danger.

To begin to answer such questions, the state Department of Fish and Wildlife is combining state, federal and donated money for a two-year, $41,000 study. What they learn could help state officials determine whether sixgills need protection from sport and commercial fishing, said Greg Bargmann, the Fish and Wildlife Department's marine fish manager.

"We think we really know about Puget Sound and the ocean, and we really don't," he said.

Last week, Bargmann chartered the Saint Peter, a 58-foot fishing boat, and its Anacortes-based crew for four days. Fishermen set miles of hooks baited with salted herring in some of the deepest waters of the South Sound.

"This seems to be the center of their population, right down here in the Tacoma area," Bargmann said. He believes sharks use the region as a nursery, but scientists don't know for sure.

"We're breaking brand new scientific ground here," said University of Washington shark expert Vincent Gallucci, who was on the boat. "It's a scientific project to die for."

Researchers watched as fishermen hauled in dozens of sharks and carefully hoisted each on deck in a specially built sling. Crew members helped technicians and scientists tag, measure and weigh the sharks as well as snip bits of fins.

The team's speedy sampling protocol was designed to ensure shark survival. Each shark was released into the water no more than five minutes after being lifted out.

The department's goal is to tag 200 to 250 sharks. Each tagging site has been assigned a different color code to help track shark movements around the Sound. They've already conducted tagging operations north of Hood Canal and close to Olympia.

Last week, Gallucci and a couple of his graduate students also planned to autopsy as many five sharks. They hoped to pick up information about shark reproduction, eating habits and pollution that scientists cannot collect from live fish.

Among other things, Bargmann would like to know if sixgills are contaminated with mercury, as are dogfish, a smaller shark which may be the sixgills' favorite food.

Also like dogfish, sixgills are believed to live a long time and reproduce late, which could make them vulnerable to overfishing, said Gallucci, a professor of aquatic and fishery sciences.

He plans to use the tagging data and reproduction information to estimate Puget Sound's sixgill population. His UW colleague, fish geneticist Lorenz Hauser, will compare genetic identifiers taken from the Puget Sound sixgills with other shark populations elsewhere in the world.

The UW is a part of a consortium of organizations that share sixgill research in the Sound. Others include Tacoma's Point Defiance Zoo & Aquarium and the Seattle Aquarium.

"This is a major step forward in the way fisheries are managed," said Jeff Christiansen, a Seattle Aquarium biologist. Last summer, he and other Seattle Aquarium divers began tagging sixgills in Seattle's Elliott Bay as part of a parallel 10-year study.

To Christiansen, sixgills are "charismatic megafauna," the kinds of animals that zoos and environmental advocacy groups build fund-raising campaigns around. "People are drawn to sharks. Sharks sell at virtually every level," he said.

Public attitudes have changed, particularly in the last 10 years, said John Rupp, Point Defiance aquatic animal curator. Before then, sharks were "trash fish, expendable," Rupp said. "It was a post-Jaws mentality," he said.

Now, in conservation circles, sharks have a cache similar to wolves, predators that play a similar role in the food chain, Rupp said.

Washington is among the only places where big sharks are not harvested by commercial fishermen, Gallucci said. Elsewhere, overfishing has nearly wiped out some populations of large sharks such as sixgills.

Conservationists first raised concerns about the future of the Sound's sixgills in 2000 when anglers in Seattle's Elliott Bay began catching the fish at night, when the sharks venture into shallow water.

Fish and Wildlife managers changed the rules. As it stands, anglers may catch sixgills, but only if they release them.

Some of the sharks tagged last week were breathtaking in size.

"Imagine catching that on a rod and reel," said Bargmann as the crew raised a 630-pound shark out of the water. As it turned out, the 11-footer was the largest of 45 fish tagged that day in Colvos Passage. Many weighed several hundred pounds, but most were considerably smaller.

Despite their size, sixgills are docile creatures. They don't attack people.

"Once you bring them up, they're lazy. They don't do much," said Curtis Beale, the redheaded fisherman who blindfolded the sharks and then yanked the big circular hooks out of their jaws.