Researchers plan to plumb Tacoma's depths this summer for giant sharks, known as six-gill sharks, to find out how far they travel and how good they are to eat.

The Department of Fish and Wildlife will bring a commercial long-line fishing vessel into Commencement Bay and try to catch sharks on hooks baited with herring or chunks of dogfish - a favorite six-gill food.

It will use a sling to bring the fish on board, where it will take measurements and body samples, then will release the fish.

"We're working with the University of Washington and the Seattle Aquarium," said Greg Bargmann, the department's head of marine fish management. "We're trying to capture some fish in the Tacoma area. ... Admiralty Inlet and on the coast" in Willapa Bay.

In Seattle's Elliott Bay, the aquarium is experimenting with underwater tagging, using divers to ambush sharks attracted by bait.

"We want to capture, tag and release fish in Puget Sound and in the ocean, and see if we get movements back and forth," Bargmann said. "We also want tissue samples for contaminants, particularly mercury levels.

"We expect six-gills may have quite high levels, especially in the Tacoma area," he said. "They feed quite heavily on dogfish shark, and dogfish are known to have high levels of mercury in them."

Six-gills grow to about 12 feet in Puget Sound, and the largest Pacific specimen measured 15 feet. Mercury tends to accumulate most markedly in large, long-lived fish, and six-gills are thought to be such fish, although no one knows how long they live.

"We are trying to answer that question," Bargmann said. "We suspect it's a long time."

One of the things the department wants to determine is whether it can allow a six-gill fishery, he said.

"There's a fair amount of interest to have a fishery, both commercial and recreational," he said. "We stopped fishing in 2000, and have a number of requests to reopen."

Before the department could do that, it would have to develop a management plan for the species.

"We don't know how long they live, how productive they are, how many are out there," Bargmann said. "So we don't have the basic information to allow any kind of a harvest."

He said the department might obtain what it needs by fall of next year. "If they have high levels of mercury, I don't think we would want to open a fishery if it's going to result in human consumption," Bargmann said. "So that's the first thing I want to look at."

A small but avid group of sport fishermen used to pursue six-gills in Elliott Bay.

"In the recreational fishery people were eating them," Bargmann said. "They're called 'snow shark' because their flesh is very white. It tastes pretty good. And the commercials want to catch them to sell them as snow shark."

Six-gill sharks inhabit the Pacific coast from Chile to Alaska. Scientists don't know if they live in separate populations or if individuals move back and forth.