Nothing marks the boundary that runs down the middle of this vast, frigid water world.

Yet dozens of fishing boats cluster around it. Coast Guard cutters prowl along it. Surveillance jets swoop over it. And sometimes, shots are fired when poachers pay no attention to it.

The Russian-U.S. maritime line slices across the Bering Sea, the richest fishing grounds left in an overfished world.

It's where the U.S. Coast Guard cutter Storis is on patrol, pitching over watery hills in a hunt for foreign trawlers that sneak into U.S. waters.

Lt. Christina Dutton, into her 16th hour on duty, balances on the slanting bridge in the green glow of a radar screen, her red-rimmed eyes checking the blips that will alert her to an intruder.

"On the line, you can't let your guard down," she says.

Out here, the line means everything to everyone except the prize: the millions of pollock swimming 60 stories below the surface. These fish represent about half of the 4 billion pounds of seafood swept from Alaskan waters each year--a $1 billion business in a good year.

On the Russian side, dozens of trawlers from Japan, China, South Korea, Thailand, Taiwan, Poland and Norway work unchecked, having paid the cash-starved Russian government for fishing permits.

With each passing year, the catch dwindles, and the vessels creep closer to the line in the sea.

On the American side, the U.S. Coast Guard watches and waits.

Day and night, helicopters, C-130 planes and four cutters patrol a line as long as the road from Boston to Miami. Mission: To keep out hit-and-run trawlers that have already depleted the waters on the Russian side.

There's no glory in being a fish cop. There are 20-hour shifts, monotonous tasks, cramped quarters, bad weather. Gale-force winds whip up swells as high as five-story buildings.

And there's the isolation that, as Ricky Dickerson of Booneville, Miss., has learned after a month away from land, chips at the nerves.

On this day, the 18-year-old "boot"--rookie--is halfway through a four-hour watch on the flying bridge. He's fighting the roll, a wind chill of 10 below zero and the thought that his sweetheart may not wait for him to finish his tour.

"If you're looking for the end of the world," he says, "you found it."

Waves wallop the hull of the Sherman as it steams across the Bering Sea to relieve the Storis.

It's Capt. Robert M. Wicklund's job to see that poachers don't outwit or out-wait them. He likes to approach the line in darkness and mingle with a pack of vessels, pretending to be a fishing boat. "We like to sneak up on the buggers, then pounce."

For a 3,070-ton vessel to pounce, all 147 men and 20 women in the crew must know their duties well enough to perform them in the dark.

It means Ryan Curry, a 19-year-old apprentice seaman from Seattle, has to be wide awake at 2 a.m. for his four-hour watch--even after a 16-hour day of fire drills, cleanup detail and messenger duty that ended at midnight.

And yet, the morale on the Sherman remains expectant as the ship approaches the line in the sea.

Suddenly, the water boils to starboard and a gray whale surfaces. "When you see more whales up here, like we have, that's a good feeling," says Chief Quartermaster Peter Hoking. "It reminds you that, hey, maybe you're doing something right, maybe you're making a difference after all."

CAPTION: Lt. Christina Dutton contacts a fishing vessel in the Bering Sea from the Coast Guard cutter Storis. "You can't let your guard down," she said.

CAPTION: Isaiah Peppard stands watch.