The giant slick from the nation's largest oil spill posed a new risk to Alaska's economy as it spread toward a major fishing port 250 miles from where the oil gushed from an Exxon tanker nearly four weeks ago.

Interior Secretary Manuel Lujan was scheduled to arrive in Valdez Thursday for an aerial tour of oil damage along the coast of Prince William Sound. Lujan also will visit the rich fishing ports of Homer and Kodiak and several Alaskan native villages affected by the spill."The Secretary wants a first-hand look before he forms any judgments," said Steve Goldstein, an Interior spokesman.

The trip was arranged by Rep. Don Young, R-Alaska. Young will accompany Lujan, as will Reps. Curt Weldon, R-Pa.; Elton Gallegly, R-Calif.; and Robert Lagomarsino, R-Calif.

A new threat to Alaska's commerical fishing industry developed Wednesday as westward winds pushed oil sludge into Chugach Pass separating Chugach Island from Kenai Peninsula, 250 miles southwest of the spill site. Homer is located just north of the pass.

The Coast Guard was rushing a custom-designed 84-inch-wide boom to protect Homer by containing the oily flow in the pass.

A 425-foot Soviet skimming vessel, larger than any Coast Guard ship, is in the Gulf of Alaska and will begin operations off the tip of the Kenai Peninsula where crude already threatens the Barren Islands and the rich fishing grounds of Kodiak.

"This is our first offensive weapon, an effort to head off the big plume," said Larry Dietrich, a commissioner with the Alaska Department of Economic Conservations.

The Soviet ship has the capacity to sweep up 5,000 barrels an hour and can store up to 50,000 barrels.

The "plume" or thick strand of weathered oil - estimated at about 10,000 barrels - has escaped the booms of Prince William Sound where 10.1 million gallons escaped from the tanker Exxon Valdez beginning March 24.