GALVESTON, TEX., AUG. 1 -- Working to mitigate an environmental disaster, salvage crews today had skimmed up to one-tenth of the oil that spilled from two barges after they collided with a Greek freighter in the Houston Ship Channel Saturday.

Coast Guard officials reluctantly agreed to allow limited use of oil-eating microbes in areas where mechanical salvage was not possible.

Crews were aided by good weather, favorable winds and the apparent mystery disappearance of a substantial portion of the estimated 500,000 gallons of "catfeed oil," a refinery oil similar to No. 5 fuel oil.

Coast Guard Rear Adm. James Loy said that in a flight today over the 17-mile-wide slick, he was surprised to see that the heavy black portion of the slick was 20 to 25 percent of the size it was on Tuesday.

Salvage crews have skimmed perhaps 50,000 gallons of oil, he said, and the owners of the three-barge tow -- Apex RE&T Towing Inc. of St. Louis -- said a substantial amount of oil may remain in one tank of a half-sunken barge that may be raised as early as Thursday. But Loy and state officials said that would not account for the missing oil.

"As to the missing oil, that is a dilemma that is puzzling all of us," Loy said.

Coast Guard Capt. Tom Greene said it is unlikely the oil sank to the bottom of Galveston Bay. He said the Army Corps of Engineers took samples from the bay bottom up to a mile from the accident site and found no oil except in the immediate vicinity of the crash. Greene said Louisiana State University is now calculating whether that much oil could have evaporated so quickly.

"If it sank to the bottom and killed marine life, we're in trouble," said Thomas E. Ryan of the Texas Division of Emergency Management. "If it evaporated, we're all right." The state suspended all oyster harvesting in the area until further notice.

So far, the impact on wildlife has been minimal. Volunteers preparing to rescue and clean oil-soaked birds had found only a few today.

But environmentalists said it is almost unthinkable that the long-term effects of the spill will not be disastrous, and Loy said they may be right.

The oil slick is spotted from the northern shore of Galveston through delicate areas of salt marsh around Pelican Island and up to Eagle Point. Such estuaries where salt and fresh water mix are nesting grounds for birds and other wildlife. At least 100 species of birds, including the endangered brown pelican, inhabit the area, some parts of which are so delicate that the Coast Guard says attempts to clean them would cause more damage than the oil.

Economic costs also may be heavy. The Houston Ship Channel, a 50-mile waterway that connects a dense concentration of refineries and petrochemical plants with the Gulf of Mexico, was closed for three days, with one damaged barge perched precariously on a ledge over the channel. At least 60 ships were backed up.

The channel was opened to inbound barge traffic on Tuesday and to outbound barge traffic today, largely clearing out the shallow-draft backlog. However, deep-draft ships still cannot pass for fear their wake would nudge the stranded barge into the bottom of the channel.

To the north at Ellington Air Force Base, the Coast Guard and the National Transportation Safety Board opened hearings into why the freighter Shinoussa and the three-barge tow collided on a sunny afternoon in the middle of the bay.

The spill at first was thought to be minor. Even the NTSB, normally on the scene of any disaster within hours, did not arrive until Tuesday.

Texas Gov. Bill Clements (R) also was caught unaware, continuing to attend the National Governors' Association in Mobile, Ala. But today he declared a state disaster area, which places his office in charge of the state response. The Coast Guard remains in charge.

The Coast Guard Regional Response Team agreed to their limited use in areas that could not otherwise be cleaned, and under tight controls. The Coast Guard has been fearful of adding new biological agents to a delicate area.