Bob Mitchell and his three fishing companions had just pulled a plump bluefish out of the Chesapeake Bay when Mitchell looked over the bow of the "Daydreamer" and saw the morning sun glinting whitely off an oily sheen on the water's surface.

When the 55-year-old retired Air Force officer looked closer, the oil slick seemed to be everywhere, extending hundreds of feet in front of the 24-foot craft and half a mile on either side, almost reaching the Chesapeake Bay Bridge to the north of them.

"The water was completely covered with it," Mitchell recalled yesterday. When he saw the extent of the spill, he tried to radio the Coast Guard's Annapolis station, but it was not until 10:30 a.m. Thursday, an hour and a half later, that he got within radio range and made his report.

As Mitchell and his friends headed in to moor the boat near Annapolis, they counted 16 or so huge ships anchored between Thomas Point and the bridge. They speculated which of the ships, if any, might have discharged the heavy waste oil onto the waters of the bay.

That is the same question officials of the Coast Guard and of Maryland's Department of Natural Resources have spent the past two days trying to answer.

Some 4,000 ships from all over the world sail into Baltimore Harbor each year carrying grain and oil and anything that can be loaded into a railroad car-sized container. At any given moment, a dozen of these are anchored between the bridge and Thomas Point, waiting for a space to open up at the Baltimore docks.

"That area's a heavily used commercial anchorage," explained John Griffith, an assistant attorney general for Maryland who spends much of his time working with Department of Natural Resources officials to identify and prosecute violators of the state's water pollution laws.

This acquatic parking lot, Griffith said, has been "an ideal place to perform shipboard maintenance. When you do that you accumulate oily waste in the bilge. And sometimes engineers just discharge whatever's in the bilge."

By late afternoon Thursday, Maryland and Coast Guard officials thought they had identified the extent of the 1,300-gallon spill and had started clean-up operations, aided by calm seas and low winds. In these conditions, removal of a medium-to-small spill like this one are relatively easy, although the cost generally runs well above $2,000.

While they were cleaning, officials of the enforcement division of Maryland's Water Resources Administration were taking samples of the goo for laboratory analysis -- the first step in finding out where it came from.

According to Coast Guard petty officer Mike Waller, there was little doubt that the oil came from a ship. It was too far down the bay to come from one of the industrial plants that ring Baltimore harbor. But there was still a possibility that it was an inadvertent, accidental discharge.

Then, yesterday afternoon, officials of Maryland's Water Resources Administration flew over the spill in a helicopter and spotted two smaller patches of oil to the south, bringing the total size of the spill well above 2,000 gallons.

"This gives us very strong evidence" of a deliberate spill, the Coast Guard's Waller said. "If it were accidental, it's hard to believe it was accidental three times. If there were a problem with a leak it would be continuous."

But the extra spills, he said, made that unlikely. "There was little wind or wave action to speak of, so it wouldn't have broken up that much."

While the clean-up operations continued, Coast Guard officials boarded two of the large freighters at anchor -- the two which, judging from wind and tide patterns, were the most likely source of the spill.Oil samples were taken from the vessels and sent to the Annapolis labs for comparison with the oil taken from the water.

This process, known as "fingerprinting" the oil, is highly reliable, Waller said, since each ship engine leaves a different concentration of metal filings on the oil that passes through it.

But, according to Waller and J. L. Hearn, chief of the enforcement division of the Maryland WRA, it will take several days before the lab analysis is completed -- and the boat which caused it might not be any of the 16 anchored south of the bridge, but rather some other boat moving through the area.

A Coast Guard information network can identify all the vessels headed in and out of Baltimore at the time -- about "20 or 25 in this case," Waller said. But unless the ship is stopped at Norfolk, Va., at the mouth of the bay, -- only a day's journey from Baltimore -- it could be well out to sea before officials could check it.

"If there's one place in this state that you don't want to be a person discharging oil, it is the Annapolis anchorage," said assistant attorney general Griffith. "We prosecute everybody we can get our hands on (who discharges in the area)."

The average number of significant "prosecutable" oil spills around Baltimore harbor and Annapolis, he said, has decreased over the past eight years from three to five annually in the early 1970s to one or two a year in the last two years.

Federal water pollution laws provide a maximum $25,000 fine and one year in jail for criminal violators, and a lesser $5,000 maximum penalty for those found culpable by an administrative law judge. Maryland's criminal penalties are identical to the federal ones.