Exhaust fumes were clouding the late afternoon air above the 14th Street Bridge. National Airport jet traffic was thundering low over the Potomac. And William Comfort was standing on a rock beside the muddy river, oblivious to the sound and smoke, fishing for his supper.

"With the economy the way it is, you've got to look forward to catching something," said Comfort, a Washington construction worker who lost his job two weeks ago. "I spend my mornings looking for work and my afternoons fishing. You've got to eat."

The urban anglers have returned to the rocky shores of the Potomac, the concrete walkway around the Tidal Basin and the jogging path along the C&O Canal--completing an annual migration as regular as any spawning run.

But this year there is competition for the choice spots beside Washington's tidal waterways. A new breed of fisherman--unemployed, underemployed and in many cases unschooled in fishing techniques--has come to the water looking not for sport but sustenance.

"People have always come to this shore to feed their families," says Ray Fletcher, one of two Fletcher brothers who own and operate the boathouse above Key Bridge that is a popular fishing spot. "This year there just seem to be a lot more of them."

Tuesday, at 2 p.m., there were 25 men and a few women, the great majority too young to be retired, casting bait into the Potomac where it flows past Fletcher's. At the same time, another dozen anglers were fishing from the guardrails at Hains Point. And eight more were working the murky waters of the Tidal Basin, in the shadow of the Jefferson Memorial and under the skeptical gaze of tourists.

"People are totally shocked that you can catch anything out of here," said a 27-year-old musician who has been between jobs so long he would rather be identified as Iron Man than give his real name. "There are fish in this water. Big fish."

Carp, catfish, winter shad, white perch, even striped bass are among the species that live in the Potomac and its tributaries and reportedly are good enough to eat. The Potomac may not be as healthful as it was in 1608 when Capt. John Smith wrote in his journal that his crew caught fish in frying pans, but compared with 10 years ago, when the river smelled all summer and wore a topcoat of suds and oil slicks, it now looks like a mountain stream.

"The Potomac River has been an underutilized resource since its recovery from a bout with pollution which lasted over 50 years," reads the draft of a report titled "Urbfish," issued last fall by a Washington "fishing committee" created by the Department of Commerce. The report suggested ways to encourage more urban fishing to "reunite the people of the Washington area with a reborn Potomac River."

The men and women fishing the Potomac this week said the economy was encouragement enough.

"Reagan has squeezed the money out of everything," said Jay Thomas, a 56-year-old out-of-work construction worker who brought two fishing rods to the Tidal Basin. "People aren't buying houses . . . and I've only caught two fish today."

Along the shore at Fletcher's, the conversation jumped from politics to basketball to fish and back to politics. In that company, President Reagan would have been as welcome as a lightning storm.

"The way Reaganomics is going, everybody will be getting a fishing rod soon," says Bobby Butler, who described himself as "lucky" because he has a job.

A few feet from Butler, Quentin Parson had filled two 10-gallon buckets with small white perch. But the 31-year-old jobless construction worker never slowed his cast.

"I figure I can sell some of these fish tonight, maybe get $25 and go down and get my (car) tags renewed," says Parson, who has been out of work since last year and has stopped job hunting. "It costs anywhere from $5 to $8 a day just riding around looking for a job. That's $40 a week that you ain't got. So you go fishing. At least you're doing something. At least you're eating."

At Fletcher's, with the white perch biting, the anglers worked their rods like assembly-line workers. Conversation was easy to come by, but nobody would stop fishing for it.

"Nobody is half-stepping out here," says Theodore Cummings, whose plaster and roofing business caved in this year. "Everybody is serious about catching fish."

At Hains Point, Carl and Mildred Harris had lines in the water and time enough to wonder whether all the fish were asleep. The couple, who met and married while at the Bureau of Engraving and Printing, has been fishing the Potomac since they retired six years ago.

These days, says Carl Harris, with fish more than $2 a pound at the supermarket, fishing is more than just fun.

"Times are tough," says Harris. "When we leave here, we're going by the White House to get a piece of cheese."