The last Saturday of 1984 was warm as summer and the boats jammed the fishing grounds north of the Chesapeake Bay Bridge until a little after noon, when the rockfish started biting, and then it seemed as if everyone was catching them.

Mike Sullivan slowed Miss Dolly and set the big charter boat on course across the flooding Chesapeake tide, the trolling lines streaming astern, gleaming in the sun. "You ought to get 'em now," he said. Almost instantly two rods went down hard as rockfish hit the lures 55 feet below and in a minute five more fish were thrashing on deck.

"I'll have to throw these two back," said the mate, Totsie Lagana, tossing the smallest overboard. "We're over the limit."

"Let's head for the barn, then," said Sullivan, president of the Maryland Charter Boat Operators Association and a 20-year veteran of pursuing the Maryland state fish in the nation's largest estuary. "Roll 'em up. That's it. The end of an era."

It was a curious feeling, Sullivan admitted, heading home with 25 of the last legal rockfish to be caught in Maryland for at least four years.

Today is the start of the state's controversial rockfish moratorium, an auspicious effort to stem a 10-year decline of its most important commercial finfish and most prized sport fish.

But the rockfish, or striped bass, is more than a resource; it's become a symbol of the Chesapeake and a talisman of its health.

The bay, where salt water meets fresh, is the place where rockfish have spawned for centuries and the nursery where they grow to maturity. In its glory years, as recently as a decade and a half ago, the Chesapeake was said to generate up to 90 percent of a burgeoning striper population that roamed the East Coast from Maine to the Carolinas.

But something is wrong. Scientists say the bay hasn't enjoyed a good rockfish spawning year since 1970, and the Maryland catch has plummeted from more than 5 million pounds a year to less than half a million pounds, prompting today's ban on all fishing, possession, trade, handling or sale of Maryland rockfish.

The scientists say the problem is the failure of eggs and young fish to survive, and the cause is thought to be declining water quality as human populations and industry expand along the bay and its tributaries, fouling the water with nutrients, pollutants and silt.

As of today, if caught fishing for rock, Sullivan or any of the other 500 commercial and sport fishermen who chase stripers for money would be liable for a $1,000 fine, a year in jail and confiscation of their equipment.

So would the most innocent recreational angler who happened to land a rockfish, even by accident, and decided to keep it, or a restaurateur with rockfish in his freezer, although the state has said it will issue warnings for illegal possession by restaurants and homeowners for the first few weeks, before assessing fines.

As of today, the Chesapeake in Maryland enters a new era of regulation. Recreational sport fishermen need a $5 license to fish there, the first saltwater sport fishing license required anywhere in the East.

Even the tidal Potomac River, which has rebounded from pollution better than many bay tributaries, is closed to rockfishing for the next six weeks, according to conservation measures put in place by the separate agency that supervises it. The river may come under stiffer regulations when the Potomac River Fisheries Commission meets Jan. 14 to reassess its stance.

Thus the only Chesapeake tidewater north of the Virginia line where fishermen may still pursue striped bass as of today is the Potomac at the District of Columbia, a fisheries no-man's-land with no oversight agency. But rockfish are common in the District only in warmer weather.

Today marks the start of a course of action charted Sept. 11, when Maryland Natural Resources Secretary Torrey C. Brown announced to the astonishment of commercial watermen, who claimed they had no warning, that the days of plundering rockfish were over.

The loss of income to net fishermen and charter boat captains is to be offset by parceling out about $1.5 million in state funds, largely for research work to be conducted by the watermen.

Brown said yesterday that despite a firestorm of protests after the ban was announced, he believes it is the right move. "The point," said Brown, "is that there's a way to beat any conservation system, and that's why a moratorium is so necessary.

"Also," said Brown, "we're not just saying 'No' to catching rock. We're saying 'Yes' to trying to improve the place that they reproduce, and to building a new hatchery to try to replenish the stocks."

Sullivan, who remembers the days when he could pull his boat out of the Rod 'n' Reel dock in Chesapeake Beach and plough through acres of rockfish so thick they broke the surface chasing bait, hopes that in the long run it's the little man, not the professional fisherman, who will profit from today's sacrifice.

"Don't you think the resource should be available to everyone?" he asked as he nosed Miss Dolly south through the warm, windy afternoon. "I'd like to see the resource revived to the point where the average guy, who works for Sears-Roebuck, can come down on a weekend with his kids and have a decent chance of catching a rockfish.

"Right now the resource is reserved for 30 or 40 charter boats, 30 or 40 serious private sport fishermen who know what they're doing, and the commercial netter.

"It's the little guy who pays the taxes to keep this bay up," Sullivan said. "He's the guy buying the fishing license, renting the little boat, staying in the motel and eating at the restaurants. If you ask me, he's the guy who deserves the opportunity."