Anyone fishing the Potomac River around Washington had better understand the rules and regulations on rockfish, because it's the one species you're almost sure to catch.

"They're everywhere," said fishing guide Glenn Peacock. "You have a hard time keeping them off your hook."

Yes, these are the same rockfish currently under moratorium in Maryland, where state officials closed the rockfish season statewide 2 1/2 years ago in an effort to restore stocks devastated by 15 years of poor reproduction and overfishing.

How thick are rock in the Potomac now? "They do get in the way," said Pete Cissel, a veteran bass angler who has fished the river for more than 20 years, "but it's a nice thing to have happen to you."

Anyone who has paid attention to Washington's river over the last five years knows it has undergone "an amazing transformation," as Cissel put it, in the area around Wilson Bridge. The resurgence of huge beds of bay grasses, including hydrilla, has turned the Potomac into a groaning board of aquatic and bird life, and rockfish are among the key beneficiaries.

To wit: During a day of largemouth bass fishing last week, Peacock, Shep McKinney and I went by boat from the Washington Ship Channel downtown to Piscataway Creek 10 miles south. At spots along the way, we hooked white perch, yellow perch and a few good-sized largemouth bass, plus a couple of monster catfish that got away. Different things bit in different places, but rockfish were everywhere.

"Those are rock there," said Peacock as big fish splashed around the entrance to Smoot Bay on the Maryland shore below Wilson Bridge.

"Rockfish," he said when McKinney's rod bent double with an 8-pounder while fishing over a submerged island off Dyke Marsh on the Virginia shore.

"Rock," Peacock said again as he dipped to net a silvery, 3-pound fish that attacked his lure as he jigged the main pilings of Wilson Bridge for bass. He carefully unhooked and released each of the half-dozen rockfish we caught.

McKinney, who grew up pursuing rock in Norfolk back in the 1960s, when they were thick, and who now fishes for sport from Canada to the Florida Keys, said he hadn't seen anything like the Potomac's abundance in years.

"So you were catching rock?" said Maryland state fisheries biologist Harley Spier. "Then you must have been fishing the river up around Washington."

The phenomenon is well known to scientists, Spier said. For the last five years Maryland's juvenile index of rockfish, the gauge by which spawning success of the troubled Maryland state fish is measured, has been better in the Potomac than anywhere else in the Chesapeake region, he said.

Environmentalists believe it's largely because of the $1 billion spent to upgrade water quality here by improving D.C.'s Blue Plains sewage treatment plant, which now removes phosphorus it used to spew into the river in its 315 million gallons-a-day effluent.

Without the excess phosphorus, the river no longer blooms with blue-green algae every summer, and without the algae, enough sunlight gets to the bottom in shallow areas to sustain rooted bay grasses, which provide shelter and food for little fishes. Nowhere else in the Chesapeake do grasses grow as they do here, and nowhere else do rockfish thrive as they do here.

But while it's nice to be catching rockfish again, the politics involved are mind-boggling. Fishing an area where three jurisdictions come together, all with different rules, means you have to know where you are to know what rules to obey, and once you figure that out you'd better stay put.

Pity the poor angler, for example, lucky enough to catch his limit of five stripers between 24 and 34 inches long in the main stem of the river below Washington.

Where will he go with his fish? If he takes them into Washington, he'll be three fish over the D.C. limit, which is two rockfish a day over 24 inches.

If he strays into Maryland waters, he will be in a heap of trouble for violating not only the state's flat prohibition against possessing any rockfish, but also the federal Lacy Act, which bars transportation of illegal species across state lines.

If he launched his boat in Virginia, he can land his five fish there legally, but if he drives home to Maryland, or even through it on his way home, he's in violation.

And what about licenses? No license is required to fish the main stem of the Potomac, which is overseen by the bistate Potomac River Fisheries Commission. But a Maryland Chesapeake Bay license is required to fish any Maryland feeder streams like Piscataway or Broad creeks, and a Virginia freshwater license is required to fish above the tide line on creeks on that side of the river.

It all makes for a bewildering mess, compounded by the vagueness of the boundaries around Wilson Bridge, where the three jurisdictions come together.

My solution is simple: Throw back any stripers I catch, on grounds these fish deserve an assist in their plight to recover from decades of plunder.

This makes Maryland officials happy. They believe the quest to restore rockfish in the Bay system is only beginning, and they see no end soon to their statewide ban on catching rock.

Fishermen have their own reasons. "My feeling," said Cissel, "is that any fish you put back is a bonus, because then someone else gets to enjoy catching it."

A host of bass-fishing guides now are working the Potomac around Washington, including Peacock, who charges $150 a day and books parties out of his home in Silver Spring. He's in the phone book under J. Glenn Peacock.

The biggest guiding outfit is Outdoor Life Unlimited, with headquarters in Beltsville, which also is in the book.

Most guides launch at Belle Haven Marina just south of Alexandria, where you can rent small boats or book guided trips by phone.