The long arm of the law reaches out Monday to squelch the best spring rockfish season in memory.

"I've been out six times this month," said self-proclaimed rockfish addict Mel Welch, "and each time it was just like this."

"This" was the sparkling day last week when he and fishing partner Art Cather, with a little help from a bug-eyed reporter, trolled around near the Morgantown Bridge and loaded a 186-quart cooler, the biggest one Igloo makes, three-fourths full with about 75 keeper rock.

Yes, these were the same rockfish (striped bass) declared a threatened species in Maryland, which imposed a statewide ban on fishing for them Jan. 1.

And no, it wasn't illegal. The tidal Potomac, though entirely in Maryland, is overseen by an independent bistate commission because it is used by both Virginians and Marylanders.

Despite pressure from Maryland, the commission has rejected a Potomac moratorium on the grounds rockfish are faring better in the river than elsewhere and thus require less stringent protection.

When the Potomac season opened June 1, the river was the only place in the state to pursue rock legally. With the fish untouched after a winter-long ban on netting, it proved a bonanza for a handful of commercial and sport fishermen who knew the hot spots.

Cather, who like Welch has a $25 commercial hook-and-line license allowing him to keep all the legal-size rock he catches, rather than quit at the five-fish sport limit, has worked the Potomac since the early 1950s. He said he never saw such a June. He fished 20 days and hit nothing but smashing success. But on Monday, both the commission and Maryland will impose laws to limit further bonanzas.

The new Maryland law makes possessing rockfish anywhere in the state an offense punishable by a $1,000 fine and up to a year in jail, plus confiscation of boats and gear. The only exception is to registered seafood dealers, who may handle rockfish if they can demonstrate it is for prearranged delivery out of state.

Thus Welch, Cather and other Potomac fishermen, unless they buy $150 dealer licenses and find an out-of-state market, could still fish the river legally but could not land their catch in their home state or even keep it in their refrigerators without violating the law.

Also on Monday, the Potomac commission imposes a new minimum rockfish size limit of 18 inches, up from the current 14. That would render illegal about 90 percent of fish caught in the river over the last month, according to Welch and Cather.

They are taking the new rules in stride, though true to their addictions, already plotting ways to keep fishing legally and to fight further restrictions.

Cather, who owns a small lure manufacturing company and guides fishing parties part time, believes the new measures are unnecessary. He said the abundance of rockfish in the Potomac proves they are thriving here and don't need more protection.

The Maryland Department of Natural Resources concedes rockfish are abundant in some places. But DNR contends the abundance is almost entirely of fish hatched in 1982, one of the only decent spawning years in the last 15, and DNR intends to protect those fish until they reach sexual maturity at age 6 or 7 and can return to the rivers and spawn.

Welch, a communications specialist with the federal government, seems almost relieved at the upcoming turn of events. He said he has mixed emotions about catching multitudes of rockfish, but loves the challenge and would keep at it as long as it remained legal.

Like many anglers steeped in the saltwater tradition of keeping what they catch and selling what they can't eat, he finds the prospect of catching fish for sport and returning them to the water alive hard to swallow.

Cather and Welch invited me along last week so I could report first-hand the abundance of Potomac rockfish, in hopes it would discourage further restrictions.

Clearly, the fish are plentiful.

During one midafternoon spree, Welch could spot on the depth-finder mounds of rockfish piled off the bottom, feeding where the rushing current smacked into a sand bar. As we crossed the mounds, he'd call a warning, and time after time all three rods bent double with multiple fish. But whether that localized abundance obviates the need for Monday's new restrictions, and perhaps more rules in the future, is another question.

I put a few fish in the catchbox during the day, and as I did I couldn't help thinking of Boston sportswriter Michael Globetti, who keeps comparing rockfish to bald eagles. The analogy is excessive, for sure. Eagles aren't fit to eat, so there's no point in killing one, and there are infinitely fewer eagles around than striped bass.

But somehow these days, killing a rockfish feels like a crime, even when it isn't.