When Maryland's moratorium on catching rockfish goes into effect Jan. 1, one large exception will likely stand out.

There will be no moratorium in the Potomac, Maryland's largest river and the Chesapeake region's second most important spawning area for the fish, after the head of the Chesapeake Bay.

The statewide ban, announced in September, is designed to stem a 10-year decline of the official state fish, also called striped bass, which has been designated a "threatened" species by the Department of Natural Resources.

But while Maryland lays claim to all tidal waters of the Potomac, the river's marine resources are harvested by both Maryland and Virginia fishermen and administered by a bistate agency, the Potomac River Fisheries Commission. That six-member commission, convinced that the Potomac's supply of rockfish is healthier than the rest of the bay's, voted this month to implement no ban.

Thus, barring last-minute revisions, commercial and sport fishing for rockfish will continue this winter on the Potomac, which, according to A.C. Carpenter, executive secretary of the commission, produces one-fourth of all rockfish captured in Maryland.

Carpenter said the commission acted despite requests from Maryland DNR Secretary Torrey Brown and his assistant, George M. Krantz, to include the Potomac in the statewide moratorium. Brown, while acknowledging the commission's independent status, said today he continues to hope that body will reverse its decision and honor the ban.

Should the commission make no changes, a sticky legal scenario could develop.

Maryland officials believe that after Jan. 1 it will be illegal to possess rockfish in Maryland. If it holds up, Maryland netters could continue to fish the Potomac under the rules of the fisheries commission, but would be in violation of state law if they brought their catch back to their home ports.

"We don't know what to make of that," said Jack Yates, who buys and sells rockfish at Cobb Island. "We might have to take our catch to Virginia to sell it."

Confusion over the impending rockfish ban is further compounded by the fact that Potomac commercial fishermen and their colleagues around the bay region are enjoying the best rockfish season in years.

Today, Yates was selling 50-pound boxes of fresh "rock," as it is affectionately known, for $45, or 90 cents a pound, and paying fishermen 50 to 70 cents a pound at the dock, a price lower than any he remembers in seven years. The low price is a direct result of abundant supply, he said.

A restaurant in Annapolis was selling rockfish dinners last week for $3.95, and retail prices for the delicacy were in the $1.50-a-pound range throughout the region as fishermen reported excellent catches from the Susquehanna Flats at the head of the Chesapeake to the Virginia tributaries near its mouth.

Brown argues that the abundance is not unexpected and reflects the last decent spawning year recorded on the bay, 1982. Rockfish spawned that spring have reached the 14-inch legal size, he said, but after poorer hatches the last two years they are the last hope for replenishing declining stocks of the fish.

"We want to protect these fish so they can come back to the bay and spawn," he said.

But the current abundance has some resource managers convinced that Maryland may have acted prematurely in banning fishing for the state's most important commercial finfish. "It looks like they might have jumped right at the wrong time," said a Virginia official.

Virginia decided last month not to institute a rockfish moratorium, citing initial success of conservation measures in reducing the commercial catch and indications that 1984 was a good spawning year in Virginia waters. In fact, though Maryland has urged all states on the East Coast to implement rockfish moratoriums, so far none has. Delaware has proposed, but not adopted a moratorium.

Carpenter said the Potomac has recorded better rockfish spawning success than the bay in general for the last five years, and commercial landings of rock in the Potomac actually increased during the first half of 1984, despite extensive new conservation measures.

He said the landings were up even though the river was closed to rockfishermen for 3 1/2 months of the year, Potomac fishermen had to reduce the length of their nets by half and the limit for sportfish was cut from 10 a day to five.

Carpenter said, however, the commission's decision against a moratorium could be revised, based on findings of the science and statistics branch of the Atlantic States Marine Fisheries Commission, which is reassessing rockfish information on a coastwide basis.

Maryland requested the reassessment after announcing its moratorium in September. Brown, the DNR secretary, said the regional body will offer its report by the end of the year. If its recommendations are for a coastwide moratorium, both he and Carpenter said they expect the Potomac commission to go along.

Maryland is considered the ancestral home of Eastern rockfish. Rockfish spawned in Maryland waters once were said to constitute 90 percent of the East Coast population, but in recent years that estimate has dropped as spawning success in the Chesapeake region declined, evidently as a result of declining water quality.