The Potomac River Fisheries Commission listened for three hours today to the impassioned pleas of commercial fishermen, then reversed itself and voted 4 to 2 to close the river to rockfishing for six additional weeks.

And while some of the 250-odd watermen leaving the public meeting in the firehouse here grumbled about the decision, many seemed relieved that the commission stopped short of adopting a total ban on catching rockfish, leaving them with some vestiges of their livelihood.

The closure of the scheduled spring fishing season, Feb. 15 through March 31, means the Potomac will be off limits to commercial and sport exploitation of the troubled Maryland state fish, known elsewhere as striped bass, until June 1, but will reopen for the rest of the year.

The reopening will again put the Potomac in direct conflict with the statewide Maryland moratorium on catching rockfish, in effect since Jan. 1.

The Potomac is considered Maryland waters, but its marine resources are overseen by the bistate Maryland and Virginia commission because historically it has been fished by watermen from both states.

The commission voted in November to keep the river open to rockfishing, despite pleas from Maryland state officials to join in the statewide ban.

The commissioners said then rockfish were plentiful in the Potomac. Figures released at today's meeting, which showed the Potomac rockfish catch in 1984 up 400 percent over 1983, to 631,000 pounds, confirmed their argument. In fact, rockfish fishing in the Potomac was so good last year that the catch there exceeded the total for all other Maryland waters of the Chesapeake Bay, expected to be under 500,000 pounds.

Potomac Commission Chairman Bill Pruitt said that while Potomac watermen argued they had "done enough" in 1984 by giving up several months of fishing and reducing the amount of net they set in order to reduce the catch, "the figures on landings obviously don't agree."

Pruitt said the additional six weeks closure may or may not significantly reduce the 1985 catch. "Who can predict landings?" he said. "But what I can predict is that the spring closure will let those fish get to the spawning grounds" during the spring season when rockfish reproduce.

Maryland Natural Resources Secretary Torrey C. Brown, the architect of the state's four-year moratorium, seemed encouraged. "Closing the window [the six-week spring season] is exactly what they should have done, for now," he said. "It amounts to a moratorium until June. At that time they could reconsider."

The Potomac commissioners were under the gun to do something after a federal panel voted last month to endorse inclusion of the Potomac in the moratorium, which is aimed at reversing a 10-year, bay-wide decline in rockfish.

The Atlantic States Marine Fisheries Commission's Striped Bass Management Board voted 11 to 1 to include the Potomac in the ban. The board's recommendation will gain enforcement strength if it is approved by the full ASMFC within the next few months, as expected.

At that time the Potomac commission could be forced to comply. But Pruitt said he would argue with the federal board that "this commission has acted responsibly with the hard decision we made tonight," and that the decision would result in significant reductions in the 1985 Potomac rockfish catch.

The watermen would agree with that. "They've put it to the fishermen again," said Willard Dutton of Charles County, Md.