The Potomac River Fisheries Commission yesterday banned fishermen from catching rockfish in the Potomac during the spawning season, Dec. 1 through May 31, and made it illegal after July 1 to net rockfish smaller than 18 inches. The current minimum size is 14 inches.

The commission, which governs Potomac fishing, approved the conservation measures to close a loophole in rules previously imposed in Maryland and Virginia that were designed to preserve rockfish. Rockfish are also known as striped bass.

The Maryland legislature last year made it illegal to bring ashore rockfish caught in the Potomac. Last week, Virginia approved a rule forbidding fishermen from landing rockfish less than 18 inches long on Virginia docks.

The regulation adopted by the fisheries commission yesterday is designed to close a loophole in those state rules, which in effect allowed fishermen to catch Potomac rockfish as long as they did not bring them ashore in Maryland or Virginia.

Conservationists and biologists who favor the rockfish ban cite figures showing the Maryland rockfish catch dropping from 3 million pounds a year to 300,000 pounds a year in the past decade. Unless a fishing moratorium is imposed, they warn, the area's rockfish stock will soon be totally depleted.

The regulation adopted yesterday will probably mean further loss of revenue for the local rockfish industry, which grossed more than $4 million last year, according to Kirby A. Carpenter, executive secretary of the fisheries commission.