A Metro board panel, seeking ways to stop an alarming decline in ridership, yesterday heard proposals that include canceling a fare rise planned for December, launching a promotional campaign and delaying the opening of new Blue and Yellow track while rail cars intended for it go to improve service at stations already open.

The panel took no action on these and other options compiled by a staff group that is studying why rail ridership dropped by about 0.7 percent in April compared to April 1981, the first such loss since trains began running in 1976. Bus ridership also fell seriously in the period.

In other action at Metro yesterday:

The full Metro board authorized the staff to begin talking with private companies interested in building commercial facilities at the Gallery Place, Ballston and White Flint stations. It also cleared the way for discussions with merchants and developers seeking to build passageways into eight different subway stations, as did the Woodward & Lothrop department store at Metro Center.

The board authorized the staff to seek a contractor to build a $15 million central maintenance facility in Alexandria. However, under pressure from the District of Columbia, which has been delaying the contract because the plant would take jobs from the inner city, the board directed staff members to reevaluate the plans for the facility, with the possibility of keeping some jobs in D.C.

Metro's ridership losses, part of a nationwide trend in mass transit, came despite the opening of three Red Line stations in Northwest Washington in December. Officials believe the losses are due largely to a fare rise last December, unemployment, cheaper gasoline and the fact that trains have been breaking down in unusual numbers in the past six months.

While rail ridership dipped below 1981 levels in April, it grew slightly in May over levels in that month in 1981--still far below projections.

Perhaps the most controversial suggestion at yesterday's meeting was to put the first of 300 rail cars now on order into a reserve fleet for track already in operation, to improve service there. Current plans call for the first 50 cars to be used to open the Blue Line through Alexandria to Huntington and a Yellow Line shuttle between Gallery Place and National Airport.

The two segments originally were scheduled to open in late 1982. Metro officials have talked of delays until sometime in the second half of 1983. Yesterday's proposal would postpone the openings even further.

Some board members called adoption of it highly unlikely due to pressure from Fairfax County and Alexandria to open their first rail stations.

Putting off the fare increase that the budget assumes for December would also be controversial, because it probably would increase the $182 million that the eight local governments in Metro already are scheduled to contribute toward Metro's operating budget this year.

Other options suggested yesterday: launching a major advertising campaign to attract riders to Metro; and pushing for "disincentives" to auto use, such as higher parking fees in downtown Washington.