Ridership on Metro's buses and subways dropped by 2.6 percent in fiscal year 1981 compared to the previous year, the first yearly decline in the system's history, the Metro board was told yesterday.

Metro planners had projected a loss. They blamed the drop of 5 million rides primarily on two fare increases in fiscal 1981 and on plentiful gasoline and fuel-efficient cars, which appear to have caused some commuters to return to private transport.

The ridership losses come as Metro, like transit authorities around the country, seeks ways to increase its revenues without alienating the public. In addition to inflation, it must cope with the Reagan administration's plans to phase out U.S. subsidies that now cover about 8 percent of operating costs.

Board Chairman Joseph Alexander expressed serious concern over the decline in ridership, commenting that "I don't think we can continue to raise fares."

General Manager Richard Page said he was only a "little worried," noting that large gains were registered in fiscal years 1978 to 1980. "I think we are doing extremely well," Page said. "We are carrying over 600,000 riders a day."

During the last fiscal year, rides involving only the rail system grew by about 3 million, but rides involving only buses dropped by 5 million and those involving both transport modes dropped 3 million, creating a net loss of 5 million rides. The total was about 182 million rides.

The Metro staff forecasts that figure will rise to 185 million in the fiscal year that began July 1, recapturing some of the business lost last year.

Metro had predicted the decline in bus ridership, caused to a large extent by commuters switching to the expanding rail system. Initially it had expected gains in subway ridership that would leave only a 2 million net loss for the system as a whole. In fact, for the first time, subway rides declined, by about46,000, creating the net loss of 5 million.

The rail system lost riders even though Metro had opened track between Stadium-Armory and Addison Road during the year, an addition of three stations and four miles of track.

Metro raised fares twice during the fiscal year, once in July 1980 to "keep pace with inflation," a Metro report said, and again in January 1981 to offset higher than expected costs and lower than expected federal subsidies. The increases meant an average 26 percent rise in bus fares and 31 percent for rail.

Metro ridership stood at 117 million trips in fiscal 1974 and grew steadily to 134 million in fiscal 1978. It picked up by 15 percent to reach 154 million trips in fiscal 1979 and grew an additional 21 percent to 187 million trips in fiscal 1980.

Metro officials feel the fiscal 1980 growth was artificially high, spurred by the gasoline shortages that year. They point out that, even though people are returning to their cars, overall ridership is still much higher than it was before the gas shortages.

In other business at yesterday's board meeting:

Carlton R. Sickles was sworn in as an alternate member of the board representing Montgomery County. A former congressman from Prince George's County, Sickles served on the Metro Board from 1967 through 1978, representing Prince George's. He recently moved to Montgomery County.

Figures released for the quarter ending June 30 showed that Metro's operating losses were $5.5 million less than predicted. This meant that federal, state and local government subsidies for that period declined by the same amount.

It was announced that Metro's architectural firm, the Chicago-based Harry Weese and Associates, won awards for the design of the Metro Center and Addison Road stations. The awards were granted under a program operated jointly by the National Endowment for the Arts and the U.S. Department of Transportation.