The number of major crimes committed by juveniles in the District of Columbia has dropped substantially in recent years as adult crimes have increased, a representative of the D.C. police department's youth division told a Senate subcommittee yesterday.

From 1977 to 1981, major offenses attributed to youths declined 26.9 percent, said Capt. David Bostrom, assistant commander of the youth division. From fiscal 1980 to fiscal 1982 the number of arrests of juveniles for all crimes that were referred to court dropped from 4,212 to 3,595, he said.

Bostrom released the figures at the first day of hearings by the Senate Appropriations subcommittee on the District of Columbia on the city's fiscal 1984 budget. He was a member of a panel of District officials who responded to questions from the subcommittee about the city's juvenile programs that are included in the budget. The budget, approved by the D.C. City Council and signed by the mayor, must be passed by Congress.

Bostrom said that some of the reduction in juvenile crime could be attributed to a drop in the city's youth population, which declined 33 percent in the last decade. But he also attributed the improvement to police department crime prevention programs.

At the same time, he charged that federal programs are going in the wrong direction under new leadership at the Justice Department's Office of Juvenile Justice and Delinquency Prevention, joining a growing number of critics of the office under acting director Alfred Regnery.

"They are more interested in making arrests than in preventing crime," Bostrom said in answer to a question by subcommittee Chairman Arlen Specter (R-Pa).

The subcommittee also heard from officials of the Department of Human Services on its fiscal 1984 budget, which DHS Director James A. Buford called stringent but compassionate. "There is no cushion to protect us if there is a serious economic downturn or a court mandate to provide more services," he said.

City Council member Polly Shackleton (D-Ward 3) urged an increase in the federal payment for St. Elizabeths mental hospital, asking that Congress appropriate enough funds to help close a projected $39 million gap in funding after the budgeted federal and city shares are paid. Some of this gap will be closed by a Medicaid cost containment agreement the District recently negotiated with city hospitals, D.C. Budget Director Betsy Reveal said.

While 1,100 patients at St. Elizabeths were supposed to be placed in the community by last October under court directives, only 300 were, Buford said. The facilities are available in the community, but more support services are needed, he said. The cost of maintaining a patient at St. Elizabeths for one year is $65,000, Buford said.

Specter expressed a particular interest in making the District a prototype for programs to help sexually abused children and asked DHS officials to submit information on what city programs there are now.