The Mall and the White House grounds are his turf. Washington's memorials and monuments, visited by more than 20 million tourists a year, and most of its marinas, golf courses, skating rinks and tennis courts - all are part of his domain.

His 50,000-acre holdings stretch beyond the Nation's Capital, to Harper's Ferry in West Virginia, to the end of the C&O Canal in western Maryland and to the Civil War battlefields of Manassas, Virginia.

But for Manus (Jack) Fish, the modest director of the National Capital Region of the National Park Service, managing most of the parkland in the Washington area is no more difficult than managing a family of 12 children, which he does in his spare time in McLean.

Because 535 members of Congress and Fish's bosses at the Department of Interior commute daily along most of his parkways and parks, and because the President lives in two of those parks - one at the White House and one at Camp David - his job is considered something of a "Fish bowl assignment." At least that's how former Secretary of the Interior Rogers C.B. Morton described Fish's job, in a letter now framed and hanging in Fish's office at Hains Point.

This summer, the present Interior secretary, Cecil D. Andrus, honored Fish with the highest award given by the Interior Department, the Distinguished Service Medal, for helping guide the massive expansion of Washington parks in the past decade, particularly during the Bicentennial.

Despite the prominence of his parks, Fish himself maintains a low profile. In fact, he takes credit for few things except his family, an effervescent collection of 10 girls and two boys who burst the seams of his small house.

And his house is as packed and popular as his parks, which are among the busiest in the nation. When Fish comes home at the end of his usual 10- to 11-hour days, and the dinner shifts are over ("my wife, Rosemary, is kind of a short-order cook") he settles down to read reports, the calm eye in the center of a swirling storm of people.

"It can get pretty busy," he says happily, "with the TV, stereo and radios going and people on the phone and others talking." But he adds that the house isn't really that crowded because only nine of his children, aged 11 to 25, currently live at home.

But only in this way is Fish able to combine family life, "which comes first," and a job that requires minute knowledge of everything happening in the parks, from the leaking roof at the Kennedy Center to the smallest change in a bike trail.

"I've got to study issues in detail," he says, to know what's going on in the huge federal enclave of parks and to be able to answer to all his bosses. "And I guess I like that. If I didn't I'd have ulcers and high blood pressure."

When Fish was hired by the Park Service in 1952, after receiving an engineering degree and graduating manga cum laude from Catholic University, Washington was still a somewhat sleepy Southern town. The Park Service, with a small budget, did little beside mow lawns around monuments and maintain the city's playgrounds.

As an engineer, Fish helped design many parts of the park system from playground swings to bridges, including Roosevelt Bridge, and roads, such as parts of Beach Drive in Rock Creek and the George Washington Memorial Parkway. He also designed a few things that were never built - such as a highway tunnel under the Lincoln Memorial and Tidal Basin.

He had known from boyhood that he wanted to be an engineer, confirmed by working summers as an assistant surveyor checking property lines in Washington. But he turned down his first job offer at the Navy Yard because he thought there wouldn't be enough variety.

Variety is the one thing Washington's parks have at least its federal parks.

Much of that variety began in the mid-1960s in a sudden explosion of activity designed to increase the use of urban parks. Lady Bird Johnson sparked a beautification campaign.

Fish, who was deputy regional director from 1970 until he became director in 1974, has made maintenance and perservation a priority for the Washington area's long-neglected parks, historic monuments, buildings and canals. "People get a good feeling when they see a park neat, clean and well-kept. And I guess I put a stamp on that," says the engineer who learned to love trees and makes certain that in Washington "when a tree is lost we put another one back in."

While much of the local Park Service budget goes for maintenance and preservation, "there also have to be programs for people," Fish says. "The parks are a platform."

And federal parks here have magnificent shows - shows that have helped boost the Park Service operating budget here to more than $60 million a year, a 1,200 percent increase over the past decade.

Wolf Trap Farm Park for the Performing Arts, the Kennedy Center (which the Park Service maintains), Ford's Theater, the Folk Life Festival (with the Smithsonian), noon-time concerts and displays in downtown parks, Shakespeare al fresco, Fort Dupont summer theater, free Carter Barron concerts and plays and living history programs at Pierce Mill, Turkey Run and other parks. All of these are new within the past 10 years.

And the list of new indoor and outdoor sports complexes is exhausting. But sports come easily to mind for Fish, the son of a New Jersey Catholic school football and baseball coach, who himself for more than a decade coached Boys Club track and football and girls softball and basketball teams near his home at 1902 Poole La., McLean.

But Fish was cited by Interior particularly for his Bicentennial efforts, where funds were used to make permanent park improvements in Washington, rather than just one-year celebrations, including the new Constitution Gardens, the National Visitors Center, a $4-million ice rink and outdoor sports complex along the Anacostia River and handicapped entrances incorporated unobtrusively, and expensively (costing over $2 million), into many monuments.

Fish is quick to say that it's Congress that is creating and funding all of this, but they are projects singled out and proposed by Fish and the Park Service.

Not all the changes have won universal approval, however.

Commuters who parked free all day on the Mall howled and went to court in an unsuccessful effort to keep the Park Service from grassing over two roads as part of a $5 million Mall beautification project. The $40-million visitors center, little used and still unfinished is generally recognized as a disaster, as is a 1970 Park Service land swap along the George Washington Memorial Parkway that preserved some marshes but is enabling a developer to build a high-rise city next to National Airport. It could flood the scenic parkway to Mount Vernon with an additional 18,000 cas a day, and has caused environmentalists to sue the Park Service.

The Park Service is still also in court over the historic Alexandria waterfront, a dispute over who owns it, the federal government or private property owners.

All of this is far from the quiet of his first day with the Park Service in 1952 when he reported to the engineer's office at the stone building near the Washington Monument, a former steam boiler house to power the monument's first elevator, now used as a souvenir shop.

"It was a real shock," Fish recalled last week, when he was promoted in 1970 from engineering and planning to deputy parks director. The job involves so many parks and so many people, all under the watchful eyes of Congress and Interior, said Fish.

Although he now appears at least a dozen times a year before congressional committees, "my first appearance before Congress was in 1972," when then regional parks director was out of town, "and a tough committee was quizzing the Park Service about its proposed management of the Kennedy Center.

"I was asked who would pay for any major building repairs if the Park Service took over. What if the roof leaks, I was asked. I said it already does leak and we pan it. What's that, I was asked. It's putting a pan, a bucket under the leak, I said. Well, I guess I was smiling and several congressmen started to snicker and pretty soon we were all laughing and I got through my first congressional hearing."

He noted, however, that the still-leaking roof is no longer a laughing matter, since it is costing almost $5 million to repair (who will pay for it, the Park Service or the original contractor, is being litigated in court).

In Fish's office on Hains Point, among many pictures of his family, all large since they include at least 14 people, are the voluminous copies of every hearing, almost every document and newspaper story concerning Washington's sprawling park system . . . "to keep me up on anything Congress might ask about." Even blueprints for the once-proposed tunnel under the Lincoln Memorial, roads and other projects he helped design.

But while Fish is prepared with detailed answers, the questions often aren't asked. During this year's budget hearing before a friendly Senate committee, the only thing he was asked about were the commuter traffic jams several congressmen saw daily on the George Washington Parkway, particularly "where Spout Run" comes in. Could Fish do something about it? He said he would look into it.