Jack Hemingway, whose achievements as a conservationist and proficiency as a fisherman were nearly overshadowed by his role as the son of a celebrated father and the father of famous children, died Friday night at Weill Cornell Medical Center in Manhattan. He was 77.

He suffered complications of heart surgery, his wife said.

Mr. Hemingway, who lived in Ketcham, Idaho, was the first son of Ernest Hemingway and the father of Margaux and Mariel Hemingway, both models and actresses.

''I spent the first 50 years of my life being the son of a famous father and am now spending the last 50 as the father of famous children,'' he wrote in his autobiography, ''Misadventures of a Fly Fisherman: My Life With and Without Papa,'' published by Taylor Publishing Company in 1986.

Mr. Hemingway picked up his father's fondness for angling, though he got next to no paternal piscatorial tutelage, and went on to fish most of North America's great trout streams, as well as working through the Nature Conservancy and other conservation groups to preserve wild areas.

As a commissioner on the Idaho Fish and Game Commission from 1971 to 1977, he succeeded in having the state adopt a ''catch and release'' fishing law. The result, said Bruce McNae, owner of Fish and Fly magazine, was that Idaho's trout stocks increased, while those of neighboring states declined.

Mr. McNae said Mr. Hemingway's vast knowledge of fishing and sheer energy on expeditions astounded him. ''Jack's Papa wrote masterfully about the great adventures of life,'' he said, using Ernest's nickname. ''Papa's Jack lived them.''

Orri Vigfusson, chairman of the North Atlantic Salmon Fund, lauded Mr. Hemingway's skill at ''presenting'' his hand-tied flies to salmon. He said that as a director of the fund, Mr. Hemingway was a leader in the campaign to pay commercial fishermen to forgo salmon fishing in Atlantic feeding grounds.

Mr. Hemingway embraced his father's legacy, serving as a judge in contests where writers try to emulate the Hemingway style; he helped create a line of furniture modeled after pieces in Hemingway's homes in Key West, Fla., Africa and elsewhere, and tried to preserve his own memories of his father.

One famous story was about the time he sat in his father's home in Havana, pouring out to his father his difficulty in finding a steady career after he left the military. He expressed despair.

Ernest Hemingway, mentioning his own father, who had killed himself, replied, ''I want to promise that you'll never do what Grandfather did, and I promise you I won't.''

The father then advised the son to pick a career and stick with it. The next day, they went buzzard hunting, and shared two pitchers of martinis. They ended the evening watching ''Casablanca.''

Jack Hemingway wrote that that day was the closest he had ever felt to his father. Six years later, Ernest Hemingway killed himself.

John Hadley Nicanor Hemingway was born on Oct. 10, 1923, in Toronto. His mother was Hadley Richardson, the first of Ernest's four wives, and his godmothers were Gertrude Stein and Alice B. Toklas. The name Nicanor came from a famous matador, Nicanor Villalta. In childhood he mingled with Ezra Pound, James Joyce, Clark Gable, Gary Cooper and Ingrid Bergman.

As a toddler in his father's crude apartment above a sawmill at Rue Notre-Dame-des-Champs in Paris, he acquired the nickname Bumby because of his plump teddy-bear qualities. The Bumby name stuck with him through life, though in Jack's autobiography, his father always addressed him as ''Schatz,'' German for treasure.

In ''A Moveable Feast,'' Ernest's remembrances about his early days in Paris, he wrote about the boy's being baby-sat by their huge, protective cat, Mr. Puss-Puss. In an interview with The Toronto Star in 1986, Jack said: ''Why not? He was a big cat and wouldn't let anyone near my crib.''

He was 5 when his parents divorced; he grew up with his mother and in boarding schools, seeing his father only on summer vacations. ''He was my hero,'' Mr. Hemingway said in a 1999 interview with The New York Times. ''When he was with you, you were the total center of his attention. But when I left to go back to school, I was out of his mind.''

The two boxed, with the father knocking the son down and the son cutting his father's eye. When Jack was a teenager, Ernest arranged for his son's initiation by a Havana prostitute, not knowing that his son had already made her acquaintance, gratis, he said in The Times interview

He attended the University of Montana and then Dartmouth College, but dropped out to enlist in the Army when World War II began. He was assigned to a military police detachment in North Africa, where he said his most extraordinary feat was maintaining the only venereal-disease-free unit in that area. After pulling some strings with generals who knew his father, he was assigned to the Office of Strategic Services, the fledgling military intelligence branch.

When he parachuted into Nazi-occupied France, he carried his rod, reel and fly box with him, and went fishing after his first mission was accomplished, nearly getting caught by a German patrol. On another mission, he was shot in the right arm and shoulder. He was captured and spent the rest of the war in prison camps from which he tried unsuccessfully to escape.

His courage impressed his father. ''His view of me changed -- the fact that I was wounded and had a nice set of visible scars,'' he said in an interview with The Idaho Statesman. ''I stopped being viewed as a nice little boy.''

Mr. Hemingway is survived by his wife, the former Angela Holvey, whom he married after his first wife, the former Byra Whittlesey, died in 1986. He is also survived by two daughters, Joan, nicknamed Muffet, of Twin Falls, Idaho, and Mariel, of Ketcham and West Lake, Calif.; two brothers, Patrick, of Bozeman, Mont., and Gregory, of Miami and Montana; and two grandchildren. His third daughter, Margaux, died in 1996 from an overdose of barbiturates.

Jack Hemingway had a number of jobs, including stockbroker and fishing-supplies salesman. Just before his death he finished another autobiography, ''A Life Worth Living,'' to be published by Lyons Press. He and his wife were working on another book, to include tales of their travels, personal reminiscences and recipes. He wrote the foreword before entering the hospital about three weeks ago. She said she would finish the still-untitled book.

But his greatest love was fishing, thanks and no thanks to his famous father. ''I know he wanted me to love fishing and hunting, and I believe that he deliberately set about to make me want to do it on my own initiative,'' he wrote. ''Tennis parents and stage mothers should take note. The kid has got to want to do it, not just to please the parent, but for himself.''

John H Hemingway