The nation's tuan fishermen could legally kill 78,900 porpoises this year under a bill passed yesterday by the House Merchant Marine and Fisheries Committee. But strict fines would be imposed on individual boats for excessive killing of the friendly, air-breathing mammals.

The compromise measure marks the first congressional action on the emotional tuna-porpoise issue since 1972 law called for reducing porpoise kills to a "level approaching zero."

Environmentalists, the tuna industry and the Commerce Department have fought in court for the past two years over how many porpoises may be killed. The animals, which swim above tuna schools, get caught and suffocate in the long nets of tuna fishermen.

The bill, sponsored by Rep. John M. Murphy (D N.Y.), sets the yearly kill quota higher than present Commerce Department regulations, which allow only [WORD ILLEGIBLE] and the Carter administration recommendation of 68,910.

Tuna fishermen, who had docked their boats in a three-month strike until last week, say they can't avoid snaring porpoises. Environmentalists say some vessels have already dramatically reduced the kill with finer-mesh nets and careful fishing techniques.

The committee bill drew mixed reaction from environmentalists, who wanted a provision to force annual reduction in porpoise killing, a measure supported by the White House.

"The bill gives industry a free pass for the rest of the year," said Toby Cooper of the Defenders of Wildlife. "There's no incentive to reduce the kill."

Industry reacted bitterly. "The whole attitude is to punish the U.S. fleet," said August Felando of the American Tunaboat Association. "We might as well sell our boats and get out of business."

Felando said tuna fishermen had already made a "tremendous sacrifice" by agreeing to a provision in the bill that would place government observers on all vessels to guard against violations.

But provisions charging the industry $2 million for a technological and biological research vessel and fining individual boats $32 for every porpoise killed above the average industry kill rate are "tremendous financial burdens," he said.

Tuna boat owners have been threatening to switch their vessels to foreign flags, but the bill would force any transferring vessel to post a bond to make sure it complied with U.S. porporise kill standards.

The tuna-porpoise issue had been heavily lobbied on both sides. The $1.3 billion fishing and canning industry flew in a group of fishermen's wives a few weeks ago to roam the halls of the Capitol.

"But that backfired," Cooper said. "They came across like upper-middle-income hussies with their spangled jewelry - not like poor fishermen's wives." Tuna men are among the most prosperous fishermen - an average boat costs $5 million.

The environmental side was supported by such groups as the Sierra Club and the Environmental Defense Fund.

And dolphin lovers, it seems, form a significant constituency. "When you get letters form five-year-old kids all over the country, you respond," said Rep. Robert L. Leggett (D-Calif.). "The TV program 'Flipper' has stimulated a deep-seated feeling about porpoises."

The tuna industry, which includes canneries like Ralston-Purina and H. J. Jeinz, predicts an increase in the cost of tuna fish. The fleet has already lost $50 million for the strike, Felando said.

However, half of the tuna canned in the United States comes from foreign vessels that are not affected by U.S. porpoise kill quotas. The U.S. catch fell 40 per cent while the West Coast fleet was grounded, but the difference can be made up by industry surpluses, Cooper maintained.