WASHINGTON—It was not the day Robert J. Kopchak had been waiting for —the day the Supreme Court let Exxon off the hook for punitive damages in the worst oil spill in U.S. history.

The Cordova, Alaska, fisherman lost a quarter of his earnings when the lucrative Pacific herring fishery crashed in the 1990s—and on Wednesday, the Supreme Court decided to cut punitive damages in the Exxon Valdez disaster from $2.5 billion to $500 million.

Under the ruling, the arithmetic works out to an average of $15,000 for each of 33,000 victims. The decision to reduce the amount owed by 80 percent was hailed by the business community and decried by environmentalists and Alaskans.

Kopchak says many families are worse off than he is and that he fears that despair could turn to suicide.

“I keep waiting for the echo of a gunshot,” he said. “This has taken such an emotional toll. My family has friends we’re really concerned about.”

A recent report by researchers at the Prince William Sound Science Center attributes the collapse of the herring fishery to the 1989 Exxon Valdez disaster. Exxon maintains there is no link between the spill and the virus that reduced the number of herring.

In the Supreme Court decision, Justice David Souter wrote for the court that punitive damages may not exceed what the company already paid to compensate victims for economic losses, $507.5 million, just over four days worth of Exxon Mobil Corp.’s profits last quarter.

The Exxon Valdez case involves reckless action that was “profitless” for the company and that has already resulted in substantial recovery for substantial injury, Souter wrote. A penalty should be “reasonably predictable” in its severity, he added.

The Exxon Valdez, a supertanker, dumped 11 million gallons of crude oil into Alaska’s Prince William Sound, fouling 1,200 miles of coastline.

A jury decided in 1994 that Exxon should pay $5 billion in punitive damages. In 2006, a federal appeals court cut that verdict in half. Exxon asked the Supreme Court to reject the punitive damages judgment altogether, saying the company already has spent $3.4 billion to clean up the spill and compensate Native Alaskans, landowners and commercial fishermen.

Plaintiffs would have collected an average of $75,000 each under the $2.5 billion judgment.

The Supreme Court was divided on its decision, 5-3. Justice Samuel Alito took no part in the case because he owns Exxon stock.

Amar Sarwal, general litigation counsel for the U.S. Chamber of Commerce, said the ruling gives an “extraordinary amount of guidance” to courts beyond the Exxon Valdez case.

Plaintiffs attorneys pushed back, saying that the ruling applies solely to cases involving maritime law.

“Those who claim it stands for a generalized punitive damage limit are wrong,” said Kathleen Flynn Peterson, president of the American Association for Justice, a national group of plaintiffs attorneys. Souter wrote that the legal landscape is filled with examples of ratios and multipliers for punitive damages versus compensatory damages, saying most of them fall short of offering reasonable limitations in the Exxon Valdez case.

Overall, Exxon has paid $3.4 billion in fines, penalties, cleanup costs, claims and other expenses resulting from the worst oil spill in U.S. history.