At least five major Japanese seafood companies may have violated U.S. antitrust laws by trying to fix prices offered to some Alaska salmon processors in 1991, according to Justice Department documents filed in U.S. District Court.

In an affidavit filed in Seattle, Justice Department attorney Richard Cohen cites witnesses, telexes and documents that indicate "certain Japanese importers of seafood processed in Alaska may have engaged in anti-competitive conduct in possible violation . . . of the Sherman (Antitrust) Act."Mr. Cohen is part of a team of San Francisco-based Justice Department attorneys conducting a wide-ranging investigation of Japanese fish buyers. They were in Toyko this month to interview Japanese seafood officials and gather documents. By as early as April, the department hopes to determine whether to file a criminal complaint against at least some of the companies, Mr. Cohen says in his affidavit.

Fishing is Alaska's second-largest industry, after oil. The federal investigation is seeking information on all types of Alaska seafood imported to Japan, including the abundant pollock harvested from the Bering Sea.

But court documents indicate the Justice Department is focusing on the Bristol Bay red salmon fishery that unfolds each July. The bay sustains the world's largest wild runs of reds and seasonally employs thousands of people. In some years, the catch is worth more than $100 million to fishermen, who sell to processors.

The processors clean and freeze or can the fish, and then sell most of it to Japanese seafood buyers.

The investigation is looking into events surrounding the bay's 1991 summer fishing season. That year was marked by a worldwide glut of salmon and a price collapse that triggered protests and a strike by fishermen. The investigation targets dealings between the Alaska processors and the Japanese fish buyers, according to Mr. Cohen's affidavit.

Attorneys for Japanese buyers say there is no conspiracy to fix Bristol Bay prices and that the U.S. government has offered no credible evidence to indicate otherwise.

"They absolutely deny that they have been talking," said Charles Routh, a Seattle attorney for Taiyo Fisheries Co. "They have not and did not."

Mr. Cohen, in his affidavit, says witnesses "had been told by employees of Japanese seafood importers that those employees were 'networking' with employees of other major Japanese seafood importers concerning the prices to offer to Alaska seafood processors."

When an Alaska processor called Japan for price quotes, he would generally receive the same price quotes from all the Japanese buyers, according to documents cited by Mr. Cohen.

Gunnar Knapp, a University of Alaska Anchorage economist who has done an extensive study of the Japanese salmon industry, cautioned that the antitrust allegations do not take away from fundamental changes that have depressed markets.

"If that is going on, and you bring all the people to justice and prosecute them, you shouldn't assume you will get substantially higher prices as a result," he said.

Indeed, Japanese salmon markets are again awash in salmon, and Mr. Knapp and industry officials are already warning of a possible plunge in prices this summer.

The Justice Department investigation is an outgrowth of a decade-old department settlement of a civil antitrust lawsuit charging eight Japanese companies with conspiring to set prices paid for processed Alaska crab. Those companies are Taiyo, Nippon Suisan Ltd., Mitsui & Co., C. Itoh & Co., Nippon Reizo Kaisha Ltd., Shinko Sangyo Trading Co., Kyokuyo Co. and Toshokuku Ltd.

The price-fixing of crab is alleged to have occurred during meetings of a trade group known as the Japan Marine Products Importers Association.

Without admitting any guilt, the eight companies agreed to a 1982 settlement that prohibited them from any exchange of information about prices to be paid to U.S. processors for any kind of seafood. And the companies agreed to monitoring by the Justice Department for the next decade.