The Supreme Court on Thursday barred Japanese salmon fishermen off the Alaska coast from casting 9-mile-long nets in which thousands of porpoise and other marine mammals are trapped accidentally.

The justices, without a recorded dissent, refused to lift a lower court order that prohibits gillnet fishing by Japanese fleets during the June-July salmon season in an area of the Bering Sea and North Pacific.The Federation of Japan Fisheries Cooperative Association and the Reagan administration had asked the high court to overturn the injunction and permit fishing to resume pending the high court's consideration of a formal appeal.

Environmental and native Alaskan groups urged the justices to leave the injunction in force.

"We're obviously jubilant," Alan Reichman, a Greenpeace spokesman in Seattle, said of the court decision.

"We know at least we're not going to have this ecological disaster played out in our waters this summer, and we're hopeful it is going to be stopped for good," Reichman said.

Government lawyers had said continuing the ban would have "immediate and dramatic effects."

The Justice Department said preventing the Japanese from fishing for salmon "may prompt Japan to withdraw from the international convention that currently prohibits Japanese fishermen from fishing for North American salmon in a large area of the North Pacific. It also, almost certainly, will lead to the removal from Japanese fishing vessels of the United States observers who now monitor the incidental taking of marine mammals by Japanese fishermen."