SEATTLE — Federal environmental regulators, citing risks to water quality and salmon spawning grounds in one of the world’s richest fisheries, moved on Friday to block the development of a giant open-pit copper mine in the watershed of Bristol Bay in southwest Alaska.
While the decision by the Environmental Protection Agency was not an outright death blow to the project, called the Pebble Mine, it left little room for the mine and its supporters to persuade the agency otherwise. E.P.A. officials said they would now start gathering additional information and public comment under a provision of the federal Clean Water Act that could end any chance of the mine project’s going ahead.
The agency’s administrator, Gina McCarthy, was unequivocal in saying that the science, including a three-year peer-reviewed study by the E.P.A. completed this year, had spoken.
The vast size of the proposed mine, with its attendant waste products, would harm the water, the economy and the culture, she said. The region’s unique ecosystem produces almost half the world’s supply of wild sockeye salmon and is the spawning ground for all five species of Pacific salmon: sockeye, coho, chinook, chum and pink.
“The Bristol Bay watershed is indeed one of the most productive ecosystems on the planet,” Ms. McCarthy said. And the Pebble Mine, as it has been proposed in size and scope, she added, would cause “significant and irreversible negative impacts.”
The mine is deeply controversial in Alaska, and had been further politicized in recent weeks, with Senator Mark Begich, a Democrat running in a hard-fought re-election campaign, coming out strongly against the project in January, calling it “the wrong mine in the wrong place.” Many other elected officials, including Gov. Sean Parnell, a Republican, have either supported the project or have argued that the permit process should at least go forward without E.P.A. intervention.
Federal officials emphasized in the announcement on Friday that their examination of Bristol Bay and the Pebble Mine had come at the request of native Alaskan tribes in the region. The resulting investigation, begun in 2010, concluded in a final draft report in January that the mine, even without accidental spills or discharges, presented a threat of serious harm.
Depending on the size of the mine, 24 to 94 miles of salmon-supporting streams and 1,300 to 5,350 acres of wetlands, ponds and lakes would be destroyed, the study said. A transportation corridor to support the mine, the study said, would also cross wetlands and about 64 streams and rivers in the Kvichak River watershed, 55 of which are known or likely to support salmon.
The mine’s developers, the Pebble Partnership, said in a statement in January, when the final draft of the E.P.A.’s scientific study was released, that the research was flawed by considering only a hypothetical proposal of what the project might be, since specific permit requests had not even been filed, and by not adequately taking into account state-of-the-art engineering safeguards that would be put in place.