The U.S. Environmental Protection Agency (EPA) moved Tuesday to block a controversial proposal for a copper and gold mine in Southwest Alaska. The remote region, home to the world’s largest sockeye salmon fishery, is a massive source of minerals believed to be worth hundreds of billions of dollars.

For nearly 20 years, the Pebble Mine project has been seeking government approval to extract millions of tons of rock ore through an open-pit mine on a square mile of land. However, the Biden administration issued a final determination blocking the plan, citing the Clean Water Act.

“The Bristol Bay watershed is a vital economic driver, providing jobs, sustenance, and significant ecological and cultural value to the region,” EPA Administrator Michael Regan said. “With this action, EPA is advancing its commitment to help protect this one-of-a-kind ecosystem, safeguard an essential Alaskan industry, and preserve the way of life for more than two dozen Alaska Native villages.”

The announcement marks only the 14th time in the roughly 50-year history of the Clean Water Act that the EPA has flexed its powers to bar or restrict activities over potential impacts to waters, including fisheries, according to The Associated Press.

However, the company behind the Pebble Mine project calls the EPA’s move an “overreach” and political, maintaining that the project can coexist with salmon. The partnership’s website says the deposit is at the upper reaches of three “very small tributaries” and expresses confidence any impacts on the fishery “in the unlikely event of an incident” would be “minimal.”

The EPA’s decision to block the mine was based on an analysis by the U.S. Army Corps of Engineers, which said discharges of dredged or fill material to build and operate the proposed mine site would result in a loss of nearly 100 miles of stream habitat, as well as wetland areas.