A stream flows through the Bristol Bay, Alaska watershed, near where the Pebble Mine would be located Bureau of Land Management via AP

U.S. environmental regulators moved on Friday to block development of the Pebble Mine in Alaska — potentially one of the largest copper projects in the world — citing the possibility of "irreversible harm" to the state's valuable salmon fisheries.

The Environmental Protection Agency said it has initiated a rarely used process under the Clean Water Act to "identify appropriate options to protect" the Bristol Bay fishery from the future effects of the proposed mine.

The decision follows a report in January that found large-scale mining would pose serious risks to salmon populations and native cultures in this pristine corner of southwest Alaska.

"Extensive scientific study has given us ample reason to believe that the Pebble Mine would likely have significant and irreversible negative impacts on the Bristol Bay watershed and its abundant salmon fisheries," EPA administrator Gina McCarthy said in a statement. "This process is not something the agency does very often, but Bristol Bay is an extraordinary and unique resource."

Shares of Northern Dynasty Minerals Ltd, which owns the Pebble project, plunged 29 percent on Friday. The stock has fallen more than 64 percent in the last 12 months.

The Vancouver-based company was swift to condemn the EPA's action, saying the agency is looking to preemptively block the project without allowing it to go through the established permitting and environmental review process.

"What the EPA is trying to do is short-circuit that process," said Tom Collier, chief executive of the Pebble Limited Partnership, a Northern Dynasty subsidiary. "That's just a huge mistake. That's not the way America works."

Northern Dynasty has said the mine can be developed in a safe manner and would provide an economic boom for Alaska. The company expected to employ about 2,500 people through the construction phase and then about 1,000 throughout the mine's operating life, and filter hundreds of millions in tax dollars to federal, state and regional governments each year.

But opponents have long said the environmental risks outweigh the benefits, citing the potential for widespread damage if polluted water were to enter streams in the region.

The mine wasn’t only potentially dangerous for the environment, but for fishing jobs in Washington state.

Two Senators from that state, Maria Cantwell and Patty Murray, applauded the EPA’s decision, saying the proposed mine could have devastating impacts on the Pacific Northwest fishing industry. Thousands of Washington state jobs are tied to Bristol Bay salmon fishing.

The EPA can use the Clean Water Act to effectively veto the mine. It has initiated similar processes just 29 times and completed the entire process only 13 times. The review will include a new consultation period, public hearings and further consultations with the Army Corps of Engineers and the company.

The Pebble project is located some 200 miles southwest of Anchorage, in a region of Alaska that produces nearly 50 percent of the world's wild sockeye salmon. The area also has a booming sport fishing and tourism industry.

Al Jazeera and wire services