The U.S. Environmental Protection Agency on Thursday proposed restrictions that would block plans for a copper and gold mine in Alaska’s Bristol Bay region that is home to the world’s largest sockeye salmon run.

A statement from the regional EPA office said discharges of dredged or fill material into the waters of the U.S. within the proposed Pebble Mine footprint in southwest Alaska would “result in unacceptable adverse effects on salmon fishery areas.”