Maine plans to sue the U.S. Environmental Protection Agency over a ruling intended to ensure that members of Maine’s Indian tribes can safely eat large quantities of fish for sustenance.

The move, disclosed in a letter this week from Attorney General Janet Mills to EPA Administrator Gina McCarthy, opens a new front in an escalating, decades-long power struggle between the state and tribes over the latter’s rights and authority under the 1980 Settlement Acts, the federally brokered compromise agreement that extinguished the tribe’s claim to two-thirds of the state.

In Tuesday’s letter, Mills charged that the EPA has acted unlawfully, usurping Maine’s regulatory powers in violation of federal law and court rulings. She advised the agency that she was giving 60-day notice of her intention to sue.

The Penobscot Nation expressed concern with the move. “While not surprising, it is disappointing that we can’t get the state to recognize or acknowledge our sustenance fishing right,” said the tribe’s chief, Kirk Francis. “We have a right to clean water so that sustenance fishing is conducive to the health of our people.”

The dispute is over the LePage administration’s proposed water quality standards, which are subject to review by the federal agency. The EPA informed state authorities on Feb. 2 that their proposed standards are inadequate to protect sustenance fishermen on the reservations from certain toxins, because they eat much greater volumes of fish than the average Mainer.

The agency, acting in consultation with the federal Bureau of Indian Affairs and the U.S. Department of Justice, asserted such fishing rights were granted to the tribes under the Settlement Acts. It ordered the state to come up with more restrictive standards for certain toxins in Indian waters within 90 days. The EPA ruling did not suggest eating fish from the state’s waters is unsafe for most Mainers.

Gov. Paul LePage subsequently blasted the EPA decision, calling it “outrageous” and an act of “retribution” against the state Department of Environmental Protection for having previously crossed the agency. The DEP won a 2007 federal appeals court case that confirmed the state’s authority to establish environmental regulations on Indian lands and in July 2014 filed another suit to force the agency to honor that decision.

DISPUTE CENTERS ON SETTLEMENT ACT

In her letter, Mills said the Settlement Act only permits “certain Maine tribes to take fish without restriction within their reservations provided that such fish takings are for individual sustenance only” and does not provide a tribal right to sustenance fish.

Penobscot Chief Francis said the tribes, not the state, are trying to properly enforce the Settlement Acts. “It’s very clear that there is no acknowledgment at all of the rights of the tribe to fish and for those fish to be conducive to eating,” he said. “It’s very clear this is about wiping that out for the tribes, not about promulgating water standards.”

In a statement, the EPA said it was committed to upholding both the Clean Water Act and the Settlement Acts in Maine. “EPA must ensure that water quality standards in Maine protect the health and environment for all populations, including the Indian tribes in Maine who practice sustenance fishing as provided for by the Indian Settlement Acts,” the agency said, adding that it and the Department of Justice would respond to the state’s allegations in detail if a suit is filed.

The Department of Environmental Protection did not respond to interview requests, and the Attorney General’s Office declined to comment.

The EPA on Monday rejected other proposed state water pollution standards in Indian territory, including ammonia and arsenic. The agency also rejected a DEP proposal to downgrade the water quality target standard for a 0.3-mile section of Long Creek in Westbrook.

The EPA rulings on tribal waters, which could substantially tighten water standards in certain sections of the Penobscot and St. Croix rivers and other water bodies, have alarmed paper companies and riverside municipalities, which fear they may result in added pollution control expenses.

Maine’s largest tribes, the Penobscot Nation and Passamaquoddy Tribe, are already locked in disputes with the state over saltwater fishing rights, tribal jurisdiction over domestic violence cases by non-Indians on their reservations, and land use regulation on the tribe’s extensive, off-reservation trust lands. The Penobscot Nation have sued Mills in federal court over the state’s assertion that the tribe has no special rights in the main stem of the river that bears its name.