The U.S. Environmental Protection Agency has struck down the state of Maine’s controversial law that blocks gaspereau from the St. Croix River.

The agency ruled that Maine is violating the America’s Clean Water Act by blocking a native fish from entering the river.

Hugh Akagi, the chief of the Passamaquoddy tribe in New Brunswick, said the federal agency’s ruling is giving them a reason to finally celebrate.

"We need victories. We haven't seen a whole lot of victories. It will be nice to share this. There needs to be a celebration on this," Akagi said.

Maine Gov. Paul LePage has not said if he will accept the ruling. But a spokesperson says the government will respond to the EPA decision early next week.

The EPA's decision is a reaction to court challenges launched against the federal government by environmental groups in New England and in Maine.

Maine has prevented gaspereau from migrating up the St. Croix River for 17 years in an effort to protect small-mouth bass, an introduced species favoured by anglers.

The Atlantic Salmon Federation is one of the groups that argued against Maine’s law.

John Burrows, the federation’s director of New England programs, said Maine's governor should now accept the federal decision.

"I certainly think this would be a fairly easy decision that would have tremendous economic and environmental benefits," he said.

When Maine blocked the river to gaspereau in 1995, there were more than two million of the small fish in the river. The fish population has dropped significantly since the state's actions.

The Conservation Law Foundation and Friends of Merrymeeting Bay launched a lawsuit in May that alleged the Environmental Protection Agency had failed to enforce federal laws designed to protect the river.

The dispute was escalated further in June when three chiefs representing the Passamaquoddy tribe in Maine and New Brunswick declared a state of emergency in the St. Croix River.

Tribal governors at Indian Township and Pleasant Point, Me., joined with Akagi, saying gaspereau "are threatened with extinction" in the river.