PLEASANT POINT, Maine — Years of efforts by the Passamaquoddy tribe and environmental activists to restore the annual migration of alewives up the St. Croix River received a boost Tuesday when the U.S. Environmental Protection Agency ruled a Maine law banning native alewife migration up the river violates the federal Clean Water Act.

For 17 years, the annual freshwater migration of millions of saltwater alewives has been blocked by a barrier on the St. Croix River at the Grand Falls Dam fishway that keeps the fish from swimming farther upstream to spawn. The Passamaquoddy nation wants that barrier removed and the issue has spawned years of legal action spearheaded by Friends of Merrymeeting Bay, an environmental advocacy organization based in Richmond.

An EPA letter to Maine’s attorney general says the status quo represents an illegal lowering of water quality that the agency cannot and will not approve. The EPA ruling, in essence, challenges Maine’s efforts to eliminate alewife access to natural spawning habitat above Grand Falls Dam near Princeton.

“This all ties back to a small but very vocal minority of smallmouth bass fishermen who had a bad year of fishing and blamed it on the alewives,” said Ed Friedman, chairman of Friends of Merrymeeting Bay. “I suspect what really happened is that there was a drawdown of the water level, which created a loss of spawning territory for the bass population.”

The Maine Department of Marine Resources was not prepared to comment Wednesday on the EPA action.

“At this point we have no comment,” Marine Resources Commissioner Patrick Keliher said Wednesday. “We’re looking at the legalities and will determine over the next week what this letter means.”

To Passamaquoddy tribal members, whose settlements on both sides of the St. Croix River separating Maine and New Brunswick date back 4,000 years, the prolific fish are seen as an important food source and a critical element in the diverse marine ecology of Passamaquoddy Bay.

In a letter sent May 24, 2012, to the International Joint Commission that works to resolve disputes involving the waters separating the U.S. and Canada, Clayton Cleeves, Passamaquoddy tribal chief of the Pleasant Point Reservation, said his people want to see all alewife barriers removed.

“I urge you to consider the free passage of alewives to their breeding grounds along the St. Croix watershed and into the northern lakes in the state of Maine,” Cleeves’ letter reads in part. “This special request will eliminate all barriers and blockades and will give alewives the freedom to breed at proportions bestowed by the Great Spirit.”

Cleaves said three major barricades shut off 98 percent of alewives’ natural spawning habitat. “They can’t get up to their maternity ward,” he said.

Under current law, the state’s Departments of Marine Resources and Inland Fisheries and Wildlife undertook an effort to prevent alewives and blueback herring from swimming up into the St. Croix River basin, using stop logs to block the dam’s fishway, which was installed some years ago and paid for by U.S. Fish and Wildlife Service.

In its finding, the EPA directs the state to restore migratory passage.

“EPA is not aware of any sound scientific rationale for excluding indigenous river herring (or other migratory species) from the St. Croix River,” the ruling reads in part. “Maine should take appropriate action to authorize passage of river herring to the portions of the St. Croix River.”