PORTLAND

The state is committed to a plan that would allow alewives to return to the St. Croix River above a dam that blocks their passage, but only in a gradual manner and not in an unrestricted fashion that environmental groups and Maine Indian tribes are calling for, Maine’s attorney general said Wednesday in a letter to the Environmental Protection Agency.

The EPA last month sent a letter to Attorney General William Schneider saying a Maine law that bars alewives, or river herring, upriver of the Grand Falls dam violates the federal Clean Water Act, and that Maine should take action to allow alewives upriver of the dam. Advocates of restoring alewives to the river’s full watershed had hoped the EPA’s letter would force the state to open the dam’s fish passages and allow the fish upriver.

But in his response, Schneider wrote that the law, known as the alewife law, is valid, and that the state never intended to apply it for purposes of the Clean Water Act. That stance is opposed by the Conservation Law Foundation, which has sued the EPA in hopes of giving alewives full river access.

“The state has always regarded the alewife law as a fisheries management measure with meaning and effect only in that context; it has never considered the statute to be a CWA water quality standard,” Schneider wrote.

Schneider’s letter is the latest step in an ongoing dispute about whether alewives should be allowed full access to the St. Croix River, which serves as a boundary between Maine and New Brunswick.

The Boston-based Conservation Law Foundation filed a lawsuit against the EPA in May to require the agency to force Maine to allow the fish to return to their natural habitat above the Grand Falls dam. The CLF claimed that not doing so violates the Clean Water Act.

The Passamaquoddy Tribe and other groups have also pushed to have the river opened to alewives.

Alewives were effectively barred from the river for more than 150 years because of dams and pollution. But with cleaner waters and fish passages on the dams, millions of fish returned to the river in the 1980s, with the annual run growing to more than 2.5 million fish.

At the request of Maine fishing guides, legislators in 1995 enacted a law ordering the fish passages at the Grand Falls dam and the downstream Woodland dam to be closed to alewives. The guides maintained that alewives posed a threat to smallmouth bass populations — and by extension to the guides’ livelihoods — in lakes and streams in the St. Croix River watershed.

Lawmakers passed another law in 2008 allowing the fish to pass above the Woodland dam, but not above the Grand Falls dam 9 miles upstream.

Alewife advocates say the fish are good for the ecosystem, would serve as a bait source for lobstermen, don’t compete with bass populations and should be allowed to return to their native habitat in full.

The state supports a compromise plan that has been proposed to the International St. Croix Watershed Board, Schneider wrote in his letter to the EPA. That plan would let alewives return to a limited area of the river above the Grand Falls dam in a slow controlled manner that would allow for an increase in number over time while their impact on the ecosystem could be monitored.

Sean Mahoney, the head of the Conservation Law Foundation’s Maine office, said he opposes the plan supported by the state because it sharply limits how many alewives could go above the dam and how far they could go upriver, while subjecting them to strict monitoring that could halt their restoration for any of a number of reasons.

“We need to resolve our federal lawsuit with the EPA, which we’re in the process of,” he said. “Assuming this is the state’s final word, we’re going to have to look at the prospect of litigation against the state, which we were hoping we wouldn’t have to.”