A recent order by the federal Environmental Protection Agency that will open a substantial portion of the St. Croix River to migrating alewives has many fans — and more than a few critics.

Both sides of the dispute may have valid points to make, but there also may be a valid compromise position that, if the EPA agrees, could offer a way around the controversy.

Alewives, also known as river herring, are a small fish with a potentially huge economic and ecological impact.

Their spawning runs upstream from the ocean now involve some tens of thousands of fish using only 6 percent of their estimated potential range.

Expanding it by opening now-closed fishways in upstream dams, however, could balloon their numbers to the tens of millions.

Alewives can have an impact on game fish such as smallmouth bass, an important resource in upstream lakes for fishing camp owners and guides.

While sport fishermen say the impact is negative, however, others say they can be favorable to game species.

Biologists say alewives consume plankton that produce algae blooms that choke up waterways. Their impact, however, is likely to be far wider than that: One former Maine fisheries official termed them “Purina Chow for the ecosystem,” noting their roe provides food for the bass that sport fishermen value, and the fish themselves are widely used as bait by lobstermen.

Their most significant use, however, could be in helping inshore groundfish stocks recover. Some experts think the near disappearance of inshore commercial species is because of the lack of alewives lower down the food chain.

Over the past two decades, the state has acted twice to close the upper portions of the St. Croix River to alewives, but the EPA has overturned that ban by demanding that blocked fishways at the Grand Falls Dam be opened.

While dams farther upstream, at Grand Lake Stream and Vanceboro, were not specifically mentioned in the EPA order, the agency did say that alewives should be allowed into the “upper St. Croix.”

An oversight board with both Canadian and American members established to deal with issues involving the river, which separates Maine and New Brunswick, says it will monitor the impact of the new fish run on the area between those two dams and the Grand Falls Dam.

Limiting alewives to this area could provide a useful test of whether the runs do more harm than good, offering the opportunity to calm the fears of sport-fishing interests.

That, however, depends on whether the EPA meant to include the two dams above Grand Falls in its order, something that could preclude a limited-area study over time.

Maine officials would be wise to determine the EPA’s intent, and, if any leeway is permitted, vow to cooperate with the agency and with their Canadian counterparts to gauge the alewives’ impact in a coordinated fashion based on science.