WESTBROOK — In a cloud of rushing bubbles, a sleek silhouette flits past, its forked tail receding into the turbid Presumpscot River.

The image is of a spawning alewife, recorded on an underwater camera positioned inside a fish passage at the Sappi paper mill in Westbrook, where for the second year in more than a century, spawning alewives, shad and blueback herring are able to surmount the Cumberland Mills Dam.

When the run is complete in mid-July, conservationists will be able – thanks to the cameras – to count how many of the fish are returning, giving a look at how quickly the species may return to strength.

“The idea that fish are now coming back, and they can begin to rebuild their population, is really important to the ecology of this river, to get it back to what it once was,” said Dusti Faucher of the Friends of the Presumpscot River, the grassroots organization that has fought for 15 years to ameliorate man-made impediments in the river. “It’s a really historic moment.”

Construction of the fish passage was completed in April 2013, after the Department of Inland Fisheries and Wildlife in 2009 ordered Sappi to construct the concrete channel. The project cost Sappi millions of dollars, and is the first step in a court-ordered process of remediation on the river, which is still one of the most impounded in the region.

Still standing between the Sappi mill and the river’s headwaters are eight dams, each a remnant of the prosperous industrial age that helped populate southern Maine and Cumberland County.

Until July 15, the three sea-run species are expected to make the swim uphill 100 yards, in the process rising more than 22 feet over the Cumberland Mills Dam. Once on the other side, the alewives and other species will have access to nearly a mile of river, bringing them to the doorstep of the next impediment, the Saccarappa Dam.

Although they live mostly in the ocean, the species spawn each year in fresh water. They are a staple food for bald eagles, river otters, osprey and striped bass.

Over the next two years, Sappi and the city of Westbrook have committed $200,000 to plan the future of the Saccarappa Dam, located in the heart of Westbrook’s downtown. Engineers and planners are still determining whether it will be a site for another fish passageway, or whether the dam should be removed, said Sean Mahoney, executive vice president of the Conservation Law Foundation.

Of the eight dams between Sebago Lake and Sappi’s Westbrook mill, the company will be responsible for constructing passageways at the next five dams upstream – a process that could take another decade or two, Mahoney said.

Forcing the company to build the fish passages has not been easy. The Federal Energy Regulatory Commission in 1996 granted licenses for Sappi to continue to generate hydroelectric power on the condition that it construct the passages at the six dams.

The company fought the order in court for years, Mahoney said, eventually appealing to the United States Supreme Court in 2006, but the nation’s highest court refused to hear the case. Now the former adversaries from conservation and industry are working together on the fish passage projects, with Sappi committing most of the funding.

It will be an expensive endeavor for the company – which has spent $5 million already on the first passageway – but one that Mahoney said it owes the region after years of profiting from the river.

Sappi has told conservation groups, including Mahoney’s, that it will continue to operate in good faith. So far, it has upheld that promise, Mahoney said.

“Sappi is committed to safe, effective passage of sea-run fish on the Presumpscot,” Donna Cassese, managing director of the Westbrook mill, said in a statement. “We look forward to many more seasons of fish passage to come.”

If plans hold, the project at the Saccarappa Dam will be a major boon for the species, representing a gateway to the most coveted habitat. “We feel very confident that if the fish can get above the Saccarappa, they’re going to thrive,” Mahoney said.