A fishway constructed on the Cumberland Mills Dam in Westbrook will lead alewives into a part of the Presumpscot River this week for the first time in more than a century.

The fishway, which the Maine Department of Inland Fisheries & Wildlife in 2009 ordered Sappi Fine Paper to build at its dam, was built over the past two years and required to be completed by May.

“As a result of significant collaborative efforts and teamwork, Sappi expects to complete the fishway on time, within budget, and most importantly, safely,” the company said in statement. Sappi declined to comment further about the fishway.

The company has two more years to create a fish passage at Saccarappa Falls, a mile upriver, and is considering removing the dam entirely instead of building a fish ladder — possibly because it would be cheaper. The company has said the fish ladder at Cumberland Mills cost $5 million.

Eventually, Sappi will have to create passageways at its four dams farther upriver, the timing of which depends on how quickly the fish come back.

The Maine Department of Marine Resources has been stocking 600 alewives per year above Saccarappa Falls since 2009, so a good number of the fish — which take four years to mature — would return to spawn just in time for the opening of the fishway at Cumberland Mills, said Gail Wipplehauser, a scientist with the department.

She said Friday she expected the run to start in about a week and for about 1,000 alewives to pass through the dam over a three-week period.

Once the Saccarappa Dam is open, she expects about 24,000 alewives, as many blueback herring and about 10,000 shad to come through.

The purpose of allowing the fish to swim farther upstream is to give them access to habitats that are conducive to spawning, said Michael Shaughnessy, president of the Friends of the Presumpscot River.

Bigger populations of alewives, shad and herring will make for better fishing in the river, where salmon and striped bass are expected to follow; and in the ocean, where cod and tuna will have more to eat, Shaughnessy said.

There’s a broader purpose, too, he said — our “basic obligation to the planet” to keep it in good condition.

The Friends of the Presumpscot River and other conservation groups have been fighting to get Sappi to build fish passages on the Presumpscot since 1996, when the licenses for its five hydroelectric dams on the river were up for renewal from the Federal Energy Regulatory Commission.

The commission granted the licenses on the condition that Sappi build fish passages — a decision the company appealed to the U.S. Supreme Court and lost in 2006.

Construction of the fishways wasn’t required to start until there was passage at the Cumberland Mills Dam, the lowest dam on the river, which is regulated by the state, not FERC, because it is used to draw water for cooling, not to generate power.

Eventually, the state required that Cumberland Mills have a fishway, too.

The design is called a Denil fish ladder and is about 300 feet long with a 4-foot-wide passageway for the fish, Wipplehauser said. It is lined with 72 baffles, which slow the water as it flows down the ladder, giving the fish a chance to get up and over the dam.

The mile stretch of river between the Cumberland Mills and Saccarappa dams doesn’t have a prime habitat for spawning, Shaughnessy said.

However, he said, “they’re not going to get anywhere else if they don’t get up through here.”

He called the expected opening of the passage this week a milestone.

“We want to leave the place as well as we found it, which is a long way to go,” he said, “but this is a short step in that direction.”

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