WESTBROOK, Maine — Right now, a rocky pool behind a small waterfall on Mill Brook is thick with alewives. Their sleek, dark bodies are pointing upstream in the thousands. In ones and twos, they dart, flop and leap up the frothing, three-foot drop. They’re on their way home, to Highland Lake, where they will spawn before returning the same way, back to the sea.

They do it every year, in this secluded, woodland pool, like nature’s clockwork. The only difference this year is that they will have an audience to cheer them on.

A new trail network, opened late last year, is giving the public a chance to see the natural spectacle for the first time. The paths run beside the brook on land recently acquired by the Presumpscot Regional Land Trust. The trust was formed 30 years ago to promote healthy lands, waters, wildlife along the heavily industrialized river.

“It’s pretty cool, isn’t it?” said PRLT Executive Director Rachelle Curran Apse. “That’s, by far, the best spot to see them.”

The trust started putting together the Mill Brook Preserve, as it’s called, three years ago. The city of Westbrook donated the first parcel of land along the brook and, with other donated parcels since then, the preserve has grown to 120 acres.

“We’re preserving it one piece at a time,” said Apse.

In December, the PRLT opened four miles of trails along the waterway, making this year’s alewife run accessible to the public. By the end of 2017, Apse expects even more land, and another mile of trails, to be added to the preserve.

Mill Brook begins at Highland Lake and ends on the Presumpscot River. It flows mostly through undeveloped forest. It’s only about five miles long but it hosts the largest alewife run on Casco Bay. Last year, over 30,000 fish made the journey to the lake.

“We expect it will be that much, or more, again this year,” said Apse.

By the time the fish reach the pool in the woods, they’ve already swum up the Presumpscot River and taken a right onto Mill Brook. That task got easier in 2002 when the Smelt Hill Dam was removed at the mouth of the river. Starting in 1998, the Maine Department of Marine Resources began stocking Highland Lake with alewives to try and re-establish the run. It worked, aided by a new fish ladder at the lake’s outflow dam.

“The adults will be in the lake for a few weeks, spawning, and they’ll come back down the river and go out to the ocean to feed,” said Gulf of Maine Research Institute staffer Zachary Whitener, who was collecting 25 fish from the pool on Wednesday to help study the health of the population.

“The juvenile alwives that will hatch, will live in the lake for a few months,” said Whitener. “In the late summer, early fall, will swim back down this brook and out to the ocean, themselves.”

A few years later, when they’re all grown up, the fish will return to their birthplace in the lake to start the cycle all over again. The annual movement of fish is important to the local ecosystem because alwives are eaten by everything — from other fish, to birds, to weasels and raccoons.

This weekend is prime viewing for the migration and Dr. Karen Wilson and Dr. Theo Willis, from the University of Southern Maine Environmental Science and Policy Department, will lead a walk to the pool.

“It’s really just about getting the public engaged — there’s a lot of life down there,” said Willis.

Willis said alewives migrate up the state’s other rivers but the little pool on the land trust land is a unique place to watch them. It’s doubly special being so close to greater Portland’s urban center.

“They migrate up the Penobscot, too,” he said, “But there’s no place in Bangor to see them.”

The guided walk to the fish pool is Saturday June 3 at 10 a.m. Space is limited and reservations can be made by contacting the Presumpscot River Land Trust online.

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Troy R. Bennett

Troy R. Bennett is a Buxton native and longtime Portland resident whose photojournalism has appeared in media outlets all over the world.