Alewife migration a fascinating natural phenomenon

By Kaysa Jalbert

It's late spring, and the waters of Mill Brook near South Windham are warming up. The brook, winding through a secluded, hilly forest, becomes the stage for an incredible natural spectacle. As you walk along the trails, you come upon a viewing pool near the southern trailhead. In the clear water, thousands of silver alewives, a type of river herring, are gathering. These fish, each about 11 inches long, shimmer in the sunlight as they move together in tight, swirling schools.

Alewives enter the Presumpscot River from Casco Bay then
swim up Mill Brook until they reach their native spawning
ground at Highland Lake. FILE PHOTO

The fish congregate at the base of a small set of rapids. Here, the water bubbles and churns, creating a white froth against the rocks. Despite the challenging current, the alewives begin their ascent. The fish move up the falls in a determined scramble, oftentimes flipping onto their sides and sliding back down; trying many times to push forward through the flow of water. Some make it on the first try, while others are pushed back by the force of the water, only to try again moments later.

This is how Brenna Crothers, Community Engagement Manager of the Presumpscot Regional Land Trust describes one of coastal Maines most fascinating phenomenon’s that is the alewife fish migration from Casco Bay to Highland Lake where they spawn, and you have a chance to witness it. Over the next few weeks, thousands of alewife fish will be swimming through Mill Brook Preserve.

Once in scarce numbers, the alewives have rebounded after Smelt Hill dam was removed in 2002, allowing migration numbers to reach over 70,000 in the past few years. This removal, along with other restoration efforts, like culvert replacements and fish ladder installations, has led to the significant return of alewife migrations.

“This is a species of fish that came back from almost a 250-year absence,” said Crothers. “Witnessing their journey, you can't help but cheer for these mighty fish. The schools of fish that make their way up rivers and brooks face many obstacles: shallow waters, fallen logs and trees, predators, swimming up seemingly impossible waterfalls without the grace of a salmon, but in a movement that can only be described as a determined scramble. These fish inspire.”

Each year, Alewives enter the Presumpscot River from Casco Bay, then swim up Mill Brook until they reach their native spawning ground at Highland Lake. This almost 11-mile-long journey typically takes two to three weeks. Adult alewives can make this round-trip about three to four times in their lives.

The migration will be visible from every one of the Mill Brook Preserve trailheads, the closet trailheads to the water being on Perry Court and north of 789 Methodist Road. The southern fish viewing pool is the most likely to spot fish, however the northern pool is more remote meaning at the right time you can see multiple fish ascend the waterfall.

The alewife migration is a family-friendly activity. Mill Brook has a two-mile hiking loop that crosses two bridges, includes nature scavenger hunt cards along the trail, and has a fairy and gnome village.

For the health of alewives during their migration, ropes have been placed around the viewing areas to ensure all people and pets stay out of the water, to stop soil from eroding into the stream and to make it clear fishing and collecting fish are not allowed.

The journey of migration is very stressful for Alewives. It begins in the ocean and estuaries, where they get eaten by larger fish and birds such as eagles, ospreys and cormorants. As they migrate into streams in the forest, new predators stand by such as raccoons and foxes.

Finally arriving at their spawning grounds, they must still be aware of larger fish like the freshwater bass. Survivors then find their way back into the ocean and their spawns follow some months later. The native range of the alewife stretches from the bottom of North Carolina up into New Brunswick, going up and around Nova Scotia as well.

Alewives have large eyes, a large black spot behind the gill cover, and their silver colors are darker on top, making for camouflage from water’s surface, and shinier from the bottom, giving them camouflage from below as well.

Another name for an alewife is “sawbelly” because of their narrow bodies that form jagged sharp scales along their stomachs. The alewife may also go as “Gaspereau” in Atlantic Canada, or “kayak fish” for its elongated shape.

To get more information or to download the alewife migration electronic guide, visit https://www.prlt.org/alewife-migration <