Alewives make their way through rapids in Mill Brook at the northern fish viewing pool in the Mill Brook Preserve in Westbrook on May 27.

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A tenacious ritual of spring returned to the waters of Mill Brook in Westbrook over the past two weeks as the largest fish migration from Casco Bay began. Thousands of alewives are making their way up the Presumpscot River into Mill Brook, where they furiously flap through shallow sections and up rocky cascades to get into Highland Lake, where they spawn.

Numerous alewives swim through the southern fish viewing pool in the Mill Brook Preserve on May 25.

Alewives enter into Highland Lake in Westbrook from a fish passage on May 27.

The alewives’ destination is imprinted on them because it’s the place they were born and instinct brings them back through Mill Brook to the lake every year. “They come back starting at four years old and the adults, if they can make it through the ocean and aren’t eaten, can return four or five times,” says Karen Wilson, an associate research professor at the University of Southern Maine who has been following the alewife runs up Mill Brook since 2014. Wilson says the number of alewives that come through Mill Brook during a run, which happens mid-May to mid-June, is typically in the thousands and that the 2018 run set a record of an estimated 64,000 alewives.

Numerous alewives swim through the southern fish viewing pool in the Mill Brook Preserve on May 25.

Mikki vanSummern checks the temperature of the water at the Highland Lake Dam in Westbrook on May 27. VanSummern is a volunteer with the Presumpscot River Land Trust and records data about water height and temperature in addition to counting alewives that enter the lake from the dam’s fish passage.

The spectacle of thousands of alewives fighting to get upstream draws crowds to the Mill Brook Preserve every spring, primarily to a few pools where the alewives gather en masse before their next arduous push upstream. “They’re quite likely resting,” Wilson says. “They can also be waiting for the water level to hit the appropriate level for that particular barrier.” The water level has to be just right, Wilson says. Too much water creates too strong of a current and too little water makes it too difficult for the alewives to climb the falls and cascades.

Mikki vanSummern, a volunteer with the Presumpscot River Land Trust, walks the fishway at the Highland Lake dam in Westbrook to make sure there is nothing blocking the passage on May 27.

The alewife run through Mill Brook is part of Maine’s success story in returning alewives to their traditional spawning grounds. Many of the alewife runs in central Maine, made possible by the removal of dams on the Kennebec River, have resulted in staggering numbers of the anadromous fish moving upstream. In 2018, a record-setting 5.7 million alewives passed through the Benton Falls Dam. This spring, completion of a new fishway at the China Lake Outlet Dam has returned alewives to that lake for the first time in decades.

Toby Jacobs searches a pool and falls for alewives in Mill Brook in Westbrook on Saturday. Jacobs, stewardship and outreach manager for the Presumpscot River Land Trust, led a group walk along Mill Brook on Saturday to look for alewives but they saw none, meaning the fish have all probably already migrated upstream to spawn in Highland Lake.

After the eggs hatch, Wilson says the juvenile alewives will stay in their lakes for a few months before heading downstream to estuaries, which afford them some protection from predators. Wilson says the juvenile alewives will then roam the Gulf of Maine as far as Cape Cod to the Bay of Fundy before returning four years later to Casco Bay to start their own journey through Mill Brook.

Alewives make their way through rapids in Mill Brook at the northern fish viewing pool in the Mill Brook Preserve in Westbrook on May 30.

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