Aaron Whitman of the Gulf of Maine Research Institute prepares to net some alewives for testing at Mill Brook Preserve in Westbrook last week. A group of migrating alewives can seen be to the right. Chance Viles / American Journal

The alewives migration is back in full swing this season, a phenomenon that draws thousands of visitors to the Mill Brook Preserve in Westbrook each year.

The alewives were first spotted last week, and the run is expected to last anywhere from a week to a month as the fish make their way from Casco Bay and upstream through the preserve to lay eggs in Highland Lake in Windham.

The migration is most visible where Mill Brook pools, attracting hundreds of fish that circle before making their way farther upstream.

Two alewives fight their way up Mill Brook in Westbrook last week, heading for Highland Lake in Windham, where they will spawn. The fish migration can last anywhere from a single week to a month. Chance Viles / American Journal

Researchers from the University of Southern Maine and the Gulf of Maine Research Institute say the return of the fish is a sign of Casco Bay’s good health.

The team catches some of the fish to analyze their diet and take DNA samples. They also collect water samples because it contains the DNA samples as well.

“This really can give us a view as to how healthy Casco Bay is, so it is important. We take samples of their diet and the water and get an idea of the health of (the ecosystem) that way,” GMRI researcher Samantha Bengs said at Mill Brook Preserve last week.

The eggs laid by the fish in Highland Lake will hatch and most of those tiny fish will make their way to Casco Bay at the end of June, according to Sharon Mann, who is studying for her Ph.D. at USM.

“Some fish do hang around over the winter, though, and will mature elsewhere,” Mann said.

While conducting their work at the brook, the researchers often act as ambassadors, explaining to visitors what they’re seeing, said Toby Jacobs, stewardship and outreach manager for the Presumpscot Regional Land Trust.

The land trust is looking for more volunteers to assist with visitors, maintain trails and do fish counts during the alewives migration.

“This really is an incredible thing and the public interest is awesome,” Jacobs said. “It’s a sight to see, the fish that are strong enough to fight their way up the rocks and through the rapids.” Jacobs counted eight per minute at one particular rapid.

For more information or to learn good viewing spots, visit prlt.org. Generally, the roped areas by the river are where the fish are seen, and visitors are asked to not go past the ropes or let their pets wander to the water.

Aaron Whitman holds a “razor belly’ alewife, named so because the line from tail to head is sharp rather than smooth. Chance Viles / American Journal

Alewives circle in a Mill Brook pool. Chance Viles / American Journal

The upstream battle can get chaotic at times, with some fish even knocking others onto the banks. Chance Viles / American Journal

A lone fish fights it’s way up a rapid. Some fish make it up, others do not. Chance Viles / American Journal

Researchers Samantha Bengs, left, Aaron Whitman and Sharon Mann prepare to take fish and water from Mill Brook. Alewife health in particular is a good indicator of ecosystem health because the fish leave DNA all the way from the Casco Bay inland to the lakes. Chance Viles / American Journal