A large-scale wildlife migration is taking place into central Maine that few people ever see unless they know where to look.

May marks the start of the annual return of alewives, a species of sea-run herring, to lakes and ponds to spawn. They gather in schools offshore, and as the weather warms, the silvery fish with the dark backs make their way to historic breeding grounds, many of them in central Maine or just a short drive away.

Alewives are considered a keystone species, which means they play a critical role in how ecosystems function.

For generations, alewives have fed people and have been used for lobster bait. The migration draws both airborne predators like eagles, osprey, great blue heron and herring gulls to prey on the fish and photographers who try to catch avian hunters catching a meal. It also attracts otters, minks, foxes and other land mammals looking for a snack. Alewives are also prey for salmon, striped bass other species of salt- and freshwater fish.

Two notable alewife-related celebrations are planned in or near central Maine this month.

On Saturday, Benton is set to host its annual Alewife Festival from 11 a.m. to 3 p.m. at Riverside Park at the town office, located at 1279 Clinton Ave.

On May 23, during Memorial Day weekend, the Damariscotta Mills Fish Ladder Restoration organization in Nobleboro plans to host the Run with the Alewives 5K race.

Maine Rivers, a membership organization that works to protect, restore and enhance the health and vitality of the state’s rivers, has put together a map of locations where alewives can be seen. Some of these spots are on private property and some have limited access.