An estimated 630,000 herring migrated through a fish ladder installed at the Mystic Dam this year, marking one of the largest herring migration groups in Massachusetts history, the Mystic River Watershed Association said.
The number of river herring had been on the decline for several decades because of habitat loss, the association said in a statement. But when the Department of Conservation and Recreation and the Division of Marine Fisheries installed the fish ladder in 2011 after a push from the watershed association, it opened up an additional 165 acres of fresh water habitat for fish to lay their eggs.
Fish ladders are designed to provide a way for fish to get around obstructions in rivers like dams, according to the National Oceanic and Atmospheric Administration. They generally consist of a series of pools moving upstream, allowing fish to leap through a stream of water, rest in a pool, and proceed past the obstacle.
This year, volunteers with the Mystic River Watershed Association counted 91,997 herring migrating through the ladder between April and June. The association used a sophisticated model to come up with an estimate of the total of 630,000, up 40 percent from last year.
“We are thrilled that the herring count has increased, and glad our volunteer monitoring program and our new underwater video monitoring program are engaging more people in the community — including local students — in the amazing river herring migration,” said Patrick Herron, the executive director of the association.
The video monitoring program is the result of a partnership between the association and six local schools designed to bring herring migration to the attention of students.
An underwater video camera films the fish using the Mystic Lakes ladder and streams the footage into classrooms, where students learned about the fish and helped document data, the association said. Several classes also took field trips to the ladder for a more hands-on experience.
The association installed a second ladder in the Aberjona River after the Mystic ladder was successful, which could allow herring to have access to habitats as far inland as Woburn next year.Alyssa Meyers can be reached at firstname.lastname@example.org. Follow her on Twitter @ameyers_.
Correction: A previous version of this story misidentified which group installed the fish ladder. The Globe regrets the error.