An estimated 815,000 river herring swam through the fish ladder at the Mystic Lakes Dam in Medford in 2025, the highest count since monitoring began in 2012 and enough to make the Mystic River the largest herring run in Massachusetts for a second straight year. The number represents a species that was once nearly wiped out in local waters, now surging back through a river system that runs through some of the most densely built suburbs north of Boston.
The estimate comes from the Massachusetts Division of Marine Fisheries, relayed by the Mystic River Watershed Association, whose community science volunteers help generate the data state biologists rely on. When counting first started at the Medford dam in 2012, the annual run was estimated at roughly 200,000 fish — meaning the population has more than quadrupled in just over a decade. As WCVB reported, conservation efforts have transformed the Mystic River into a healthier ecosystem for both people and wildlife, and officials say the herring numbers prove those efforts are working.
A Fish Ladder Built to Undo Decades of Damage
The turnaround traces back to 2012, when the Massachusetts Department of Conservation and Recreation and the watershed association completed a permanent fish ladder at the Mystic Lakes Dam between the Upper and Lower Mystic Lakes. That structure reconnected migrating herring with more than 200 acres of ancestral freshwater spawning habitat they had been locked out of. Before the ladder existed, volunteers had to manually carry migrating herring over the dam in plastic buckets, according to the watershed association's account of the project.
River herring — a category that includes alewife and blueback herring — spend three to four years in the ocean before instinctively returning to the freshwater streams where they were born to spawn, according to the Massachusetts Division of Marine Fisheries. That biology explains the delay between habitat restoration and visible population growth: the dramatic doubling of the run observed in 2015 reflected fish born just after the 2012 ladder opened, finally returning as adults.
Winchester's Ladder Opened the River Further Upstream
A second fish ladder at the Center Falls Dam in Winchester was completed in 2017 and became fully operational the following year, letting herring push further upstream into Winchester's rivers and ponds. During its first full year running in 2018, an underwater video camera recorded nearly 100,000 herring crossing that passage, per the same watershed association reporting. The corridor's expansion has continued incrementally ever since, each new passage unlocking another stretch of historic spawning ground.
The next milestone is already funded. Construction financing was finalized in March for a new pool-and-weir fish passage at Woburn's Scalley Dam, with construction slated to begin later this year ahead of the spring 2027 migration season, according to the City of Woburn. The project, paid for through environmental settlement funds, federal grants, and a private foundation donation, will open access to more than 100 acres of spawning habitat in Horn Pond — and state biologists estimate it could eventually push the Mystic's total run beyond one million fish.
From Near-Collapse to Statewide Record
The recovery follows a period when river herring populations crashed coastwide to as low as one percent of historic levels, prompting the state to impose a strict moratorium in 2006 on harvesting, selling, or possessing river herring, a prohibition the Barnstable Clean Water Coalition notes carries fines of $50 per illegally possessed fish for recreational and commercial fishermen alike. River herring were once abundant throughout New England, and WCVB's report notes their decline was driven by dams, pollution, and habitat loss — the same forces the Mystic watershed's restoration work has spent years reversing.
By 2024, the recovery was already outpacing every other monitored run in the state: an estimated 640,000 herring passed through the Mystic Lakes Dam that year, with roughly 37,500 reaching Horn Pond, a full rebound from drought-related dips seen in 2020. The Mystic River Watershed Association has highlighted that restoration projects spanning the 21 cities and towns within the watershed have helped bring herring back to local rivers and streams, and officials there say healthier herring populations demonstrate the long-term payoff of protecting local waterways.
Volunteers and an Underwater Camera Do the Counting
Every spring from April through June, more than 160 community science volunteers fan out across watershed dams to conduct 10-minute visual fish counts, while an underwater camera installed in 2017 uploads video clips that the public can review and tally online at mysticherring.org. State fisheries biologists build their statistical population models directly from that volunteer-collected data, making the record-setting count as much a civic achievement as a biological one.
The stakes reach well beyond the fish themselves. River herring act as a keystone species in regional food webs, ferrying energy from ocean plankton to predators including striped bass, bluefish, bald eagles, ospreys, river otters, and seals, according to the watershed association. Historical 17th-century accounts described herring so thick in New England rivers that settlers claimed they could walk across the fishes' backs — a scale of abundance the modern recovery is still working to approach.
The watershed itself covers 76 square miles across 21 municipalities north and west of Boston, serving more than 600,000 residents in what the association describes as the most densely populated and urbanized watershed in New England. The nonprofit was founded in 1972 to combat severe industrial pollution in the river, and its restoration campaign now stretches across multiple towns and decades. Conservation groups reconnected waterways to support spawning fish and worked for years to improve water quality, according to WCVB's reporting, and the return of the herring, per officials, marks a major milestone for regional biodiversity.
