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River Herring in the Mystic_Credit Patrick Herron
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According to the official estimate from the Division of Marine Fisheries (Mass DMF), more than 425,000 river herring passed through the fish ladder at the Mystic Lakes Dam in Medford in 2022, and an additional 20,000 passed into Horn Pond. This the highest documented herring run this year in any river in Massachusetts!

River herring — Alewife and Blueback herring — are migratory species that spend most of their lives in the open ocean, but return to freshwater every year to spawn in the river system they were born in. They are threatened by overharvesting in the ocean and — crucially — by the loss of inland habitat for breeding caused by dams blocking access to inland lakes and streams. The recent count represents the latest chapter in an ecological restoration success story that has included the installation of multiple fish passages on the river resulting in a sharply rebounding population of river herring in the Mystic River.


This year’s total is a bit lower than last year’s estimate but consistent with the sense that the Mystic population remains robust. (Fish populations fluctuate over time for multiple reasons). Often the Mystic is among the largest documented herring runs in the state, but this year it was the largest.

This is both remarkable and a sign of some less positive changes elsewhere.

It is remarkable because one might not expect the largest herring population in the state to be one that has to run the urban gauntlet of Boston Harbor and the Amelia Earhart Dam — not to mention the Mystic Lakes Dam — to get to their breeding grounds. Usually, the largest runs in Massachusetts are on rivers with relatively unobstructed streams and protected estuaries like the Herring River in Wellfleet on Cape Cod Bay.

But this year, numbers from herring counts from other usually high-count Massachusetts sites were notably lower than last year. The Herring River count, for instance, was 290,000, after counts of over 1,000,000 as recently as 2019.

As Ben Gahagan of the Division of Marine Fisheries says, “Indeed it was a bad year across most of southern New England.” Similar patterns were seen in Rhode Island and Connecticut. “On the other hand,” Gahagan adds, “Maine had record runs and New Hampshire was actually pretty decent.” That is, there may be some phenomenon at work disproportionately affecting southern populations in recent years. Multiple causes may be at work, and state scientists say it is too soon to point to a single reason.

One major factor in reducing Massachusetts herring populations in recent years might be the droughts the area has experienced. In many river systems, droughts late in the summer can prevent juvenile migration back to the ocean by drying up streams, reducing productivity in the population in a way that is seen years later in the fish counts of returning adults. The Mystic system may be relatively immune from this effect, at least currently. There are fewer dead ends for fish to end up in, even in relatively severe drought.


A fish ladder at the Mystic Lakes Dam in Medford was built in 2012 — allowing fish in the Mystic River to reach Upper Mystic Lake for the first time in decades.

At that time, the Mystic River Watershed Association launched a volunteer herring count in collaboration with Mass DMF. Dozens of volunteers annually visit the dam to make 10-minute sampling counts of fish passing into Upper Mystic Lake every daylight hour from April to June, and the data are used to estimate the total number of fish passing through the fish ladder.

In the first year, an estimated 200,000+ fish passed through the dam, based on data generated by volunteer counters. But river herring first return to freshwater to reproduce only at age 3 or 4, when they are sexually mature. When the first cohort of fish born in the newly expanded freshwater habitat reached reproductive age and returned for the first time to Mystic Lakes in 2015, volunteer counts showed that the population of fish loyal to the Mystic River had doubled (Figure 1). And numbers continued to climb over the next few years to an estimate of as many as 780,000 fish in 2019.

This is a huge success story: the single simple fish ladder at Mystic Lakes doubled — and even tripled — a significant wildlife population. For a sense of scale: 500,000 of these foot-long fish end-to-end would stretch 100 miles!


Volunteer fish counters also monitored fish entering the next big lake upstream in the Mystic River watershed: Horn Pond in Woburn. Currently the Scalley Dam at Horn Pond mainly blocks fish passage into the lake, although a small percentage of fish who arrive make it up a small cascading stream that goes around the dam when water levels are high enough.

The estimated number of fish able to enter Horn Pond this year, based on volunteer counts, was approximately 20,000. Many more fish were seen at the base of the dam, most of which presumably turned back downstream.

The good news is that money from two federal environmental damages settlements — including from famous Superfund cases in Woburn — will bring millions of dollars of investment in building a fish ladder at Horn Pond. The City of Woburn is also investing in this project, expected to be completed in the next few years. Fisheries scientists believe this will expand the population of river herring in the Mystic system even further.

Horn Pond will also likely be a place where the general public will be able to view this migration directly. “Woburn has proposed expansive improvements to the park area to integrate the ladder and viewing opportunities for the public,” says DMF’s Ben Gahagan. “I think all parties see public engagement as integral to long term success.”


Will the Mystic herring run reach a million fish? Will the Mystic continue to lead the state in herring populations? Stay tuned for more data from a remarkable urban wildlife migration.

In the meantime, keep an eye out for volunteer opportunities to participate in our in-person and video counting programs. Registration to be a herring monitor for the 2023 season will open in February/March.

And finally, this can never be said enough: all the data we have about the river herring population on the Mystic River we have because of volunteer community scientists. This is public knowledge generated by residents of the watershed. Thank you, all.

Figure 1. Mystic Herring Data 2012 to 2022

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