Mystic River logoMore than 550,000 river herring passed through the fish ladder at Mystic Lakes Dam in Medford in 2021, official estimate from the Mass. Division of Marine Fisheries show.

This count, reported March 10 by the Arlington-based Mystic River Watershed Association, represents the latest chapter in an ecological restoration story in the heart of greater Boston.

This data also represents the commitment of hundreds of community science volunteers.

Figure 1. Mystic Herring Run, 2012-2021.In 2020, there was no in-person count because of Covid restrictions. Video monitoring data for that year is shown. Short lines on the bars indicate 95-percent confidence interval, or margin of error.

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, the watershed association explained.

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, an association news release said.

Fish ladder aided access

After 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 — the watershed association launched a volunteer herring count in collaboration with the Mass. Division of Marine Fisheries. Dozens of volunteers 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.

River herring first return to freshwater to reproduce 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 (see Figure 1, above)

And numbers continued to climb over the next few years to an estimate of as many as 780,000 fish in 2019.

A success story

This is a remarkable success story: the single intervention of a fish ladder at Mystic Lakes doubled — and even tripled — a significant wildlife population. For a sense of scale: 780,000 of these foot-long fish end-to-end would stretch 150 miles.

Figure 2. Horn Pond, Mystic Herring Run, 2018-2021.In-person monitoring was called off in 2020 because of Covid, but there is evidence from our (separate) video monitoring system and from around the state that river herring populations suffered declines from the effects of drought in 2016-2017, when conditions led to fewer juveniles surviving to maturity that year because of dried up streams, higher water temperatures, and other factors.

So, in 2020, river herring counts on rivers were dramatically down all across Massachusetts, and fisheries scientists attribute this to those drought conditions three years earlier.

The latest estimate from 2021 represents an apparent increase in the Mystic population from the previous year, and this is what those at the watershed association say they might expect. Recoveries take time. There is every reason to expect that the number may well recover to previous levels.

Another leap?

What would it take to make another leap in the population of fish that migrate up the Mystic River? The next big lake upstream — the next big tract of potential breeding habitat for river herring the Mystic system — is Horn Pond in Woburn.

In the past few years, volunteers at Horn Pond too have been counting the comparatively fewer fish that are able to enter that lake. There is ne now three years of data from Horn Pond. In addition, there are four years of video monitoring from Center Falls Dam in Winchester, where another fish ladder — made possible by the advocacy of residents and public funds — makes it possible for fish to pass through the center of Winchester and on toward Horn Pond (see Figure 2).

The graphs from Mystic Lakes, Center Falls and Horn Pond together tell a story of how river herring spread out across a landscape. Given the means, river herring will swim many miles inland seeking suitable (and uncrowded) habitat to spawn. But some stay back to use the breeding habitat downstream. So, we see more fish at Mystic Lakes than at Center Falls, and more fish passing at Center Falls than make it into Horn Pond.

A promise

The big promise of Horn Pond is the fish passage that will be designed and built in the next two to three years, funded by federal grants and the City of Woburn. This year there were dozens of reports of hundreds and even thousands of fish at the base of Scalley Dam at Horn Pond on many different days. The state has worked to make the current small bypass stream at the outlet of Horn Pond accessible to river herring, and some fish obviously find their way. But monitors know many more do not pass that point, and return downstream.

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. Fisheries scientists believe this will expand the population of river herring in the Mystic system even further.

Will the Mystic herring run reach a million fish? Will next year be the biggest year yet? Stay tuned for more data from a remarkable urban wildlife migration.

In the meantime, you can learn more about volunteer opportunities to participate in our in-person and video counting programs. Volunteers are sought to help monitor the herring migration in 2022: Details and registration here >>

March 8, 2022: 26th annual Mystic Herring Run

This news announcement was published Friday, March 11, 2022.