The numbers are in! According to the official estimate from the Massachusetts Division of Marine Fisheries (DMF), more than 550,000 river herring passed through the fish ladder at the Mystic Lakes Dam in Medford in 2021. This count 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.
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.
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 Mystic River Watershed Association (MyRWA) launched a volunteer herring count in collaboration with DMF. 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 three or four, 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. Numbers continued to climb over the next few years to an estimate of as many as 780,000 fish in 2019. 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.
In-person monitoring was called off in 2020 because of the COVID-19 pandemic, but there is evidence from a 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 River population from the previous year, and this is what we might expect. Recoveries take time. There is every reason to expect that the number may well recover to previous levels.
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 in 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 currently enter that lake. There are now three years of data from Horn Pond. There are also 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.
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.
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. We know now that fish try to get into Horn Pond. 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. DMF 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 we know many more do not pass that point and return downstream.
Money from two federal environmental damages settlements – including the 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 MyRWA’s in-person and video counting programs. MyRWA is currently seeking volunteers to help monitor the herring migration in 2022.
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.