Photo Credit: David Mussina
Arlington, MA—An estimated 378,000 river herring reached their breeding grounds at the Upper Mystic Lake in 2020, according to estimates based on data provided by thousands of volunteer observers. The Mystic herring run continues to be one of the largest in the state.
Each spring river herring migrate from the Atlantic to Boston Harbor and up the Mystic River, to the Mystic Lakes and beyond. Since a fish ladder at the Mystic Lakes Dam opened up the Upper Lake for spawning in 2012, the population of fish that use the Mystic River system jumped from 200,000 to an all-time high of 780,000 in 2019.
The estimated count for 2020 is dramatically lower, but this is not an isolated event. According to data from the Massachusetts Department of Marine Fisheries, herring counts in rivers across the state saw steep declines. Six rivers monitored by electronic counters recorded an average of a 51% decline in 2020 compared to 2019, including the other largest runs in the state on the Herring and Monument Rivers.
“Though it’s hard to watch the population of Mystic fish going down in a given year, given all people have done to help these animals, this statewide pattern is striking and interesting,” said Andy Hrycyna, Watershed Scientist at MyRWA. “The question is what caused it. State fisheries scientists are pointing to a drought.””
According to the MA Division of Marine Fisheries, the main hypothesis explaining low herring counts across the state this year has to do with a severe statewide drought that peaked in 2016. In many rivers across Massachusetts, fish passages over dams actually ran dry at the height of the summer. Even in systems where streams did not dry up entirely, the drought could have other negative effects on the juveniles born that year. igher water temperatures lead to more stress on river herring through increased energetic demands and lower oxygen levels.
“Fish may have gotten trapped, or parts of the lakes and rivers that usually form good habitat for spawning and growth may have been effectively taken off the table that year, leading to fewer juveniles being able to make it out to the ocean to return as adults,” says Hrycyna.
Historically, when river herring populations were larger, breeding populations included fish that were three to nine years old. Today, evidence shows that river herring population’s “age structure”--how many fish there are of different ages-- is severely truncated, making the population more sensitive to environmental pressures. One bad breeding season will tend to result in the loss of a third to half of the population that migrates upriver when the affected age group reaches maturity. If there were fewer three- and four-year-old fish this year because of the drought in 2016, the population seen migrating would go dramatically down for that reason alone. According to Ben Gahagan, a diadromous fish scientist at the Division of Marine Fisheries, “if you have one bad year, you can lose half your fish.” Scientists believe this is what happened on the Mystic and regionally.
Despite this dip, volunteers and residents should be assured that in general, the Mystic continues to be viewed as a success story by agencies and advocates from across the state. Since the construction of the fish ladder at the dam at Upper Mystic Lake, hundreds of acres of new habitat have been made available for herring to return to. According to Gahagan, “the fish have returned rapidly, and we’re even getting a lot of fish per acre of habitat as well.” And the population will likely bounce back from its drought-induced decline.
This habitat will soon get another boost, with the building of an improved fish passage at Horn Pond, thereby adding 100 acres of high quality breeding habitat that fish have not been able to reach for decades. The fish can reach the dam at Horn Pond right due to the construction of a fish ladder at Center Falls Dam in 2016, however most cannot then make it through the inadequate fish passage currently at Horn Pond.
“Despite the dip this year, MyRWA and our partners at DMF remain positive about the Mystic’s herring populations,” said Patrick Herron, Executive Director. “We will continue to push for improvements, as well as run our successful counting programs to document the run size. This year shows how important data is to monitoring the health of these populations over time.”
This numbers provided would not be possible without the robust herring monitoring programs run by MyRWA. During 2020, MyRWA engaged 85 volunteers at Horn Pond, and a whopping 5,500 volunteers who counted videos online using an underwater ‘fishcam’ through the Mystic Herring website: https://www.mysticherring.org/. MyRWA hopes to have the official count for Horn Pond this fall.
