The Mystic River Watershed Association has been tracking the number of river herring migrating through the Upper Mystic Lake Dam every year since 2012. Each day during the migration season, a dozen volunteers count the number of fish passing through the fish ladder at the dam. Each volunteer counts for 10 consecutive minutes at a random time during the day. At the end of the season, we use statistics to estimate the total number of fish that migrated each day based on the counts from those random 10-minute periods.

This chart shows the total number of river herring that migrated on each day since 2012. Click and drag to zoom in to a specific year.

To see how the herring migration run has changed over the years, we can plot the number of fish by the day of the year. Each line shows the estimated daily count for a single year. Hover your mouse over the chart to see the specific values for each year.

In which year did the migration start the earliest? What was the largest number of herring that migrated in a single day, and in which year did that occur?

Another way to compare the yearly migrations is to plot the cumulative number of fish by day of the year. The value on each day is the total number that migrated from the beginning of the season until that day. The value at the end of each line indicates the total number of migrating fish for that year.

Which year had the largest total number of river herring? Which year had the least? Are there some years that are more similar than others?

Finally, we can compare the total number of herring that migrated each year. Because these total counts were estimated from the random 10-minute counts by our volunteers, we don't know exactly how many fish passed in each year. The error bars represent this uncertainty and show the most likely range for the total of each year.

Even with the uncertainty, this chart clearly shows that the total run nearly doubled from 2014 to 2015. This increase in run size is of special interest to MyRWA and state biologists who study river herring. Proceed to the next step for one possible explanation.

One hypothesis for the increase in run size from 2014 to 2015 is that it shows the return of adults who were born in 2012 when the new fish ladder was installed at the Upper Mystic Lake dam. The new ladder increased the amount of spawning habitat by 165 acres, allowing adult herring to migrate and spawn upstream of the dam for the first time in decades. The eggs that were laid in 2012 then hatched and became juvenile fish, which swam back to the ocean later that summer. Over the next 3 years, those young fish grew into adults and returned in 2015 to spawn in the same habitat where they were born.

What do you think will happen this year? Will we see more herring than last year? Will the population continue to grow over time? Or will it reach a new equilibrium as all of the habitat becomes utilized? What other factors may affect the run size each year?

The End!

Thanks for visiting and we hoped you enjoyed learning about how the river herring migration has changed over time based on our volunteer data from previous years.

This year, we are excited to be collecting data through both our volunteers counting in person at the dam plus folks like you counting the videos on this website. Check back next year to see what we learned and how this year's migration compare to previous years!

Want to explore more data? Continue on to the next story to learn about what causes the river herring to begin migrating each year!