How has the river herring migration changed over time?

The Mystic River Watershed Association has been counting the number of river herring each year since 2012β€”in 2018 an estimated 589,000 herring migrated up the river! How has the total number of herring changed from year to year? Are there any differences in the herring run between the years?

What causes the river herring to start migrating each year?

Each spring, river herring begin their migration up the Mystic River. But what causes them to start migrating? Why do they start earlier or later in some years relative to others?