The official final estimate of the number of river herring migrating up the Mystic River is here from the Division of Marine Fisheries: 589,924 +/- 74,087. This is the second highest estimated run ever recorded on the Mystic River, and the second year in a row above 500,000 fish.

This is more evidence of what we believe is a great urban ecological success story.

A fish ladder was installed in the Upper Mystic Lakes Dam in 2012. Its purpose was to increase the freshwater breeding area with hopes of increasing the population. We started counting fish that first year and have continued every year since.

If the ladder did have an impact on the population by expanding breeding habitat, when would we expect to see the signal? We would not expect to see a jump in the population right away. Juveniles go out to the ocean at the end of their first season and first return to spawn to the river they were born in at sexual maturity, age 3 or 4. So we would expect to see evidence of an expanded population not in the first three years, but in the fourth.

Right on schedule, we see a spike in 2015 in the population of adult fish migrating annually to the Mystic River. And then the number climbed and has stayed high.

This is very strong evidence that a single fish ladder caused the Mystic population of river herring to more than double, making it in 2017 the largest in the state, despite the Mystic being the most urbanized river in the state.

The formula seems to be pretty simple in this case: more breeding habitat means more fish. Removing dam impediments can increase migratory fish populations.

(For more on why this is estimated number is different from the video model estimate see our other blog post on that topic.)