How Plymouth is Blending Tradition and Technology to Restore its Herring Population  

Countless fish species rely on an interconnecting network of freshwater waterways to reach spawning and rearing sites. However, many face a challenging journey that involves navigating dams and other barriers.  

Plymouth, Massachusetts, is one of several towns across the state working to improve passage for local fish species. Their target is the Town Brook, a 1.5-mile stream connecting the freshwater pond Billington Sea to the Atlantic Ocean. Each spring, Alewife, blueback herring, American Eel, and rainbow smelt rely on this brook to migrate or “run” upstream from the ocean to the 285-acre pond to spawn.  

A Troubling Trend

Historically, six dams blocked the alewives’ path, leading to a slow and steady decrease in the herring population. In 2000, it was estimated that only a mere

20,000 herring ran upstream, down from 500,000 to 1 million at its peak.    

 Since the 17th century, Herring has been essential to Plymouth’s history, sustaining Indigenous communities and supporting early fisheries. They were so valued that towns once appointed officials to oversee the harvest and protect runs. 

Reopening Routes 

In 2002, Plymouth, in collaboration with the NOAA Restoration Center, began an ambitious restoration project. The initiative was to remove three dams, lower a fourth, and improve fish passage at two additional sites – including the historic Plimoth Grist Mill. 

The project took over two decades, and altogether, Plymouth has addressed six dams combined with work around wetland restoration, daylighting, land protection and stormwater improvements.  

Counting the Comeback 

To monitor progress, Plymouth installed underwater cameras in 2020 to track counts and assess each improvement. 

These cameras capture footage 24/7 during the 70-day herring migration season (April-June). The footage is then cut into 10-60-second clips and uploaded to a dedicated website where anyone can count the number of herrings that swim by.   

This past year, more than 175 hours of video footage was analyzed by a group of dedicated volunteers.  

“The commitment of our volunteers has been instrumental in tracking our recovery efforts. Their unwavering support has allowed us to validate each improvement,” added Nate Cristofori, Town of Plymouth National Resources Specialist.  

AI Joins the Count 

Despite the herculean effort of volunteers, Plymouth still had over three-quarters of their video feed that needed to be reviewed. To automate this labor-intensive, manual counting process, the town turned to HydroAI™ – Innovasea’s AI-powered fish counting solution.  

With HydroAI™, sites get around-the-clock automated real-time fish counting.  Plymouth was able to upload the previous year’s footage into the system, and in less than one hour, HydroAI™ had fully analyzed the remaining clips, a total of 175 hours of video.  

Now, Plymouth has a more complete picture of its efforts and how best to address future planning.  The town has used this data to validate its visual counters and gain greater insights into the brook’s ecosystem.  

Following the success of this project, Plymouth will install a HydroAI™ camera at the Plimoth Grist Mill site ahead of this year’s herring run. The system will track the success of an ongoing multi-million-dollar project to restore the natural flow of Town Brook for the first time since 1636.  It will also complement the town’s dedicated network of volunteers who will continue to monitor and support the fish counting efforts. 

“River restoration is an incredibly rewarding undertaking and working with Innovasea to document the positive impacts of that work has been a wonderful learning experience and a great way to share the results of that work with the public,” added David Gould, Plymouth’s Director of Energy and Environment. “Being able to provide incredibly accurate population numbers during the spring migration supports our goals to restore Town Brook to a thriving and vibrant diadromous fish run.” 

 Watch the video below to learn more about Innovasea’s HydroAI™ solution