Multispecies Response to Enhanced River Connectivity in the Mill River, Taunton, Massachusetts.

Berkman, Stephanie¹, Bradford C. Chase¹, Sara M. Turner², Michael Bednarski³, Trevor Burns⁴, Mike Trainor¹, ¹MA DMF, New Bedford, MA, ²NOAA Fisheries, GARFO, Gloucester, MA, ³VA DGIF, Henrico, VA, ⁴MA DOT, Boston, MA

The Mill River, Massachusetts, underwent extensive restorative efforts, including three dam removals in 2012, 2013, and 2018 and installation of a fishway to Lake Sabbatia in 2012. In order to evaluate the effects of restoration on migratory fish in the Mill River, this study quantified population-level responses of river herring to restored passage into Lake Sabbatia, assessed pre- and post-restoration information on occurrence and abundance of American eels in Lake Sabbatia, and described sea lamprey use of restored habitat for spawning activities. Underwater video was used to monitor river herring and other diadromous and resident fish species during and after restoration. The river herring run sharply increased after removal of the last dam, with estimated run counts increasing from 900 in 2013 to 31,789 in 2021. American eels have been monitored in Lake Sabbatia since 2013 through an eel ramp and a mark-recapture survey of yellow-phase eels. Ramp passage results indicated high annual variability in number of young-of-year and age 1+ eels over the time series. Population models from the mark-recapture survey suggested that abundance of yellow-phase eels increased with the removal of downstream barriers, although occurring at relatively low densities that may reflect habitat limitation. Lamprey redd surveys, conducted to assess use of newly accessible habitat by spawning lamprey, found that redd counts varied annually, ranging from 33 in 2014 to 10 in 2022. Results from this study indicate that migratory fish respond rapidly and positively to both dam removal and enhanced fishway passage with recolonization of previously inaccessible habitats.