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Dams, Fish, and Why They Don’t Mix
Diadromous fish spend part of their life in freshwater, and part in saltwater. Prior to pervasive dam construction in the late 18th and early 19th century, they were abundant in the Merrimack River watershed. Species that were seasonally present in great numbers included:
American shad, river herring (alewife and blueback herring)
Sturgeon (Atlantic and shortnose)
The annual migrations of these fish were anticipated by local people and wildlife alike. Eagles, osprey, otters, and many other native species would rely on this seasonal abundance of fish for food. They often adapted their breeding season to coincide with the arrival of the vast schools of fish. In fact, diadromous fish are one of the few food sources that can travel inland from the ocean, against the flow of rivers and streams. When the young fish and surviving adults swim back to the ocean, they also play a crucial role in the marine food web as both predators and prey.
Dam construction began throughout the watershed shortly after the arrival of European settlers as a way to harness the power of the river. They reduced habitat connectivity and the abundance of diadromous fish. Widespread industry in the watershed, including many paper and textile mills, resulted in degraded water quality. This further exacerbated the effects of decreased habitat connectivity and lack of access for fish to their natal waters. These combined factors resulted in a severe reduction in fish abundance. It effectively removed diadromous fish from the habitats upstream of Essex Dam, the first dam on the Merrimack River. For Atlantic salmon, the habitat alteration and associated impacts proved too much, with 10 individuals or less returning to the river in each of the last five years. Atlantic salmon are now listed as endangered and are one of NOAA Fisheries’ Species in the Spotlight. Modern restoration efforts have resulted in some improvements; however, the abundance of diadromous species remains a small percentage of historical levels.
Over the past year, NOAA Fisheries has worked with partner agencies to develop the Merrimack River Watershed Comprehensive Plan for Diadromous Fishes, including:
U. S. Fish and Wildlife Service
Massachusetts Division of Fish and Wildlife
Massachusetts Division of Marine Fisheries
New Hampshire Fish and Game Department
The plan is primarily focused on the restoration of American shad, river herring, American eel, and sea lamprey, based on their historical presence, and the suitability of remaining and potential habitat in the watershed.
The purpose is to create a framework that balances diadromous fish restoration efforts with other water resource uses and ecosystem services in the Merrimack River watershed. The plan was designed to protect, conserve, and restore the habitat and natural resources of the Merrimack River. It supports NOAA Fisheries’ goals for the watershed.
With more than 3,000 barriers in the watershed, habitat connectivity remains the biggest challenge facing the restoration of diadromous fish. Most of these dams are relics that no longer serve their designed purpose. However, several of the large dams that originally provided power for mills have since been fitted with powerhouses containing modern turbines that generate clean energy. NOAA Fisheries and our partner agencies engage with dam owners to ensure effective fish passage is maintained at these facilities. The goal is to balance restoration of diadromous fish with the need for sustainable energy production.
Talbot Mills Dam (Concord River)
Pepperell Dam (Nashua River)
McLane & Goldman Dams (Souhegan River)
Kelley’s Falls Dam (Piscataquog River)
Hooksett & Garvin’s Falls Dams (Merrimack River)
With effective fish passage at these dams, it is estimated that spawning habitat for American shad and river herring could be nearly doubled. It would increase, from the current 4,200 surface acres to more than 7,200 acres. Providing access to more habitat results in increased fish production and will hopefully accelerate their recovery to historical abundance. While implementing fish passage at these dams will take time, successful engagement regarding these barriers will ensure a high return on investment of restoration resources.
There is much work to be done in order to restore diadromous fish populations to their former levels. With dedicated partners, and a comprehensive plan, NOAA Fisheries is poised to meet the challenges facing these important natural resources in the Merrimack River watershed.
To read the plan and learn more about ongoing restoration efforts in the watershed please visit the website
and search for Accession Number 20210617-5016.