August 11, 2021 — The following was released by NOAA Fisheries:
The Merrimack River watershed is the fourth largest in New England, and historically supported great runs of migratory fish. Over the past 200 years, their numbers have dramatically declined, negatively impacting the ecology of the watershed and coastal waters. Along with our partners, NOAA Fisheries has developed a comprehensive management plan for the watershed aimed at restoring these important fish, and the habitats on which they rely.
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)
- American eel
- Striped bass
- Atlantic salmon
- Sea lamprey
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.