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Science Update

Of Fish and Dams: Restoring Connections Between Fresh and Salt Water 

Jane Tucker

The plight of diadromous fishes – those that spend part of their lives in the ocean and part in freshwater – is well-documented in coastal watersheds of the Gulf of Maine, North America, and globally.  Population declines have been linked to habitat loss and degradation, pollution of waterways by agriculture and industry, climate change, and critically, by blockage of river passageways by dams, culverts, and other barriers.   

Recognizing the ecological, cultural, and economic importance of these fish populations, many efforts are underway to restore them – which means restoring the rivers that support the seasonal migrations necessary to their life cycles. 

This article aims to provide an introduction to the issue of diadromous fish and dams, and provide links to more detailed and nuanced resources than can be summarized here. 

Diadromous fish fall into two categories:

anadromous- those that live most of their lives in the ocean but migrate to rivers, ponds, or lakes to spawn, and

catadromous  -those that live mostly in freshwater habitats but spawn in the ocean. 

Several diadromous fish species have long been important to the Gulf of Maine region.  In the anadromous category are river herring, American shad, Atlantic salmon, striped bass, American and shortnose sturgeon, sea lamprey, and rainbow smelt.  There is one catadromous species, the American eel.  These fish are critical components of food webs, and as a traveling food source, they link the coastal ocean to watersheds.  River herring, an inclusive term for blueback herring and alewives, are known by the Passamaquoddy of Maine as “the fish that feeds all” and are likewise considered keystone species by ecologists studying Gulf of Maine food webs. They are eaten by a variety of predators all along their migratory pathways: by larger fish such as cod, striped bass and tuna, by birds like egrets, herons, ospreys, and eagles, by mammals like racoons, otters, and bears, and by marine mammals like seals, porpoises, and whales.  When they die, they are fed upon by decomposers like microbes and many invertebrates, which recycles nutrients from the sea back into the watersheds.   The significance of river herring to Indigenous culture and as bellweathers of riverine health are highlighted in the film Swimming Upstream, which focuses on dam removal as a fundamental tool for river restoration and achieving Indigenous environmental justice.

Diadromous fish have also been a critical resource for humans.  For thousands of years, Indigenous Peoples throughout the Gulf of Maine watershed depended on their abundance to supply a large portion of the protein in their diets, fishing according to the time of annual migration for each.   In early spring, nets and weirs were used to fish for smelt, the first fish of the season, followed by river herring.  Late spring brought the prized Atlantic salmon, or redfish, which were generally speared.  In the fall, eels migrating downstream were trapped. Eaten fresh and smoked and dried for winter stores, fish were a mainstay of Indigenous diets.


Guided by traditional knowledge as well as respect and gratitude for all components of the ecosystem to which they belonged, the People were aware of the seasonal cycles of high and low abundance of fish, and the fishes’ dependence on healthy and flowing rivers.  In keeping with a principle of reciprocity, they were careful not to overfish, protecting the populations of both fish and people into the future.

In contrast, when Europeans witnessed the migrations, they reported on the amazing abundance of fish, there to be taken, and mistakenly considered the supply to be limitless.  As colonization progressed, fishing pressure increased to serve European markets.  At the same time, riverbanks became settled and cleared for agriculture and lumber, and rivers began to be dammed to provide power for grain and lumber mills, and later for power generation. 

Fish populations began to decline but remained abundant enough to support a profitable fishing economy along the GOM coast until the combination of barriers, overfishing, habitat degradation, and climate change led to dramatic losses by the end of the 20th century.   A 2009 assessment of diadromous fish populations of the western North Atlantic found an over 90% decline in the anadromous species discussed in this article and an over 70% decline in American eels (1). 

Almost 300 years before this assessment, on the Presumpscot River in Maine, the Wabenaki Peoples and their leader, Polin, understood the consequences of dams that had been constructed in the river. In 1729, Polin traveled to Boston, the seat of the Massachusetts Bay colony, to seek aid in protecting the river passageway for fish they depended on: “ It is the Presumpscot [River], which is barred up, and the fish is therefore barred up, which is our food.” (2), and asking only that passageway for the fish be provided during the time of their migrations.  This became the first dam protest in colonial America (Brooks and Brooks, 2010).  Indigenous tribes continue to lead many dam removal and river restoration efforts, including that led by the Cowasuck Band of Pennacook-Abenaki People that won approval for removal of the Mill Dam on the Oyster River in 2022 (after the release of  Swimming Upstream).

By the 1900s thousands of dams of varying types and sizes had been constructed throughout the GOM watershed, ranging from road crossings and culverts to mill dams to very large dams at hydroelectric power plants.  In addition to being barriers to upstream passage to breeding habitats, the large dams in particular pose challenges for fish on downstream journeys, where injury or death may be caused by entrainment and passage through turbines, hazards well-documented for eels (3). Dams also alter river flow characteristics that fish (and other organisms) are adapted to and may degrade water quality by accumulating sediments and pollutants in impounded waters.  

In an assessment of over ~14,000 dams (about half of estimated total) in New England states, The Nature Conservancy found that the removal of many of the dams on GOM rivers had very high potential for benefits to diadromous fish (see map).  Two removals in Maine have set examples for others in the region as well as nationwide, but may still be seen as works-in-progress.   


Kennebec River Diadromous Fish Restoration

As reported by American Rivers, the removal of the Edwards Dam from the Kennebec River in 1999 was the first removal ordered by the Federal Energy Regulatory Commission, citing that the costs of keeping the dam outweighed the cost of removal, and marking the first time the FERC had denied a hydropower license renewal for environmental reasons.  This order changed the thinking of regulators and communities about the value of maintaining outdated and often deteriorating structures; over 75% of dam removals in the U.S. have happened since (4).  However, it was only about 30 years after the Edwards Dam was constructed in 1837 that Maine fisheries commissioners had concluded that impassable dams, overfishing, and pollution were the causes of anadromous fish declines in the state, and yet it was over 100 years later that that a settlement was reached between the state and the Kennebec Hydro Developers Group with a schedule for creation of fish passage and restoration (1986 Agreement).  During this time, the cumulative effects of the Edwards Dam at head of tide plus multiple other dams upstream had restricted lake and river/stream habitat in the watershed to less than 5% and 7%, respectively, of what had been historically accessible (5,6). 


Removal of the Edwards Dam, followed by the Fort Halifax Dam in 2008 on the lower Sebasticook River (eastern tributary watershed of the Kennebec), together with other fishway improvements, restored access to about 40% of river and 46% of lake habitat in the Kennebec watershed.  In relatively rare long-term data on post-restoration effects, Wipplehauser (6) shows that diadromous fish recolonized habitat quickly, but the level of response depended on species. River herring in particular responded strongly, with average fish counts increasing by 1,425% after removal of both dams (2008).  American shad also responded quickly, but because shad are not able to use most technical fishways effectively, their response was limited to free-flowing segments.  Striped bass and both sturgeon species now have access to historical habitat and abundance of American eel has likely increased.  These results show the effectiveness of dam removal and provide support for continued efforts to restore access to the remaining historical habitat of the Kennebec, as well as to other watersheds.


Penobscot River Restoration Project

In a precedent-setting example accomplished by a collaboration of multiple stakeholders, including the Penobscot Indian Nation, the dam owners, regulators, and NGOs, a project on the Penobscot River, ME, removed the two lowermost dams on the river, the Veazie and the Great Works Dams, constructed a natural bypass around a third and a fish lift at a fourth.  When completed in 2015, it opened thousands of miles of spawning habit to fish for the first time in 200 years. Fish counts of river herring went from hundreds to thousands before the project to over 2.8 million during the 2018 spring run.  American shad passing via the fish lift at the Milford dam, now the lowermost dam, numbered nearly 12,000 in 2021, continuing an increasing trend since restoration (~1000 before 2004; 7) (potential estimated to be 1.5 million).  Atlantic salmon and sturgeon are also present again in reopened segments of their historical habitat.   By boosting power production at remaining and nearby dams, power generation remained constant.


Considerable progress has been made in reopening miles of rivers and streams to fish passage.  In one assessment, NOAA reports:  “Across the Northeast region, the NOAA Restoration Center has supported 302 fish passage projects since 1991. Together, these projects have opened more than 2,100 miles of river and stream habitat to access by fish. This work has included removing 141 dams, installing 105 fish passage structures, and removing or modifying 38 culverts”.

However, this represents only a small fraction of total existing dams, many of which are no longer needed for their original purpose, are in disrepair, and could be removed. According to American Rivers, in the three states that border the Gulf of Maine (ME, NH, MA), 167 dams (of all sizes) were removed between 1988 and 2021. This is only about 2% of all existing dams in those state’s records (8).  Even in large and successful projects like the Kennebec, remaining dams block passage to large areas of habitat.  Fishways add to migration time for many species as they search for entry to them, may cause injury during passage (lifts), and are not passable by all.  

In addition, restoration efforts have often targeted a particular species, often salmon or herring, with less attention being paid to the life histories and needs of other diadromous species that play equally important roles in the ecosystems and foodwebs all along the migratory pathways – from the ocean, through estuaries, up rivers and streams to lakes and ponds.

We need to know more about the life stages of all these fish, their habitat requirements during each stage, how they interact with each other and with resident species, and with their environment.  As documented by Tim Briggs in Downriver and out to Sea, there is much left to learn.

One “forgotten” fish is the rainbow smelt, or “frost fish”, whose abundance during early spring migrations supported many other animals in the ecosystem as well as being a favorite for human consumption.  Declining numbers of this small fish led to its listing as a federal Species of Concern in 2004.  Featured in “Downriver and Out to Sea”, rainbow smelt are now being studied by a collaboration of scientists and agencies in GOM states in a regional conservation effort.   The loss of these fish also inspired the silk painting “ Ghost Fish” by Susan Quateman, featured as our cover image.

Conservation of American eel would also benefit from more complete knowledge of its full life history and consideration of its migration requirements.  Many adults are injured or killed during downstream fall migration through turbines, for example, and their upstream migration may be blocked by fishways designed for other species.   Although the fall migration downstream is well known, and the presence of larval eels in the Sargasso Sea was discovered about a century ago, the long ocean leg of the migration and spawning behavior once in the breeding area had not been documented. It was only in the last decade that an Atlantic eel adult was tracked to near the breeding area (9), and only very recently (October 2022) that an adult eel (European) was documented in the Sargasso Sea (10).  The spawning step remains a mystery.

Luckily, the plight of diadromous fishes has been recognized and is receiving attention from many Federal and Tribal management entities and conservation groups.   Engagement by communities and individuals can also help:

·       Repair/replace/enlarge culverts

·       Maintain and improve fishways

·       volunteer to help conduct fish counts during migrations,

·       help prevent pollution of rivers and streams by avoiding runoff into street drains:

minimize fertilizer and pesticide use on lawns

pick up after your pets

avoid washing cars on impervious surfaces

·  help in community efforts to clean rivers and streams 


The more we all recognize the cultural, economic, and cross-ecosystem importance of the diadromous fish community, the better our conservation efforts can be, and the better we can address the problem in a holistic way.   It’s not about a single fish; it's about restoring a network of species and their interactions that contribute to the health of complex and interconnected river and coastal ecosystems.



1.     Limburg, K. E., & Waldman, J. R. (2009). Dramatic declines in North Atlantic diadromous fishes. BioScience, 59(11), 955-965.

2.     Brooks, L. T., & Brooks, C. (2010). The reciprocity principle and traditional ecological knowledge: understanding the significance of indigenous protest on the Presumpscot River. International Journal of Critical Indigenous Studies, 3(2), 11-28.

3.     Mensinger, M. A., Blomberg, E. J., & Zydlewski, J. D. (2021). The consequences of dam passage for downstream-migrating American eel in the Penobscot River, Maine. Canadian Journal of Fisheries and Aquatic Sciences, 78(8), 1181-1192

4.     American Rivers (2022). Free Rivers: the state of dam removals in the United States.  Final Report, pp. 22 content/uploads/2022/02/DamList2021_Report_02172022_FINAL3.pdf

5.     Hall, C. J., Jordaan, A., & Frisk, M. G. (2011). The historic influence of dams on diadromous fish habitat with a focus on river herring and hydrologic longitudinal connectivity. Landscape Ecology, 26(1), 95-107.  DOI: 10.1007/s10980-010-9539-1

6.     Wippelhauser, G. (2021). Recovery of diadromous fishes: A Kennebec River case study. Transactions of the American Fisheries Society, 150(3), 277-290

7.     Clarke, George Aponte Aponte,(2016) "Penobscot I: Looking After the Leap: Reflections on the Penobscot River Restoration Project"   International Conference on Engineering and Ecohydrology for Fish Passage. 21.

8.     Magilligan, F. J., Graber, B. E., Nislow, K. H., Chipman, J. W., Sneddon, C. S., & Fox, C. A. (2016). River restoration by dam removal: Enhancing connectivity at watershed scalesRiver restoration by dam removal. Elementa: Science of the Anthropocene, 4.

9.     Béguer-Pon, M., Castonguay, M., Shan, S. et al. (2015) Direct observations of American eels migrating across the continental shelf to the Sargasso Sea. Nat Commun 6, 8705.

10.  Wright, R.M., Piper, A.T., Aarestrup, K. et al. (2022) First direct evidence of adult European eels migrating to their breeding place in the Sargasso Sea. Sci Rep 12, 15362.


Additional Resources


Edwards Dam and Kennebec River Restoration:

Wabanaki perspective on Kennebec dam removal:

Penobscot River Restoration Project:

Restoring the Penobscot River:

Anadromous Fish:

What are anadromous fish? In “Constraints on River Restoration Potenial”, U. of Southern Maine, includes short description and image of alewife, blueback herring, American shad, Atlantic salmon, rainbow smelt, Atlantic tomcod, sea lamprey, shortnose sturgeon, Atlantic sturgeon, and striped bass:


River Herring:

River herring life history information (including differences in alewife and blueback herring), reasons for decline, and how to help count using video images from the Nequasset Dam, Kennebec R.

In Maine River Herring Runs:

River Herring Info:

The River Herring Decline:

Help with the count:


Nequasset lake fish ladder and counts

Detailed review of river herring science includes life cycle diagram:

Hare, J. A. et al. (2021) A review of river herring science in support of species conservation and ecosystem restoration.  Marine and Coastal Fisheries: Dynamics, management, and Ecosystem Science 13:627-664.  Detailed review includes life cycle diagram.


American Shad:

Broad overview from New Hampshire Fish and Game:



Atlantic salmon:

NOAA overview and species spotlight with good video

Penobscot River salmon increasing:

Rainbow smelt:

Rainbow smelt: an imperiled fish in a changing world

(Maine Department of MarineResources, Massachusetts Division of Marine Fisheries, New Hampshire Depart-ment of Fish and Game, and National Oceanic and Atmospheric Administration)

Learn about rainbow smelt (

American Eel:

American Eels: Restoring a Vanishing Resource in the Gulf of Maine (Gulf of Maine Council on the Marine Environment)


The American Eel: International Fish of Mystery (Alliance for the [Chesapeake] Bay) gives life history of the eel and includes the interesting relationship with freshwater mussels




Jane Tucker

 Jane grew up on the coast of North Carolina, and holds a B.S. and M.S. in Marine Sciences from UNC-Chapel Hill. Now living on the Massachusetts coast, she works at the Marine Biological Laboratory’s Ecosystems Center in Woods Hole. She is part of a team of scientists that studiesthe ecology and biogeochemistry ofthe marshes,estuaries, and watersheds inthe Plum Island EcosystemsLongTerm Ecological Research program and at other coastal sites. Her dry-land interests include gardening and dressage.

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