Sea-Run Fish of the Mousam and Kennebunk Rivers

For millennia, the Mousam and Kennebunk River watersheds were home to seasonal migrations of native diadromous (sea-run) fish.  Every spring and summer, large numbers of Atlantic salmon, American shad, alewife, blueback herring, American eel, sea lamprey, sea-run brook trout and other species migrated into these two rivers. Species like Atlantic salmon and blueback herring would spawn in the riffles and faster moving water of the rivers, while alewives would travel into the watersheds’ lakes and ponds to reproduce.  Each of these species was incredibly important to the overall ecology of the watershed, bringing important nutrients into the freshwater environment and serving as a food source for other fish, birds, and wildlife. These fish were also important economic and cultural resources for the human populations in the watershed, providing both sustenance and extremely valuable commercial fisheries.


Unfortunately, over time, these once great fish runs were decimated by dams, pollution, and over-harvesting.  Today, we are fortunate that species like American shad, American eel, alewives and blueback herring still persist in our rivers, but they have been relegated to a very small amount of spawning and rearing habitat and occur at a fraction of their historic abundance.


The restoration of these sea-run fish species faces many obstacles, including:


  1. Absence of fish passage at multiple dams on the Mousam River (which has 15 dams) and Kennebunk River (which has 4).  While the Mousam has a much larger watershed (122 sq. mi.) than the Kennebunk (38 sq. mi.) and thus greater habitat potential, the health of both rivers can be dramatically improved.  In recent years, removal of dams and/or installation of fish passage have revitalized many rivers throughout Maine.

  2. Low minimum flow requirements at many dams, which degrade and alter downstream riverine habitat for aquatic insects, mussels and fish species.

  3. Urban sprawl and development contribute harmful run-off and non-point source pollution to our waterways. Reestablishing better riparian buffer zones would help to improve water quality.

  4. Presence of introduced non-native fish species, like northern pike and smallmouth bass, compete with and prey on both native freshwater and diadromous fish species.


While there are many obstacles facing these species, there is also great potential to rebuild many of these fish populations in our two local watersheds.  By removing dams or providing adequate fish passage to allow fish to migrate to their spawning and rearing habitat, protecting riparian habitat, creating adequate buffer zones between the rivers and agriculture and human development, and fixing non-point source pollution sites, we can provide a great boost to the recovery of these species and provide great ecological, economic, and recreational benefits to our communities.



What Exactly is a Diadromous Fish?


Diadromous fish are species that spend part of their lives in both freshwater and saltwater.  Diadromous fish are broken down into two categories: anadromous and catadromous.  Anadromous fish are spawned in freshwater rivers, streams, lakes and ponds and then migrate to the estuaries and oceans to grow into adults. After spending anywhere from 2 to 6 years at sea, these fish then return to freshwater to spawn, often in the very same body of water in which they were spawned!  Catadromous fish have the exact opposite life-cycle: they are spawned in the marine environment and then enter freshwater to mature to adulthood before returning to the sea to reproduce.


Maine is home to one species of catadromous fish (the American eel) and eleven species of anadromous fish: Atlantic salmon, American shad, alewives, blueback herring, striped bass, rainbow smelt, tomcod, sea lamprey, sea run brook trout, Atlantic sturgeon and shortnose sturgeon.  Both Atlantic salmon and shortnose sturgeon are federally protected endangered species, and the National Marine Fisheries Service is currently considering ESA protection for Atlantic sturgeon, American eel, alewives and bluebacks. American shad only occur in a handful of Maine’s rivers and lower numbers of alewives, blueback herring, and American eel have raised concerns among fisheries managers in recent years.  Each of these species occupies an important niche in Maine’s freshwater ecosystems and each serves several important ecological roles within these systems.  Their loss from our rivers, streams, lakes, and ponds has wide-reaching implications, not just for the freshwater environment but also the estuarine and marine environments.


Consider the alewife.  For millennia, alewives have been central to the web of life in Maine. They have served as the foundation for healthier, more diverse ecosystems by providing vital nutrients and forage throughout every stage of their lives.  Every May and June, tens of millions of adult alewives would return to Maine’s rivers on their journey to their spawning grounds in our inland lakes and ponds.  With them came a massive influx of nutrients from the marine ecosystem. In the summer and fall, both the surviving adults and the new generation of juvenile alewives make their journey downriver through our estuaries and into our bays and the Gulf of Maine. 


For the next four years, juveniles will stay in the Gulf, traveling as far as 120 miles offshore.  On their migrations and during their stays in the freshwater and marine environments, alewives provided a critical food source for a diverse array of species ranging from birds like heron, eagles, osprey, kingfishers, gulls, and cormorants to fur-bearing mammals like mink, otter, fishers, raccoon, and fox to other fish like striped bass, blue fish, trout, salmon, cod and haddock.  From bald eagles to seals, whales to ground-fish, just about everything eats alewives!


Take a look at this excerpt, featuring Nate Gray from Maine DMR: He xplains why River Herring are so important to our oean and river ecosystem.