Home / Blogs / The American Eel: International Fish of Mystery
January 12, 2021
An American eel (Anguilla rostrata) captured for tagging in Union County, PA. (Photo by Steve Droter/Chesapeake Bay Program).
The American Eel (Anguilla rostrata) is a fish that has fascinated me since childhood. It began at an early age while fishing at my family’s cabin. We were night-fishing in a small stream for bullhead catfish when I hooked into something that was obviously too large to be a bullhead. By the time I hauled the fish to shore, we were shocked to discover that it was (a very large) American eel. My family had been fishing this creek for generations, and no one was aware that such a large, mysterious creature inhabited this small waterway. I remember being in awe of a fish that looked so different (and almost foreign) compared to the bass and pickerel I was accustomed to catching. At the time, we had no understanding of this creature, and quickly released it in an effort to avoid its jaws and the profuse slime that covered its body. This chance encounter would lead me to pursue a better understanding of the eel’s life history and ecology. As a result, I would come to realize that my small creek (and the eels that inhabit it) are part of a much larger, interconnected global ecosystem.
The life history of freshwater eels had long been a mystery to science. For centuries, philosophers and scientists in Europe could not explain the occurrence of eels within their freshwater habitats. Eels were found in many European waterways, including ephemeral (temporary) waters and landlocked impoundments. But these eels were never observed spawning, and the presence of reproductive structures could not be identified upon ‘closer inspection.’ Notable figures like Aristotle and Sigmund Freud pondered the ‘eel question,’ and others developed some interesting theories as to how eels reproduce. If eels living in these freshwater habitats have no reproductive structures, how were they reproducing and where did they come from? The answer is actually much more complex than most of these earlier researchers could likely imagine.
An American eel (Anguilla rostrata) captured for tagging in Union County, PA (Photo by Steve Droter/Chesapeake Bay Program).
Thanks in part to Danish researcher Johannes Schmidt, we now know that all European eels (Anguilla anguilla) and American eels (Anguilla rostrata) originate in a region of the northwest Atlantic Ocean known as the Sargasso Sea. In the early 20th century, Schmidt set off to search the ocean for juvenile eels, mapping locations of smaller and smaller larvae, until after almost twenty years he was able to trace them back to their point of origin in the Sargasso Sea. The Sargasso ‘Sea’ is a region of the Atlantic that is not bordered by land masses. It lies between ocean currents that form what is known as a ‘gyre.’ Gyres are circular ocean currents that are influenced by wind patterns and the rotation of the earth. The name ‘Sargasso’ is derived from descriptions by early explorers of the brown Sargassum seaweed commonly found throughout the region.
It is within the Sargasso Sea that all adult American (and European) eels congregate to spawn. Eels are known as a catadromous fish, which means they live their adult lives in freshwater habitats before returning to a marine environment to reproduce. The eels then begin a complex life cycle that includes multiple physical metamorphoses and a round-trip migration of thousands of miles. After hatching in the Sargasso, eels emerge as a tiny, laterally compressed, transparent larvae. This type of larvae is known as Leptocephalus (meaning ‘slim-headed’) a common condition in juvenile marine eels. These larvae then begin a long journey following ocean currents toward freshwater habitats. (Both American and European eels are born in the Sargasso Sea, but it is still unclear how their respective larvae know which way to go). As eels begin to reach the shores of their destination, they morph into a form known as a ‘glass eel.’ The eels are still transparent, just slightly larger. As they gain pigmentation, they are sometimes called ‘elvers.’ The eels then enter freshwater habitats along the coast, morphing again into a more familiar form, known as a ‘yellow eel.’ During this phase, their bodies become serpentine and muscular. They migrate hundreds or thousands of miles upstream until they find a home. During this journey the eels are capable of moving over land and persisting for extended periods of time within dry or drying habitats. It was this ability that led the eels I caught at my family cabin to arrive at their destination. They had the ability to circumvent a substantial waterfall that exists between my stream and a larger river downstream.
Photo of waterfall between author’s eel catch and larger downstream river (Photo by Jim Kauffman).
Adult eels then live out their lives as freshwater predators, feeding at night on a variety of prey items (in my experience, it was a dead minnow on a hook). Eels are capable of living within their freshwater homes for fifty or more years, but at some point in their lives they begin to migrate back downstream toward their place of birth in the Sargasso Sea. (What triggers them to reproduce is still not fully understood). The eel’s body transforms again, developing physical traits that are more conducive to a marine environment. It is only now that eels finally develop reproductive structures (which explains why early European scientists were unable to observe them). Eels in this final reproductive form are known as ‘silver eels.’ Once they reach the Sargasso Sea, the eels spawn and die, completing a compelling and mysterious journey that likely spanned decades.
American eels face numerous threats that have impacted population numbers throughout the Chesapeake Bay watershed and beyond. Since the 1970s, eel harvest numbers have declined globally, an indication that populations are likely decreasing. This can be attributed to a number of factors, including human development, commercial harvest, climate change, pollution, impediments to migration, and invasive species. Dams have altered the waterways that eels use to migrate between freshwater habitats and their spawning grounds in the Sargasso Sea, preventing passage and impacting reproduction. Invasive species like the flathead catfish (recently introduced into the Chesapeake Bay Watershed) may be impacting American eels through predation. Changes in climate that impact ocean temperatures and currents may impact migration and reproduction. Pollutants and effluent entering freshwater systems through runoff are degrading habitats used by adult eels. Microplastics have been identified in their breeding grounds within the Sargasso Sea, in addition to accumulations of trash and macroplastics (see North Atlantic Garbage Patch). With the species facing so many threats, it isn’t surprising that the International Union for Conservation of Nature (IUCN) has listed the American eel as ‘Endangered’ due to decreasing population trends and the decline of mature individuals.
Eels in the Chesapeake Bay
Historically, the American eel was found throughout the Chesapeake Bay Watershed. Eels would have moved freely through the bay, migrating between their birthplace in the Sargasso Sea and the many freshwater tributaries throughout the bay watershed. Eels were an important food source for Native Americans who constructed weirs within freshwater river systems to capture eels during migration. Some of their stone weirs are still visible today. The Swatara Creek in Pennsylvania, a tributary of the Susquehanna (and ultimately the Chesapeake) was named for a Susquehannock word, meaning ‘the place where we feed on eels.’ After European settlement, commercial eel fisheries developed in many areas, and millions of pounds of eels were taken each year. Eventually, dams were constructed on many river systems within the bay watershed that blocked the migration routes of eels. Eels still exist in many of these water systems, but their numbers have plummeted due to historic commercial fishing and the presence of dams. Fisheries managers are now working to restore these migration routes through the installation of fish passages. American eels, European eels, and Japanese eels are still used for table fare, particularly in traditional sushi dishes. And American eels are commonly used as bait by anglers in the Chesapeake, especially those fishing for ‘stripers’ (or if you’re south of Pennsylvania) ‘rockfish.
Staff from the Alliance’s PA office and Lancaster County Conservation District gather data on specimens caught while electrofishing for monitoring purposes. An American eel was captured here on the Octoraro Creek, which meets the Susquehanna River below the Conowingo Dam (Photo by Jenna Mackley).
An Interesting Relationship
The eastern elliptio (Elliptio complanata) is a common freshwater mussel found throughout the Chesapeake Bay Watershed. Mussels are bivalve mollusks, and are known as ‘filter feeders.’ To feed, the mussel siphons organic particles from the water through its gills, collecting bacteria, algae, and other microscopic organisms. Mussels generally require clean, cool waterways that are low in nutrients without excessive sedimentation. Eels also require unpolluted freshwater habitats with an abundance of prey, in addition to unobstructed waterways for migration. Historically, these habitats were found throughout the bay watershed, and both mussels and eels were common. But with the loss of riparian forests came a decline in water quality, and a decline in mussels and eels. I have spent many days snorkeling at my family cabin, observing eastern elliptio mussels buried in the stream sediment, with their siphons protruding just slightly from the shell. But it wasn’t until recently that I came to understand the complex life history of this mussel species, and its relationship with the American eel.
Eastern elliptio mussel tagged during a survey in West Virginia’s Cacapon River (Photo by Andy Meyer).
Eastern elliptios are still widespread, but were historically more abundant throughout the bay watershed. Recent mussel surveys in some Susquehanna River tributaries have revealed a lack of juvenile eastern elliptios, indicating a decline in reproductive success. Many adults continue to persist, with some being estimated to be 100 or more years old. So why the dramatic decline in juvenile mussels? Most mussels in North America rely on fish hosts to complete their reproductive cycle. Eastern elliptios are no exception. Male elliptios release sperm into the water, which are later taken up by females as they take in water. Eggs are fertilized and begin to develop inside the female mussel. The young mussels develop into larvae, called ‘glochidia,’ and are then released into the water. It is here that the glochidia attach to the gills of a host fish where they develop into miniature mussels. The mussels then release from the fish and find a home in the stream sediment, where they can live for decades. Eastern elliptios rely on only a handful of fish species to reproduce, with American eels as their primary host. While they can reproduce using other fish species, reproduction through eels as their host has been shown to be the most productive. With the presence of so many dams along the Susquehanna blocking their migration route, eel numbers in many tributaries have declined, leading to a corresponding decline in elliptio reproduction. To mitigate this decline, biologists and fisheries managers have begun releasing eels in waterways above these dams. This strategy, coupled with the development of fish passages appears to be having a somewhat positive effect on eel migration and elliptio reproduction.
The Future of Eels and Mussels in the Chesapeake
There is a very clear connection between our work to restore the Chesapeake Bay Watershed and the future of eels and mussels. The Alliance strives to improve waterways through the implementation of conservation practices that promote clean water and healthy ecosystems throughout the bay watershed. Our riparian buffer and reforestation initiatives protect these waterways through streambank stabilization, decreased sedimentation, trapping of nutrients, and filtering runoff. Tree canopies provide shade and create cooler water temperatures. Our agricultural team also works with farmers and producers to install Best Management Practices that prevent livestock from entering waterways and better manage manure. These practices combine to create a healthier ecosystem and brighter future for eels and mussels.
There was a time when every American eel living in the bay watershed traveled freely through a pristine Chesapeake Bay at some point in their lives. Although the journey is now more perilous, there’s hope for their future. A better understanding of their life history, coupled with dam removal/fish passage structures and habitat initiatives can only improve the future of this species.
My chance encounter with an eel so many years ago and the subsequent understanding of its complex life history would be just one of the many factors that influenced my decision to enter the field of natural resource management. As the Alliance’s new Pennsylvania Forest Projects Coordinator I now have the opportunity to promote and implement habitat initiatives that will benefit eels, mussels, and many other organisms throughout the bay watershed. It’s my hope to protect these resources so others can enjoy them the way I did, when I was just a kid fishing at my family’s cabin and collecting mussels in the creek.
Forests Projects Coordinator
(717) 517 8698
Forests for the Bay