No Simple Swim: The Mysterious Migration of the American Eel (Anguilla rostrata)
- Jill DeVito
- 1 day ago
- 10 min read
Updated: 10 hours ago
A Storybook Sleuth on a Slippery Trail
When I learned that 6,481 juvenile American eels had passed through the eel ramps at the Holyoke Dam fishway in 2025, I wanted to understand what that meant for the Connecticut River population. It seemed like a straightforward question, but the more I learned about eels, the more I started to feel like a storybook sleuth on a slippery trail. For starters, there are not many fishes more elusive than the American eel. You might very well have eels in a river near you, but they’re hiding under cover during the day and hunting at night, so you’re not likely to meet one. When I finally tracked one down at an aquarium exhibit, I looked into her huge blue eyes—and I found it really hard to look away.
This individual was a silver phase eel—an adult American eel at the final stage of its epic life cycle. The sinuous body of a silver eel has changed from yellow-brown to mostly gray. And its eyes have grown from ordinary size to enormous, in preparation for navigating the dark ocean depths ahead on its harrowing journey. If this one were swimming in a river instead of a tank, she would be on her way downstream toward the Sargasso Sea in search of the mysterious place where the life of every American eel begins (and the life of the hardiest and luckiest among them will end).
The Sargasso Sea is a two million square mile swath of the eastern North Atlantic Ocean bounded by ocean currents rather than land. Scientists have been unable to identify the exact spawning location of the American eel (or that of its close cousin, the European eel, which also spawns there) despite more than a century of searching the Sargasso Sea. What we do know is that both of these species are catadromous – that’s the opposite of the anadromous life cycle you may be familiar with in salmon, sea lamprey, and other migratory fishes that spawn in rivers and grow large in the ocean. Instead, catadromous species like the American eel spawn in marine systems and migrate to fresh water to feed and grow for the majority of their life.
When eel eggs hatch in the Sargasso Sea, the resulting larvae are tiny, transparent, and flat (think: flat like an angelfish rather than flat like a stingray). They are called leptocephali, and they drift on ocean currents and feed on the organic particles they encounter there, until finally their bodies take on a more familiar tubular, eel-like shape. These “glass eels” are still nearly transparent, but they’re large enough to swim toward their next destination. For the American eel, that could be an estuary or river mouth anywhere from Guyana to Greenland.
Glass and elver phase eels photos by Creative Commons.
When it reaches coastal habitat and begins to feed, it takes on a brownish body color; the now-earthworm-sized eel is called an elver. In places like the Connecticut River system, juvenile eels will make their way upstream in search of the slow-moving, soft-bottomed habitats where they’ll grow for years (or sometimes decades) as “yellow eels” before their transformation to the reproductively mature and migratory silver phase.
Onward and Upward (the Upstream Obstacle Course)
As you might imagine, eels face challenges to their survival at every turn along this vast migration route. If a larval eel survives running the gauntlet of predators and other hazards (such as climate change altering conditions and productivity) in the open ocean, it may gather with other glass eels on their way toward a river mouth. At this point, they are vulnerable to commercial harvesting for human consumption. Locally, the Atlantic States Marine Fisheries Commission has measured dramatic declines in glass eel stocks. As a result, they’ve banned fishing for this life stage in all New England states except for Maine (but even there, quotas have been reduced in recent years).
The eels that make it into freshwater as elvers won’t just be swimming against the current; they’ll find their path obstructed by dams of all sizes. These one-to-three-year-old juveniles are surprisingly good climbers, so they’ll crawl over smaller dams on warm, wet nights from spring through fall. Sooner or later, though, they’ll come to a substantial barrier like the Holyoke Dam where those 6,481 elvers were counted this year. So how do these very small fish find their way to the other side of a very big dam?
A few juvenile eels somehow wriggle around or climb over large dams without help; and they may swim through fish ladders and lifts designed for other species. But for elvers to pass upstream in significant numbers, a barrier must be fitted with ramps for eels to climb. The climbing ramps come in many designs, but in general imagine a Rube Goldberg style pegboard made of rubber and tilted like a ramp; the elvers wind their bodies around the pegs and slither over. From there they fall into a tank (the “trap”) where they can be counted before being released upstream of the dam.
Left: a ramp designed to help eels over barriers like dams. Center: a juvenile eel climbs a wetted ramp. Right: Alex Haro works with American eel in the lab (source: Alex Haro, US Geological Survey).
You can’t see these eel ramps as a visitor to the public viewing areas at the Holyoke fishway, so I asked an eel expert for help understanding them. Dr. Alex Haro is a scientist emeritus at the USGS Conte Anadromous Fish Research Laboratory in Turners Falls, Massachusetts. And he’s enthusiastic about sharing what he’s learned over the course of more than forty years spent studying his favorite “forgotten fish.” About twenty years ago, Alex was involved with the complicated process of trying to figure out how to pass elvers effectively at Holyoke. Finally, after almost a decade of improving the functionality of the ramps and figuring out the best places to put them, the facility passed up to 50,000 elvers in a single year.
Annual American eel ramp/trap counts reported by Holyoke Gas and Electric, at Holyoke Dam, for the period 2003-2024 (Ken Sprankle, US Fish and Wildlife Service Connecticut River Fish and Wildlife Conservation Office).
Fish biologists like Alex hoped the number of elvers passed at Holyoke would continue to trend upward over time, leading to a healthy population of yellow eels growing large in the northern portion of the Connecticut River system—and the potential for an increasing number of silvers to make their way back to the ocean. But the upstream passage counts over the past decade have fluctuated from year to year in a trend that has been generally downward. A low count year could result from local flooding events that force a temporary shutdown of the passage facility. Or there could be fewer elvers arriving at the dam due to environmental factors that affect larvae and glass eels in the ocean, before they enter the river. Even year to year variation in Atlantic Ocean currents can affect larval survival—or it could simply redirect larvae to other river systems across the Eastern Seaboard.
One thing Alex Haro is careful to explain is that the annual fish count at Holyoke only tells us how many eels passed the dam. We don’t know how many showed up below the dam trying to make it upstream in the first place because to determine that would require a “massive mark recapture study over five to ten years.” In science speak, massive translates to labor intensive and expensive. Finding funding and multi-year commitments for science research is generally difficult; it’s even harder when the study is massive and the subject is a forgotten fish. Until then, we just won’t know what proportion of the juvenile population is able to make it north of Holyoke, or the subsequent barriers at Turners Falls/Great Falls, Vernon, Bellows Falls, Wilder, and onwards. Historically, American eel were found all the way to the Connecticut Lakes in northern NH, but getting there now requires a much more challenging journey, often without dedicated eel ramps like those at Holyoke.
Double Jeopardy (the Downstream Obstacle Course)
Once they’ve taken up residence in the Connecticut River and its tributaries, yellow eels will spend roughly five to twenty years foraging and growing; sometimes reaching lengths of more than three feet. But their biggest challenge may still be ahead of them. Eels that have made it this far in life don’t face as many dangers as they did when they were young but they won’t contribute to the future population unless they transform into silver eels and find their way back to the Sargasso Sea.
To do this, first, they have to pass through every barrier they climbed as juveniles again; this time in the downstream direction. At Holyoke, Alex Haro explains, there are several routes they can take. A silver eel may spill over the dam (at Holyoke, that’s a thirty-foot drop). A plunge pool at the foot of the dam increases the chances that a fish will survive the fall. Or an eel may pass through the intake to the spinning turbines that generate power. Since this has the potential to result in injury or death, some facilities (like Holyoke) have reduced the spacing between bars intended to prevent large objects and animals from being sucked into the turbines. But some eels still manage to wriggle between the bars, putting them at risk. Finally, an eel that finds its way into the adjacent Holyoke power canal may be diverted through a special downstream fish bypass, away from the intakes of several smaller turbine units that draw water from the canal. From the bypass eels are transported via a pipe to the pool below the dam and released to continue their journey downstream.
The Hidden Life of a Forgotten Fish
Alex Haro explains that there’s no single program in place for collecting eels throughout the watershed to study how many there are (or how fast they grow, or even where they live). “Counting all of them or even developing an accurate habitat-wide estimate is virtually impossible because there’s so much habitat.” At present, eels occupy virtually all the freshwater habitat in Connecticut, the mainstem and larger tributaries in Massachusetts, and the mainstem and lower portions of the larger tributaries in Vermont and New Hampshire, a total area of about 4,000 to 6,000 square miles. As a result, he’s had to be creative about using the upstream elver count at Holyoke to predict how many yellow eels are swimming around the northern portion of the watershed, and how many of those will turn into departing silvers like the ones that can be sampled at the power canal filter.
He's produced a model that incorporates the Holyoke passage counts with survivorship information from other eel populations where scientists have documented how many elvers make it to adulthood. Using the model, Alex estimates there are 50,000 eels upstream of Holyoke. At 25 eels per square mile across 2,000 square miles of occupied habitat, that’s quite a low density for a fish that was once abundant. One thing that improves the accuracy of this population model is knowing at what age our local eels are coming and going at Holyoke. Alex has gleaned some of that information from the otoliths (ear bones) of elver and silver specimens collected at the fishway in past years.
But wait, how does a scientist figure out the age of an eel from ear bones? I recently visited the USFWS Connecticut River Fish and Wildlife Conservation Office to find out. Dr. Corey Eddy, a fish biologist, explained that his more recent collection of elver and silver specimens from Holyoke would beef up the data set Alex Haro is using in his model. “You know how you can cut a tree to see how old it is? We can do the same thing with otoliths.” Because, like trees, fish ear bones have annual growth rings.
First, however, the ear bones extracted from each specimen need to be prepared for viewing under a microscope—and this is a painstaking task. Fortunately, the project is able to move forward with the help of a local volunteer through Veterans Affairs named John McLaurin. John has taken over the processing of the otoliths; first he embeds them in epoxy resin, then he cuts a thin section to mount on a microscope slide. Finally, he adds a blue stain that makes the rings more visible. Then he’s ready to estimate the age that eel had reached when it was collected at the dam.
USFWS biologist Corey Eddy (left) and volunteer John McLaurin, Jr. (right) count microscopic growth rings on ear bones to estimate age in American eels (photos by Jill DeVito).
“We’re aging American eel,” Corey explains, “so we can begin to understand the relationship between age and size. We’re going to take that data and give it to Alex so he can put it into his population assessment model, which will give us an estimate of how many American eel there are above the Holyoke Dam.” This should help scientists use future annual counts at Holyoke to estimate the total eel population above the dam, so they can make more informed management decisions, including improving passage at upstream dams.
“Today there are substantially more barriers to American eel migration than in pre-colonial times," notes Kate Buckman, the Connecticut River Conservancy's Aquatic Ecologist. "There is dedicated upstream eel passage at Holyoke, but between Holyoke and the Connecticut Lakes there are ten hydroelectric dams on the mainstem, none of which have passage facilities designed for eels. The research being conducted helps us to better understand how many eels there are, where they are, and what they are doing currently. Yet impediments to habitat access will continue to negatively impact survival and reproduction, and prevent us from reaching population targets outlined in management goals. Working to require the installation of both upstream and downstream eel passage is a critical component of rebuilding the population of American eel in the Connecticut River and something that both CRC and the Connecticut River Migratory Fish Restoration Cooperative are actively addressing.”
I, for one, would love to see people care enough about the fate of the American eel to advocate for its conservation. But I’ve learned firsthand that inspiring affection for this forgotten fish can be a challenge. When I told a family member they might have eels in the small river that runs behind her backyard (fully expecting her to be delighted), her response was “eew.” As Alex Haro points out, “They’re slimy, they’re ugly, and nobody around here eats them, so why should we care about them?” American eels are a vital keystone species that enhance aquatic ecosystems, act as a crucial link in nutrient cycling between oceans and freshwaters, and provide food for predators such as birds and otters. “Appreciating eels isn’t hard once you know more about them,” says Alex. Perhaps, after learning more about the epic adventure that is the life of the American eel, you might agree?
This article was the third in a series related to documenting 2025 fish migration in the Connecticut River. You can also read the first article about sea lamprey and American shad, or the second about alewife and blueback herring.