What’s the Deal with Eels?

Under a velvety dark sky on the night of a new moon, an American eel (Anguilla rostrata) slithers from her watery home in Lake Champlain to embark on a fateful mission. To mate, she must venture on a journey that stretches north through the Richelieu River, down the broad St. Lawrence, into the cold sweep of the North Atlantic, and then thousands of miles south to the Sargasso Sea near Bermuda. All American eels, whether they grow in the rivers of Venezuela or the streams of Greenland, are spawned in this single birthplace. For those who dwell in Lake Champlain, their migration begins on the first cool nights of September. 

Unlike the more familiar anadromous fish, like Pacific salmon who swim upstream to spawn in freshwater, American eels are catadromous. Catadromous fish hatch in the ocean, migrate into freshwater to grow and mature, and return to the sea to spawn. During migration, eels face human-created barriers, particularly dams, that can harm or kill them. Eels change form with each life phase, so a fish passage that accommodates a juvenile eel swimming upstream may prove challenging for a mature eel swimming downstream. 

According to Dr. Rose Stuart, a post-doctoral student at the University of Vermont who researches eel biology, eels in Lake Champlain are typically at the “yellow eel” life stage, and they spend six to 30 years here.  Dr. Stuart’s recent research demonstrates that eels use habitat flexibly during this life stage, occupying environments from deep lake basins to small tributary streams. Eels are nocturnal; other studies show that their activity levels fluctuate with the lunar cycle, with increased movement and predation under cover of a moonless night. 

Another commonality of eels in Lake Champlain is their sex: all are female. Male eels remain near the coasts, not migrating as far, so only females make the trek to Lake Champlain. Why? “It’s a chicken and egg question that researchers don’t quite have the answer to,” says Dr. Stuart. Eels in the marine stage (the first few years of their life after hatching) are sex indeterminate with no clear reproductive organs—those come with the physiological changes eels make to prepare for freshwater. “[The question is] do male eels migrate shorter distances than females, or is the sex determined by how far the eel migrates?” 

That is not the only question associated with American eels. Little is known about how they mate in the Sargasso Sea. Also unresolved is how yellow eels know when to depart the lake to begin their migration. The exodus of American eels around the world happens annually, with staggered start times depending on the distance travelled. Eels residing in Lake Champlain leave earlier than eels whose freshwater homes are closer to their breeding grounds.  

“Once a yellow eel ‘decides’ it is ‘ready’ — we do not know the exact mechanism of what causes this, but it is likely related to size — ,”  says Dr. Stuart, “it begins to undergo a series of physiological changes to shift from the ‘yellow eel’ stage to the ‘silver eel.’” 

These changes amount to a remarkable metamorphosis. As they prepare for their journey, silver eels’ color shifts from brownish yellow to sleek grey, and their already large eyes grow even bigger. They reabsorb their digestive tract, because they don’t stop to eat on the trip. Their swim bladders expand to withstand deep-water pressure, and the females prepare for reproduction by taking the extreme measure of demineralizing their own ribs to divert nutrients towards egg production. A study of silver eels from Maine in 1994 found that egg numbers vary greatly with size of the female, ranging between 1.8 and 19.9 million eggs.  

Despite these adaptations, American eels’ migration is hindered by human activity, particularly dams. “In-stream structures, both hydro-electric and non-generating, create significant issues for recruitment and habitat accessibility, and hydro is particularly lethal for out migrating silver eels as the length of an eel makes them particularly prone to turbine strikes,” says Dr. Stuart. “The two dams on the Richelieu River both have eel ladders to allow upstream migration, and they are not hydro structures, so these concerns are lesser in Lake Champlain, but [since American eels are so widely distributed], dams present throughout the whole range (of which there are numerous) are certainly a concern for long term survival of the species as a whole.”  

Eels are also threatened by climate change, pollution, and illegal fishing, and their population has declined approximately 50% over the last 36 years. Despite this sobering statistic, Dr. Stuart points out that eels’ persistence and adaptability provides a buffer in their competition with other species. “You can find [yellow eels] in the shallows and in the deep parts of the lake, and as generalists, they act as anchors for the entire ecosystem.” Eels eat fish, aquatic insects, snails, crayfish, and occasionally mollusks. In areas where invasive round goby are present, eels will readily consume them, which may provide some natural population control.  

We share a lake with creatures whose lives depend on voyages that are difficult to fathom and shrouded in mystery. Ensuring safe passage for eels through dam removal, fish ladders, and other conservation efforts is essential for protection of the species as well as for the freshwater ecosystem of which these enigmatic, slippery creatures are a part.