Between a rock and a hard place
Creating upstream and downstream passage around dams helps American eels complete their challenging migration.
American eels go through a lot in a lifetime. As if it wasn’t difficult enough to migrate thousands of miles while morphing through different life stages, these champion migrators face challenges all along the way, when swimming both upstream and downstream. Unfortunately, populations of American eels are declining, partially due to habitat loss and migratory obstacles, like dams.
The U.S. Fish and Wildlife Service is working to stabilize populations of these fascinating fish through creative engineering solutions.
Give me a boost
Hydroelectric dams, designed to harness renewable energy, are not always fish-friendly. They’re often more than just roadblocks to migrating eels; they can be deadly. Our biologists and engineers work closely with dam operators to create safe passage for eels up, over, down, and around these structures.
Ever heard of an eel ladder? What about an eel airlift? Or a fish pipe? The names might sound funny, but these are important types of fish-passage structures.
When dams are built, and when hydroelectric dams are licensed and relicensed, our biologists work with dam owners and operators to prescribe the right kind of fish passage so as many migratory fish as possible — including eels — can pass safely, both upstream and downstream.
Mature eels migrating downstream are attracted to fast-flowing water to conserve energy while swimming. Unfortunately, at hydroelectric dams, the fastest flows are often through the dam’s turbines. When the dam is generating energy, eels can be sucked in and killed by the rotating blades.
Deterrents like trashracks keep fish away from danger. Designed to filter trash and other debris from flowing water, these metal bars allow water to pass into the dam’s powerhouse but can stop eels from swimming into the turbine. Biologists and engineers design the trashracks so the width and spacing of the bars allow the majority of mature eels migrating downstream to avoid an untimely end.
But the fish still need a way around the dam. One option is an engineered fish bypass system, which guides eels up over, or around, the dam through tube-systems.
When the system is used with trashracks, eels looking for an alternate way to pass the dam will swim towards the water flowing through the tubes and find a safer route, avoiding the turbines. The eels swim through the fish bypass system and come out on the downstream side of the dam. If there are more obstacles farther downstream, the fish emerge from the tubes into a facility to be transported beyond the blockages by truck or barge.
Creating a secondary flow next to the dam has the same effect. This strategy diverts some of the water flow around the dam and creates the same scenario as a fish bypass system by allowing eels to follow an alternate route.
What about the upstream journey? Despite being good climbers — thanks to the sticky mucous that covers their skin — getting upstream is still tough for the elvers. They can climb up and over rock faces and other barriers but are met eventually with their own personal Everest, in the form of a dam.
Here is where eel ladders come in!
Pretty much what they sound like, eel ladders, either manufactured or carved into rocks, are like steps or rungs of a ladder that elvers can traverse more easily than the vertical face of the dam. Because eels are attracted to flowing water and wet surfaces, these structures usually have water flowing over them.
Fin-novation
When none of these solutions does the trick, our biologists and engineers do what they do best; they innovate.
On the St. Lawrence River, a massive waterway in the Northeastern U.S. that crosses the border into Canada, two extraordinarily large hydroelectric dams about 50 miles apart harness nearly the entire flow of the river to generate energy. Together, the two facilities produce enough electricity every second to power about 20,000 homes for a month.
With dams of such caliber, typical fish-passage structures just won’t do the job. Yet, on the St. Lawrence River, incorporating passage for eels is essential.
Monitoring at these sites over the last 50 years shows a more than 95% decline in the number of eels in the upper St. Lawrence River and Lake Ontario. Fewer eels are successfully completing their full migration to these headwaters. Overfishing, instream barriers throughout the watershed, and high mortality rates for adult eels at dams on the river all contributed to the population decline.
Such a stark drop in population numbers is cause for alarm and has increased pressure to find a solution for downstream migration. Studies have explored novel methods of facilitating fish passage, testing deterrents such as light, sound, and electromagnetic fields to guide adult eels away from the dangerous turbines. As of now, a combination of light and sound is the most promising solution for getting the eels safely on their way back to the Sargasso Sea to spawn.
In 2022, researchers from the U.S. and Canada plan to study using light to guide eels on the St. Lawrence River. The group hopes to guide, collect, and move eels downstream of dams to reduce turbine mortality.
“We’re excited by this work,” says Scott Schlueter, a Fish Enhancement, Mitigation and Restoration biologist for the Service. “A solution is necessary because all eels migrating downstream from Lake Ontario/Upper St. Lawrence River are female and are valuable to the overall eel population.”
Come one, come all
Though we’ve been talking about eels, most of these structures and methods can be modified to facilitate passage of other fish, too! American shad, alewife, blueback herring, brook trout, walleye, bass, and sturgeon are examples.
Engineered fish passage at hydroelectric dams increases fish survival. This benefits fish populations significantly, allowing migratory fish to travel more freely to critical spawning habitat. Regaining and retaining free-flowing waterways for aquatic species is important to the health of entire watersheds.
Improving fish passage is also beneficial for recreational fishing because it allows more species to be present in more places within a watershed.
With more than 240 hydroelectric dams in the state of New York alone, the need for engineered fish passage is widespread. Populations of species like the American eel are declining. Though they aren’t on the Endangered Species List, the Service works to help them survive, hoping they never reach that point.
American eels instinctively know the way from their spawning grounds in the Sargasso Sea to inland waters thousands of miles away. But, often, human activities get in their way. Whether going with the flow or swimming against the current, our biologists work hard to get these fish where they need to go.
