This article was originally published on The Revelator, an initiative of the Center for Biological Diversity. It is reprinted here with permission.
In September 2018 an explosion rocked the Bloede Dam on Maryland’s Patapsco River, a few miles west of Baltimore. The breach and subsequent removal of the long-dormant, decaying and dangerous dam allowed the river to run free for the first time in more than a century and opened 60 miles of habitat. Experts hailed the project as a boon to public safety, recreation and ecological restoration efforts — especially since it eliminated a key barrier to migratory fish such as shad, herring and American eels.
But another small, inconspicuous animal may also get a boost from the dam removal: eastern elliptio, a freshwater mussel that’s common to the region but has long since disappeared from the river.
Sadly, its plight is becoming the norm.
North America boasts nearly one third of the world’s 900 freshwater mussel species. But pollution, invasive species, and changes in stream flows and water quality from development have all taken their toll on mussel populations. Scientists estimate that 38 of the continent’s species have gone extinct in the past 100 years, and 95 are listed as threatened or endangered in the United States.
One of the biggest factors in the U.S. decline has been the construction of dams — more than 84,000 of them, which has fragmented rivers and altered water flows, sediment, habitat and temperatures. (Note: Recently, an endangered species of mussel was discovered in the St. Croix River above the St. Croix Falls dam, bound for extirpation due to being cut off from its host fish.)
Fortunately, the tide seems to be turning on dams, as many have outlived their usefulness and their ecological consequences have become better understood. In the past two decades, more than 1,200 dams have been demolished across the United States.
Can more removals help protect what’s left of our freshwater mussel diversity? Recent river restoration efforts show reasons for hope.
But, like with most ecosystems, it’s complicated.
“When people look at rivers, they might think about the fish swimming in them, but they really don’t think about all of the other creatures that are found there,” says Emilie Blevins, a biologist working on freshwater mussel conservation at the nonprofit Xerces Society.
Because they spend their lives submerged and often are dusted with sediment, freshwater mussels are one of those often overlooked, she says. But they’re also one of the most important.
The mollusks act as tiny, but efficient water filters, helping to improve water quality as they consume algae, phytoplankton, bacteria and organic particles from their surroundings. “When we lose mussels, the quality of an ecosystem goes down, and then we see those effects later downstream — you might see increased nutrient pollution, followed by algae blooms,” she says.
In addition, mussel beds are known as “hotspots of biological activity” because they support so much other life. They can stabilize streambeds, provide habitat for other aquatic species, and serve as prey for an array of wildlife including birds, muskrats, turtles, otters, skunks and fish.
Their sedentary lifestyle serves all these roles — with only a single foot, freshwater mussels aren’t very mobile. “They can move meters over the course of their lifetime, not miles,” says Cody Fleece, an aquatic ecologist with the engineering services company Stantec.
But mussels do still get around, as they’ve developed an evolutionary fix to expand their ranges. Females release larva — known as glochidia — that then attach to the gills or skin of a passing fish. Some species have even developed special adaptations to lure or grab potential hosts.
As larvae-carrying fish swim up and down rivers, the glochidia hang on for weeks or months until they undergo a metamorphosis to the juvenile stage. That’s when they finally drop off — ideally, in suitable new habitat.
Success is a game of numbers. A female freshwater mussel releases thousands of glochidia, few of which will make it to the juvenile stage. But the ones that survive can be long-lived, with lifespans that can stretch across decades or even a century, depending on the species.
Of everything that needs to go right in a mussel’s life history, finding a host fish is the crux. Some mussels can rely on multiple kinds of hosts, but others prefer one particular species. For example, the endangered pearl mussel (Margaritifera margaritifera) relies on juvenile trout and salmon as their glochidia hosts.
“Freshwater mussels really don’t exist without their host fish,” says Blevins. “And that’s why they can be really sensitive to a range of impacts that affect fish, including dam building.”
Barriers to reproduction
Eastern elliptio disappeared from the Patapsco River over the past 50 to 100 years because dams blocked the passage of migratory fish — particularly their favorite, American eels.
“When you have a blockage to migratory fish — from a series of dams or one really large dam — and you can’t get migratory fish upstream to where the mussels are, eventually reproduction just stops,” says Matt Ashton, an aquatic biologist at Maryland’s Department of Natural Resources. “You need to have the right fish and you need to have them in enough numbers.” He says the problem is prevalent in mid-Atlantic and Northeast states.
It’s a similar story in the Pacific Northwest, where freshwater mussels such as western pearlshells rely on seagoing salmon that have been blocked by dams from reaching upstream habitat — and the mussels who live there. “Based on what I’ve seen, dams are a real concern for mussels in the western U.S.,” says Blevins.
In areas of the Midwest and Southeast where migratory fish aren’t the key host for freshwater mussels, the story is a little different.
“For the most part, [freshwater mussels] are using hosts that have small ranges,” says Ashton. “So the dams don’t necessarily fragment those mussels and stop reproduction, they just alter habitat.”
But that can also mean trouble.
The transformation of a free-flowing river into a pool of slack water behind a dam often creates unsuitable conditions for mussels (and sometimes their host fish, too). Mussels can also become stranded when water recedes in the reservoir during low flows.
“Mussels aren’t very happy right upstream of a dam in general,” says aquatic ecologist Heidi Dunn, who works as a science coordinator for the consulting firm EcoAnalysts. Right below a dam isn’t great either, she says. High-velocity water releases can become a scour zone where the fast flow prevents mussels from getting a foothold. Cold water coming from very large dams can also disrupt mussel reproduction.
That all takes a toll. “Altered flow regimes and reservoirs caused by damming have resulted in the local extirpation of 30 to 60 percent of the native freshwater mussel species in many United States rivers,” according to a report from the Natural Resources Conservation Service.
When it comes to helping freshwater mussels in already altered environments, there’s no one-size-fits-all solution.
“There’s an argument that downstream of dams can be some of the most stable habitat because it isn’t as affected by changes in flows,” says Julie Devers, a state biologist in Maryland for the Natural Resources Conservation Service. “And so it can be a stronghold, sometimes, for freshwater mussels. But most of the aquatic biologists are on the same page that dam removal will result in a more connected ecosystem where fish can move freely, and the mussels can interact with their host fish in the most productive way.”
Because of that, more dam removals are being done with an eye to freshwater mussel protection or recovery — although fish are still often the species of primary concern.
The nonprofit American River reports that dam removals are expected to benefit imperiled mussels on the Tallapoosa River in Alabama, on the Brandywine River in Delaware, on the Cane and Tuckasegee rivers in North Carolina, on Conewango Creek in Pennsylvania, and many more.
Dunn is involved in a project still in the permitting stages in Grand Rapids, Michigan that would involve the removal of four low-head dams in an area of the Grand River with 27 species of freshwater mussels. Restoring the river, “should be a good thing in the long run,” for those mussels, she says.
But dam removals also need to be done in ways that protect existing freshwater mussel populations.
With the 2020 removal of Six Mile Dam on the Walhonding River in Ohio, for example, teams scoured areas above and below the dam to find — and relocate — some 12,000 bivalves, including 640 threatened rabbitsfoot mussels and 720 endangered sheepnose mussels.
Water levels behind the dam were also lowered in stages so that mussels in the reservoir could be recovered before they stranded in the mud.
Once they were relocated to other suitable habitat in the river, monitoring efforts have continued. “We put pit tags on them and last summer we went out with a pit tag reader and looked for all the mussels to make sure that the ones that we had moved had survived, which they did,” says Fleece.
In some cases where freshwater mussel populations have already been lost — like on the Patapsco — researchers must go to greater lengths to try and restore them.
After three dams, including Bloede, were removed on the Patapsco, and a fourth added passage for American eels, eel numbers began increasing. Since the snakelike fish are the preferred host for eastern elliptio, researchers started an assisted migration process.
Eastern elliptio were taken from a reproducing population and relocated into areas of the Patapsco where they were likely to encounter eels. “We moved adults and essentially founded a new population in the hopes that we can jumpstart the natural process,” says Ashton.
The first group of mussels was moved in 2018, and a couple of hundred have been translocated every year since. Researchers track the mussels with tiny microchips affixed to their shells and most are staying in place and surviving, which is the first goal of the project.
Next is to have those mussels reproduce and expand the population, he says, which will still take several more years.
Conservationists encountered a different situation on the Susquehanna River.
In Pennsylvania, eastern elliptio still existed in good habitat, but four downstream dams blocked migratory fish, so they hadn’t reproduced in a long time.
To help them, scientists relocated 120,000 American eels to two tributaries upstream of the dams. And it did the trick: Over a few years they saw an increase in the number of juveniles.
“In biological terms, it’s almost as close to flipping a light switch as you can have,” says Ashton.
As part of a settlement over the relicensing of the first dam on the river, which is in Maryland, the hydroelectric company Exelon is now required to move American eels by truck around the four dams, which researchers are hopeful will continue to aid freshwater mussel recovery upstream.
A Bigger need
Ashton calls the fact that we’re beginning to understand river ecology more holistically, and beginning to manage fish and mussels together, “a big win.” But when it comes to protecting mussels, there’s still a long way to go.
There are ongoing water-pollution problems from farms, industry and water-treatment systems; in the West ongoing drought is being exacerbated by climate change, and competition for water resources among users is draining streams, further imperiling mussels.
“We also still don’t understand how certain chemicals and pharmaceuticals affect mussels,” says Ashton.
But many dam removals and river restoration projects can have big benefits for mussels — and the whole ecosystem. And Fleece, who has been involved in 19 dam removal projects so far, thinks removals are one of the restoration techniques that has the greatest benefit for the expenditure of resources.
“We can affect miles and miles and miles of habitat as part of these removals,” he says. “And sometimes for organisms that we don’t even think about, or that we’re not even aware of, it’s one of those circumstances where the ecological benefits and the societal benefits align to pull in the same direction.”