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Experiment: Freshwater Mussel Restoration

February 19, 2013

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Conservancy ecologist Tharran Hobson sifts through silt for freshwater mussels. Matt Miller/TNC

By Matt Miller, senior science writer

Tharran Hobson dons a wetsuit and wades into the wetland. Soon he is swimming, diving, fishing around—“pollywogging,” he calls it—finally emerging with his muddy bounty, a small wire cage.

As underwater treasures go, it doesn’t look like much: A wire structure with a silt-covered tray in the bottom.

But this contraption may hold the key to restoring some of the least understood and most abused creatures in the Mississippi River system: freshwater mussels.

Conservancy scientists and partners are currently testing whether restored wetlands might be suitable sites for freshwater mussel propagation efforts. If it works, it could be another important tactic in the restoration of freshwater mussels that once existed in Midwest rivers by the millions.

I’m joining these scientists at the Conservancy’s Emiquon Preserve, a 6800-acre restored floodplain along the Illinois River (a significant tributary of the Mississippi). This project has already been successful in drawing in thousands of migrating waterfowl, and in restoring 30 native fish species.

Could it also serve as a breeding project for mussels?

That’s what I’m here to find out.

A Multitude of Mussels

Researchers (left to right) Sally McClure, Tharran Hobson and Dan Sallee check cages for mussels. Matt Miller/TNC
Researchers (left to right) Sally McClure, Tharran Hobson and Dan Sallee check cages for mussels. Matt Miller/TNC

The freshwater mussel could be considered the Rodney Dangerfield of river conservation. It doesn’t have the fans that bald eagles or ducks or largemouth bass do.

Mussels have suffered for it.

The Mississippi River system holds an incredibly diverse array of mussels, including evocatively named but largely unknown species like the butterfly, Higgins eye and winged maple leaf. They existed by the billions, with mussel beds covering large swaths of river substrates.

This bounty set off three waves of “mussel booms” along the rivers. In the late 1800s, “prospectors” searched for pearls in the wild mussels. “It was like a Gold Rush, with people going through millions and millions just to find a few pearls,” says Dan Sallee, a fisheries biologist and mussel expert for the Illinois Department of Nature Resources.

This was followed by the harvesting of mussels in the early and mid-1900s for the pearl button industry. Mussels from the Illinois River supplied most clothing buttons in the country until World War II, when plastic buttons dominated the market.

Finally, beginning in the 1960s, mussel shells were shipped to Japan, where they were used to culture pearls (the mussel shell was implanted in oysters, creating more rapid if less valuable pearl growth). “They were overharvested each time,” says Sallee. “And each time it became harder for them to bounce back.”

That was in part due to other factors, most notably water pollution from pesticides, fertilizers and raw sewage. Damming of rivers changed the river’s ecology, making it more difficult for mussels to reproduce. And then came non-native zebra mussels, which out-competed the natives.

Today, freshwater mussels exist as a tiny fraction of their original population. “In the 1970s, I tried to take silt samples in one section of the river, and couldn’t because there were so many mussels covering the bottom,” says Sallee. “I went back to the same areas in the late 1990s, and found one live mussel in three hours of searching. It was like that everywhere.”

Bringing ‘Em Back

Researchers found a surprising number of clams in their mussel cages. Matt Miller/TNC
Researchers found a surprising number of clams in their cages. Matt Miller/TNC

Scientists are now looking at ways to restore mussel populations. But their rather strange life histories present both challenges and solutions.

Mussels are sedentary creatures. For their species to spread, they need a fish host. Mussels use a variety of surprising tactics to accomplish this. Some species, for instance, stick out a fleshy protuberance that looks an awful lot like a minnow when it whips around in the current. A fishing lure.

When the fish comes to bite the lure, expecting an easy meal,  it receives instead a mouth full of baby mussels (also known as glochidia). The mussels encyst in the fish’s gill—without harming it—until they mature. At which point, they drop off and begin their sedentary life.

“The problem is, the environmental conditions for mussels to reproduce don’t come every year,” says Conservancy ecologist Tharran Hobson. “In natural conditions, their breeding can be really sporadic. Some species can live fifty years or longer, and they used to exist in such numbers that it wasn’t a problem. But with certain mussels, it’s a big problem.”

With fewer mussels, there are also fewer chances that fish hosts will find their “lures.”

If the fish don’t come to the mussels, though, you can always bring the mussels to the fish.

And that’s the basis for the experiment at Emiquon Preserve. Fish hosts—with this experiment, largemouth bass—are “infected” with hundreds of glochidia or the parasitic phase of the mussel, and then placed in a cage.

When the mussels mature, they drop off onto a tray, and the fish is released by researchers. When the mussels can fend for themselves, they are taken to an existing mussel bed.

“When you put them back on the bed, the hope is they’ll thrive,” says Hobson.

The Emiquon Preserve is host to a wide array of restoration and research projects. Could it also be used as a mussel nursery? Wetlands purify water, so in theory mussels could thrive there.

On this particular day, Hobson dove down for cage after cage of mussels. When the fish were removed from the cage several months ago, a sandy substrate covered the bottom — perfect conditions for mussels.

The trays that held mussels were covered in thick silt. Not ideal. The researchers pointed to the thousands of ducks kicking up the wetland bottom, wind and organic matter from decaying plants. Could this doom the mussel propagation effort?

Researchers found muddy substrate in their mussel cages--not idea conditions for successful propagation. Matt Miller/TNC
Researchers found muddy substrate in their mussel cages–not idea conditions for successful propagation. Matt Miller/TNC

 They found plenty of mussels in the silt, but whether they will be able to survive and thrive well enough for release back into rivers remains to be seen.

Researchers already know that mussels bred in this way can do well when the cages are in rivers. But this comes with its own set of challenges. Cages can be disturbed or destroyed by floods, river currents, zebra mussels and humans.

Fish can also be infected with baby mussels and just set free. But having a wetland breeding facility would add another option in restoring these imperiled species.

“If we knew it would be a success, it wouldn’t be an experiment,” says Salee.

Whatever the outcome, the overall future is looking brighter for mussels on the Illinois River. Water quality has been steadily improving thanks to the Clean Water Act. Zebra mussels are killed off each summer due to high water temperatures, so they’re not much of a factor on the Illinois (they still wreak major havoc on most of the upper Mississippi).

“People have overharvested and then overlooked mussels for decades,” says Hobson. “Now they need a little help to rebound. Each propagation project teaches us something new, something we can apply so that we can save and restore these overlooked but fascinating creatures.”

Mussel research requires a lot of wet and muddy work. But the results can help restore these creatures to the Illinois and Mississippi river systems. Matt Miller/TNC
Mussel research requires a lot of wet and muddy work. But the results can help restore these creatures to the Illinois and Mississippi river systems. Matt Miller/TNC

Opinions expressed on Cool Green Science and in any corresponding comments are the personal opinions of the original authors and do not necessarily reflect the views of The Nature Conservancy.

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  1. Nice piece. One small correction though. The photo of the “mussel” on the fingertip is actually a fingernail clam. A freshwater bivalve to be sure, but a member of the Family Sphaeriidae, whereas mussels are members of the Unionidae. Not even closely related. Sorry just a stickler for details. If you really want to delve into the particulars see: Graf, D.L. 2013. Patterns of freshwater bivalve global diversity and the state of phylogenetic studies on the Unionoida, Sphaeriidae, and Cyrenidae. American Malacological Bulletin 31(1):135-153.