MADISON, Wis. — The first statewide survey for native mussels in 40 years reveals these water-filtering clams are facing mixed fortunes.

Mussel populations and diversity were highest in the St. Croix River, with 24 different species found at one site. High species diversity was also found in the Manitowish, Chippewa and Peshtigo rivers.

“On the St. Croix River, the abundance and species richness was very impressive,” said Jesse Weinzinger, a conservation biologist with the Wisconsin Department of Natural Resources who coordinated the surveys. “There were times you'd be pulling up 200 to 300 mussels and 12 or so species in our 15-minute timed surveys.”

Mussel populations are also rebounding in the Wisconsin River as clean-water regulations enacted during the last 50 years are paying off. Native mussels are even starting to be found in the lower Fox River and Green Bay, where a massive cleanup project is underway. The DNR said improved water quality is making it possible to consider reintroducing more mussel species.

At some sites, however, surveys revealed declining mussel populations. Ten sites had no mussels, and Weiznzinger said major rivers in southern Wisconsin, including the Pecatonica and Rock rivers, are experiencing large declines.

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Conservation biologist Jesse Weinzinger holds a native mussel with invasive zebra mussels attached. Jack Silverberg photo
Conservation biologist Jesse Weinzinger holds a native mussel with invasive zebra mussels attached. Jack Silverberg photo

Stretches of the Pecatonica River where DNR surveys 15 years ago found four species listed as either threatened or endangered now held none of those rare mussels and Weinzinger found scores of dead mussel shells.

While the mussel surveys didn't dig into potential causes of the declines, water quality trends tracked through other DNR programs are showing that levels of nitrates and ammonia in the river are above the threshold mussels can tolerate.

Among the world’s most threatened animals

Freshwater mussels are one of the most imperiled animals on the planet, with 70% of all species declining. In Wisconsin, 24 of the 50 native mussel species are endangered, threatened or listed as a species of concern, says Lisie Kitchel, another DNR conservation biologist who works with native mussels.

“Native mussels are important for healthy lakes and rivers,” she said. A freshwater mussel can filter gallons of water a day, removing mercury and other contaminants. They also are food for raccoons, muskrats, otters, herons and other wildlife. They are even food for fish when the mussels are young, and dead shells can provide safe places for fish to lay their eggs.

Mussels declined in the 20th century due to factors including water pollution, dams that blocked the flowing water mussels need and overharvest. From the 1880s to the 1940s, mussels in Wisconsin were used to make buttons before plastic buttons replaced them.

After that era, mussels from the Upper Mississippi River were shipped to Japan where they were cut up and turned into the seed from which pearls were cultured. Overharvest of mussels on the river eventually led to closing the commercial harvest of mussels, Kitchel said.

Now, mussel populations are increasing in some of these waters again, thanks to protections afforded by the state and federal endangered species acts. Improved water quality since the 1972 Clean Water Act started controlling wastewater discharges to streams and rivers also had an impact.

The DNR, U.S. Fish and Wildlife Service, dam owners and other partners have also sought to save mussels stranded from reservoir drawdowns by propagating them at the Genoa Fish Hatchery for release back into state waters.

Survey samples 99 sites, finds 39 species

Before the DNR surveyors began to look for mussels, they reviewed historical mussel surveys dating between 1928 and 2015. While DNR and other researchers had conducted surveys on specific waters in recent years, the statewide survey was the first such effort since the 1970s.

The DNR statewide surveys started in 2016 and conservation biologists donned waders, and in many cases SCUBA gear, to collect, identify and record more than 21,000 living individuals representing 39 species before returning the mussels to the water.

Spikes were the most commonly observed species by number with 25% of the total catch, followed by mucket at 20% of all observations. Fatmucket, spikes, plain pocketbooks and giant floaters were the most widespread.

Biologists have used the survey information to identify and map 16 areas where they will conduct long-term monitoring and focus conservation efforts.

"The survey has been very important in helping us gather information on the distribution, population demographics and habitats of native mussels, all important information to help us focus our monitoring and other conservation efforts," Weinzinger says.

Water enthusiasts asked to photograph and report native mussels

Volunteers helped survey for Wisconsin's most recently discovered native mussel, the eastern pondmussel, in 2018. Stephanie Boismenue photo
Volunteers helped survey for Wisconsin's most recently discovered native mussel, the eastern pondmussel, in 2018. Stephanie Boismenue photo

Wisconsin DNR biologists encourage paddlers, anglers, and other water enthusiasts to photograph and report native mussels they see while on the water.

"Wisconsin is known for having 15,000 lakes and more than 84,000 miles of rivers. We need your help to have a better picture of what mussels occur where," Weinzinger said. "Share photos and associated information on where you found them, when, and the habitat they are in."

After photographing closed mussels with the hinge side up, volunteers are asked to return live mussels to the water, then report that information to DNR's Wisconsin Mussel Monitoring Program. It’s illegal to harvest mussels in Wisconsin.