Painstaking efforts to restore Minnesota’s freshwater mussels are starting to payoff
Published 7:04 pm Friday, November 29, 2019
By Kirsti Marohn and Daniel Ackerman, Minnesota Public Radio News
On a sunny day this fall, biologist Lindsay Ohlman shuffled around a knee-deep section of the Cedar River near Austin, Minn. She used her feet to feel the streambed, searching for the perfect spot to release 200 muckets — a species of freshwater mussel — into a stretch of river where they have been absent for a century.
After a few minutes: She found it.
“I like this substrate,” Ohlman told her colleagues waiting eagerly on the riverbank. “It’s not super compacted, so it allows [the mussels] to bury in, but it’s also not super loose so it’s not going to let them get flushed away downstream.”
The group — all scientists with the Minnesota Department of Natural Resources — spent the next half-hour planting the inch-long muckets like seeds into the riverbed.
The juvenile mussels had been collected about four years earlier, farther down the Cedar in Iowa, where the species is still hanging on naturally. DNR biologists raised the muckets — first in tanks in a lab, then in enclosed cages in a lake. Now they’ve grown big enough to fend for themselves in the wild.
In the coming years, Ohlman and her colleagues will return to this stretch of the Cedar River to check on their charges. They want to see these muckets grow and eventually reproduce on their own.
And they plan to reintroduce more mussel species here and elsewhere in the region.
Ohlman is among a group of biologists, most of whom are based in Minnesota, leading a nationwide push to restore dwindling populations of native freshwater mussels. The hard-shelled mollusks have struggled in the upper Midwest since the late-1800’s, first thanks to commercial harvest by the button-making industry, and later because of dams and pollution.
Mussels are important to river ecosystems because they filter water, leaving it cleaner, and provide food for other aquatic creatures. And they’re also good indicators of a river’s health.
Now the DNR is bringing mussel species back to streams and rivers across southern Minnesota.
It’s going to take a lot of work, but for now, Ohlman’s team is celebrating the day’s success. Muckets are officially back in the Cedar.
“It’s a win for mussels!” Ohlman cheered. And the Cedar River isn’t the only Minnesota waterway where mussels are winning.
Success on the St. Croix
A propagation effort on the St. Croix River has helped restore another mussel species, this one endangered, that once lived in about 38 rivers from Minnesota to Arkansas.
Now, the winged mapleleaf is found in only five rivers, including the St. Croix, which is home to the species’ only known reproducing population.
Every fall, scientists collect pregnant female mussels from a stretch of the St. Croix River near Taylors Falls, Minn., which forms the border with Wisconsin. It’s a joint effort involving multiple agencies, including the U.S. Fish and Wildlife Service and the National Park Service.
The scientists take those female mussels to a lab. There, they harvest their larvae, known as glochidia, and expose them to a host fish, which is necessary for mussel reproduction. For winged mapleleaf mussels, the host is a catfish. The glochidia attach themselves to the fish’s gills or fins until they transform into juvenile mussels and drop off.
“We’re trying to intervene in certain periods of their life cycle in order to give them a boost, give them a helping hand,” said Byron Karns, natural resource program manager for the St. Croix National Scenic Riverway.
The goal is to “try to make sure that we can get as many of these juveniles on as many catfish as possible,” he said.
When the propagation is complete, the female mussels are tagged with a unique identification number, so they’re not reused every year. That helps diversify the population’s genetics.
Divers then return the tagged female mussels to the same location, placing them carefully on the river bottom so they can burrow down for the winter.
“We want to keep these girls happy,” said Beth Glidewell, a mussel biologist and diver with the U.S. Fish and Wildlife Service.
‘A very special place’
The St. Croix River has always been a gold mine for mussels. Of Minnesota’s roughly 50 native mussel species, 40 live in the St. Croix. And the stretch near Interstate State Park, where the propagation effort is underway, is a hotbed of mussel diversity, Karns said.
“There’s some mussels that are down here that are so common that you would think that they are common everywhere,” he said. But: “If you go anywhere else in the states of either Minnesota or Wisconsin, you won’t find them, either at all or in any kind of abundance.
“This is a very unique spot, a very special place.”
Most U.S. rivers have seen mussel species decline or even disappear over the past century, due to dams, land disturbance or pollution.
That’s not true of the St. Croix, Karns said. Scientists believe the river had the same number of mussel species now as it did hundreds of years ago.
“We haven’t lost anything,” he said.
The benefits of a natural shoreline
Part of what helps mussels thrive in the St. Croix is that the river has been protected since 1968 as a scenic riverway, meaning the land on both sides is undeveloped.
“We have a very — relatively speaking — pristine shoreline for many, many miles that a lot of other rivers don’t have,” Karns said.
All that forest soaks up rain and stormwater, keeping nutrients and sediment out of the river.
Filter-feeding mussels thrive in clean water. And because they live so long — some species as long as 100 years — these hard-shelled invertebrates a good indicator of the river’s health over time. Scientists can tell that the St. Croix has been in good shape for a long time because of the age and quantity of its mussel populations, Karns said.
“If you have even several months of really bad, nasty conditions that these mussels can’t survive in, they’re going to die, and you won’t find 80-year-old mussels anymore,” he said.
The St. Croix isn’t pristine. The stretch from Taylors Falls to Stillwater was recently added to the state’s impaired waters list due to high phosphorus levels.
But Karns said there are lessons here, about how human activity in a river’s watershed affects the creatures that dwell in it.
“We all have to be responsible for what we do on the landscape,” he said.