By Tom Eisenhart
Spend enough time diving river bottoms, and you will find all kinds of 'treasures.'
Mike Davis, project manager for the Minnesota Department of Natural Resources' Statewide Mussel Survey, can testify to that. He has a large aquarium full of oddities such as a buffalo skull, part of a wagon wheel and a cask, all found during his mussel survey work.
Yet the biggest surprise of his career took place in the late 1990s, where the Mississippi River flows through Minneapolis and St. Paul. Native mussels had returned to this stretch of river, an area where they hadn't been found in decades.
Today we join Mike and his team as they survey mussels at this site. It seems like a highly unlikely place to find any wildlife thriving. High above us semis barrel over the Lafayette Bridge; alongside, barge tows push their loads by. The smell of bread from a local bakery mixes with diesel fumes.
Diving on the Mississippi's bottom here is eerie. Visibility is negligible; divers find mussels by touch. And, it can be unnerving to hear the growling motors of barge tows as they pass.
As we watch, Ben Bosman, a graduate student and diver for the day, emerges from the water and climbs up the ladder into the dive boat. He unloads two handfuls mussels from a pouch slung over his shoulder. Sorting through them, Bernard Sietman, team leader for the project, is surprised to find a Fawn's Foot, a species the state is considering listing as either endangered or threatened.
Since mussels are stationary creatures, they couldn't have found their way back here by themselves.
Mike suspects that based on the age of the mussels found here, the floods of 1993 could have played a role. High waters may have enabled fish that had mussel larvae attached to their gills to move upstream past the dams and drop juvenile mussels into the waters here.
A researcher of aquatic invertebrates would probably find the mussels return fascinating. But many people might wonder: why all the fuss about lowly 'clams?'
Mussels are good indicators of the health of our rivers and other freshwater systems. For one, they live a long time, some species for more than 100 years, most for 10 to 40 years. Second, they can't swim away from harmful sediments, nutrients, pollutants and invasive species.
"You can tell something about the health and history of a river by virtue of their presence or absence," Mike says.
Beyond their ability to help gauge the biological health of rivers, mussels play other vital roles. Mike notes that a healthy mussel bed can contain upwards of 30 species, creating a "synergistic effect in a river that's not unlike a coral reef."
Mussels ingest plankton and other small debris. In turn, they excrete nutrients in a form that algae can use. Aquatic insects feed on algae, small fish feed on insects and big fish eat the small ones. "It creates a positive feedback loop," Mike says.
While the return of mussels to places like the Mississippi River near Minneapolis-St. Paul is encouraging, other threats loom. Zebra mussels, for example, have literally suffocated native mussel populations in Lake Pepin, located south of here.
Other threats to mussels include non-point source pollution, such as run-off from agricultural fields and urban areas. And land use changes affect the force and speed at which water flows into the river from tributaries.
Still, Mike is hopeful for mussels' future. He and his DNR colleagues have started reintroducing mussels in the Mississippi and other rivers. They are using techniques developed by biologists in the early 1900s who sought to produce more mussel shells to supply the button industry.
In one effort, they harvested the larvae of Higgin's Eye mussels in places where they are threatened. They then took the larvae to a fish hatchery and "infect" the gills of bass and walleyes. Those fish were brought to rivers like the Mississippi and kept in cages until the larvae dropped off and formed juvenile mussels. Using this procedure, hundreds of Higgin's Eyes have been reintroduced.
Monitoring of the Higgin's eyes has shown they are producing viable larvae. Yet in the river at Minneapolis-St.Paul, Mike hasn't found juvenile Higgin's Eyes. "Something isn't quite right, or perhaps we haven't been doing this long enough," he explains.
Mike is committed to finding ways to help protect and restore its mussels. As he notes, improving the health of mussels benefit nature and us. "Getting them back will help absorb some of the nutrient load we are putting it the river. It's nature's way of cleaning our water."December 06, 2010