Freshwater mussels really matter to river's ecosystem
MONTEVIDEO -- As far back as the turn of the century, there were "clammers" who dug freshwater mussels in the Minnesota and Mississippi Rivers in search of pearls.
Later, they raked the upper Minnesota River to harvest mussel shells in quantities large enough to fill rail cars. They shipped the shells to Muscatine, Iowa, to be made into buttons.
Today the job of hunting for mussels in Minnesota belongs almost exclusively to two men, Mike Davis and Bernard Sietman.
The value of their work might exceed anything all those years of unregulated harvesting ever produced.
"Mussels can be the canary in the coal mine,'' said Davis, as he sloshed in chest waders on Oct. 14 through the chilled waters of the Chippewa River downstream of the Minnesota Highway 40 bridge east of Milan.
Davis and Sietman, both with the Minnesota Department of Natural Resource's ecological resources division, have been working together since 2002 surveying freshwater mussels in the state. Along with assessing the status of the state's mussels, the two are establishing monitoring sites where the health of individual mussel populations can be tracked over time. They've already established sites in the Mississippi and Cannon Rivers. Now they've established a third site in the Chippewa River.
"The (Chippewa) river is doing well compared to the other rivers in the Minnesota River system,'' said Sietman, of what they found.
Assisted by volunteers recruited by the Chippewa River Watershed Project, the two spent three days examining and marking with a Global Positioning System over 100 different locations under the river's waters. They were excited to find two species of mussels in the Chippewa River -- the spike and black sandshell -- that have disappeared from the main stem of the Minnesota River and are listed as species of special concern in the state.
They also found juvenile mussels, evidence that mussels are continuing to reproduce in the river.
Minnesota is home to 49 species of mussels, but 26 of them are now listed as either extirpated, endangered, threatened, or of special concern.
The Minnesota River watershed was once home to 40 of the state's mussel species. Today, only around 20 species can be found, said Davis.
Over-harvest, chemical pollution, sedimentation, dams, channelization, dredging, wetland drainage and practices that cause rapid bounces in river levels are all harmful to these aquatic animals.
Take dams for example. They prevent fish from migrating. Mussels rely on fish to be hosts and disperse their young. Some mussels use specific species of fish for the task.
Mussels have evolved sophisticated methods for making fish the underwater storks for their young. The female plain pocketbook mussel has a part of flesh that looks like a minnow to attract fish. When a walleye is about to chomp, the mussel's chamber explodes and fills the mouth and gills of the fish with its young. The young take nutrients from the fish's blood and will ride along for a week or more before leaving the fish.
The fish does not seem to be harmed by the experience and may benefit, said Davis. Fish that have hosted mussel young are less vulnerable to other parasites.
"It's nature's vaccination for fish,'' said Davis.
Mussels will move or burrow into the river bottom when water levels drop, but they have virtually no defense against contaminants. Compared to fish, mussels can be more sensitive to chemical contaminants by a full order of magnitude, said Davis.
Young mussel larvae are extremely sensitive to ammonia, which washes into rivers from farm fields. "We're discovering that water quality standards designed to protect fish in some waters are not sufficient to protect mussels," said Davis.
Mussels are coming to our attention for their value as gauges of water quality. In the process, we are discovering just how important these animals are to the overall health of our lakes and rivers as well, said Sietman.
We've always considered mussels to be the filters of our waters. They feed on plankton and organic materials.
But recent studies are suggesting they also play an important role in the quality of our fisheries as well, said Sietman. He cited one study showing that the biomass in a river's substrata is much greater where live mussels are present. They provide feed for the microscopic organisms on the lower rungs of the food chain, which in turns attract larger predators.
There is also research suggesting that mussels help stabilize a river bottom and reduce scouring. Other research points to the benefits of oxygenation that occur as mussels burrow in a river's substrata. They may play a role akin to that of earthworms in soil, said Sietman.
In some very real ways, beds of mussels are to rivers what coral reefs are to our tropical seas, noted Davis.
Unlike coral reefs or the colorful fish that inhabit them, mussels have never benefited by widespread public concern or attention.
That's starting to change. Davis and Sietman were joined on the Chippewa River by dozens of interested volunteers during a an earlier September visit.
The Clean Water Act and improvements to water quality have allowed mussels to make comebacks in some waterways, said Davis. He and Sietman said they are "cautiously optimistic'' that we can continue to improve water quality and help mussels rebound.
They will continue to survey and monitor mussels and hope to get the public involved in voluntary mussel monitoring programs. Sietman and Davis said they are also hoping to start re-introducing species of mussels to certain waterways.