Decline in freshwater mussels an indicator of poor river and stream health in South Dakota
Many species of critical members of freshwater ecosystems may be vanishing within state
Nestled in the silt, sand or fine gravel of South Dakota’s rivers and streams live some of the state’s least appreciated yet most ecologically important creatures — freshwater mussels.
Their names spark the imagination: Fatmucket, White Heelsplitter, Higgins Eye, Round Pigtoe, Giant Floater, Plain Pocketbook, Fawnsfoot.
Usually hidden beneath the water’s surface, mussels do the quiet work of filtering water in South Dakota’s rivers and streams, helping other aquatic species such as fish thrive. They are a natural food source for otters, ducks, herons and fish.
Many species of these critical members of freshwater ecosystems may be vanishing within South Dakota. Recent surveys of the state’s 14 major river basins — comprising the first comprehensive assessment of living mussel species and their population sizes in South Dakota rivers and streams — found only 17 of the 36 species once known to live in state waters, a 53% decline.
The decline of freshwater mussel populations in waterways in South Dakota and across North America is a major concern on several environmental levels.
Freshwater mussels are powerful filter feeders, consuming phytoplankton, algae and even bacteria from rivers and streams while also filtering out particles at rates measured in gallons per day. At least one mussel species can clear lake water of significant amounts of E. coli, a bacteria that can cause serious illness in humans. Research continues into their promising abilities to ‘treat’ manmade contaminants.
Experts say the reduction in mussel populations in South Dakota waterways is further evidence of largely poor water quality in a state where 78% of South Dakota stream-miles and 85% of lake acres are considered “impaired” in some way.
Freshwater mussels have been on the decline for two centuries — all for reasons related to the actions of man.
In the late 1800s and for several decades, mussels were harvested for their pearls and shells from South Dakota waters, including the Big Sioux and James rivers. Tuscan, located four miles southwest of Menno, was a center of mussel harvesting, according to a 2009 article in South Dakota Magazine.
Mussels were boiled to open their shells and remove the meat. While some people ate the mussel meat, often it was fed to pigs, or used as catfish bait if rotten. Boxcars filled with tons of shells were shipped by rail to Iowa factories to be made into iridescent buttons. Plastic replaced shell for buttons in the 1950s.
South Dakota’s mussel populations have yet to recover from that decimation.
After over-harvesting came land-use changes that altered water quality and stream bed stability, further harming mussel populations.
Accelerating land-use changes — often tied to expansion of agriculture — lead to soil runoff, sedimentation and non-point pollution from manure, fertilizer and pesticides. Water clouded with clay, silt and other particles, including algae, can affect the fish hosts mussels rely on to reproduce. Increased sediment smothers mussels. Pesticides can poison them. Fertilizer runoff causes excessive algae growth that depletes oxygen.
Thirty-six percent of tested water in South Dakota rivers and streams has excessive amounts of total suspended solids, according to the 2020 South Dakota Integrated Report for Surface Water Quality Assessment prepared by the state. Suspended solids, which can include soil particles, can increase turbidity and water temperatures, decrease oxygen levels and generally degrade conditions for fish and other aquatic life.
“Similar to previous reporting periods, nonsupport for fishery/aquatic life uses was caused primarily by total suspended solids from agricultural non-point sources and natural origin,” the report states. “Non-point source pollution is the most serious and pervasive threat to the water quality of South Dakota’s waters.”
The South Dakota Department of Agriculture and Natural Resources and the Department of Game, Fish & Parks have worked for decades with the U.S. Environmental Protection Agency, U.S. Fish and Wildlife Service, farmers, ranchers and other organizations to improve water quality in South Dakota’s rivers, streams, lakes and reservoirs.
Farming and ranching organizations say that their members are good stewards of the land on behalf of future generations, and that those who work the land are the “original environmentalists.” Many South Dakota landowners participate in conservation efforts, such as the reduction of sediment flowing from the Bad River basin into the Missouri River. But state data tell a story of high levels of agricultural pollution of surface waters. As of 2019, 78% of assessed stream-miles were impaired. E. coli, a bacteria living in livestock and wildlife feces, and total suspended solids, which often include materials from soil erosion, were the contaminants in first and second places.
After poor water quality come physical barriers. Thousands of impoundments on tributaries restrict the natural volume and velocity of water that mussels need to reproduce.
During 2014 and 2015, Kaylee Faltys and her research team waded in streams, feeling the muck for mussels with their bare hands at 202 sites within the 14 major river basins across the state.
After visiting the 202 sites, Faltys produced a grim tally: only 15 species of 36 anticipated species were found, 11 as live specimens and four in the form of recently used whole or half-shells. Of the 202 survey sites, only 91 total sites had live or empty-shell evidence of mussels. No evidence of mussels was found at 111 of the sites, more than half.
A silver lining appeared later in 2016, when Faltys and her colleagues separately assessed population sizes at the 44 locations with living mussels. A live Spike mussel and a half-shell of the Ellipse mussel were discovered, the first time each species has been found in South Dakota. Two additional known native species also were found in 2016: a Plain Pocketbook and a Fawnsfoot, bringing the study total to 17 out of 36.
“This stark decline in species richness may suggest that habitat conditions in South Dakotan streams and rivers are degrading, possibly due to a variety of factors such as land-use changes, impoundments, habitat destruction and host fish availability,” she said.
Chelsey Pasbrig, a GFP aquatic biologist, said in an email that her agency is concerned about the decline of freshwater mussel populations in South Dakota, and it is aware they are among the most endangered animals in North America.
“GFP has begun collaborations with other states to explore the option for augmenting populations with propagated individuals; however, this is in its infancy” she wrote.
Pasbrig added that no current mussel monitoring efforts are underway in South Dakota.