The Iowa River, from the Coralville Dam to Hills, was the focus of this year’s annual research project to inventory and map the distribution of Iowa’s mussels, often called clams.
More than 50 biologists, students, county naturalists and volunteers collected 22 species of freshwater mussels in the Iowa River during the three day event held each August since 2005.
Live mussels were inventoried, measured for growth; and then returned to the water. Most were found using a technique known as pollywogging, as researchers and volunteers crawl along a stream bed, probing the bottom with gloved hands. Trained divers inventoried deep water spots.
“These studies help us learn more about mussels and the areas where they live and thrive,” said Scott Gritters, fisheries biologist with the Iowa Department of Natural Resources (DNR). “The Iowa River by Coralville is one of our most studied resources. We have been tracking mussels here every few years since 2005 to watch the population trends, including after the historic floods of 2008 and one extreme low water period.”
The Iowa River mussel population does not appear to be as abundant as it was 10 years ago, but this year’s Mussel Blitz documented pockets of rich and diverse mussel beds. Many areas of the Iowa River have few mussels, but short stretches are still very rich and diverse, Gritters said.
“The majority of the mussels were found in the coarse gravel with one to three-inch rock with very little sand,” said Paul Sleeper, fellow fisheries biologist with the Iowa DNR responsible for managing this section of the Iowa River. “These stretches, easily searched in two to three feet of water, are perfect fish spawning habitat for shovelnose sturgeon and smallmouth bass.”
Together, the Iowa DNR, the U.S. Fish and Wildlife Service and the U.S. Army Corps of Engineers started researching the disappearance of native fresh-water mussels in Iowa 13 years ago. This includes searching for the federally endangered Higgins-eye pearly mussel. Once ranging across most of the Upper Midwest, this species has been eliminated from many of the rivers in which it once thrived.
"About 54 species of native mussels were once found in Iowa. Now, there’s about 42. Nine of these are endangered. Another six are threatened and several more species are very hard to find any more in Iowa,” said Gritters.
Stretches of the Cedar, Wapsipinicon and Iowa rivers have been stocked in the past several years with walleyes and bass whose gills had been injected with the mussels’ larvae.
“Stocking fish, something we commonly do anyway, is one way to reintroduce mussels into our rivers,” said Gritters.
Mussels are a good indicator of the health of a river. The better the water quality, the more mussels there are in that water. Mussels compact the algae they filter then kick out the crushed pellet to waiting fish; much like how fish are fed at a fish hatchery. Native mussels generally do not do well in soft substrates or fine sand. Impoundments which block migrating fish, also are a major hindrance to native mussels which are transported by hitchhiking on the fish.
“The whole river ecosystem runs better with native mussels living in it. Fish and mussels depend on each other,” said Gritters. “Our fish populations and the opportunities people have to enjoy clean water improve when mussels are presents.”