Mooneye, sturgeon and microplastics in trout

Highlights of the St. Croix River-related science presented at this fall’s Research Rendezvous.




4 minute read

Angie Hong is the coordinator for Minnesota’s East Metro Water Resource Education Program, a local government partnership.

Lake Sturgeon (MN DNR)

In the swiftly flowing waters of the St. Croix River, a federally endangered species of mussel called the spectaclecase shelters among boulders on patches of gravel and sand. It’s not easy to make babies when you’re spectaclecase. First, the male must release sperm into the river current, hopefully upstream of a lady mussel. If a female that is siphoning water for food and oxygen intercepts mussel sperm in that water, she can fertilize her eggs. To grow into adult mussels, however, there is one last step. When the female spectaclecase expels her microscopic larvae into the water, the glochidia (baby mussels) have to attach to the gills of a passing fish – but only the right kind of fish – in order to grow into juveniles that can live independently on the river bottom. For years, scientists have wondered which species of fish acts as the larval host for spectaclecase. At this year’s St. Croix Research Rendezvous, they finally revealed the answer.

Freshwater mussels have a complex life cycle. Adult males release sperm into the water. After being siphoned by adult females, these sperm fertilize the eggs. The females will then release microscopic larvae into the water. If they attach to the gills of a fish, these larvae will grow into mature mussels that drop off and live on the river bottom.

The annual St. Croix Research Rendezvous is a unique one-day event organized by the St. Croix Watershed Research Station and hosted by Warner Nature Center to share the latest research on water, wildlife, geology, and botany in the St. Croix River Basin. During the conference, scientists give brief 20-min summaries of their latest projects, providing attendees with an overview of how they did their work and what they learned in the process. At this year’s Research Rendezvous, attendees learned the answers to several zoological conundrums, including, “Who carries the spectaclecase babies until they’re grown-up?” “How far do sturgeon swim?” and, “What happens to the microfibers from our fleece jackets when we do our laundry?”

Warner Nature Center hosts the St. Croix Research Rendezvous each fall.

To determine the larval host fish for the spectaclecase, a team of researchers from the Minnesota Department of Natural Resources (DNR) and University of Minnesota considered three possible species that had not yet been studied – American eel, mooneye, and goldeye. After collecting fish from the St. Croix and Mississippi Rivers and inoculating them in a laboratory, they discovered that both mooneye and goldeye could potentially act as hosts for the spectaclecase. Best of all, some of the mooneye found in the St. Croix River even had spectaclecase larvae already growing on their gills.

Mooneye. Hiodon tergisus. (NOAA Great Lakes Environmental Research Laboratory)

Minnesota DNR also led the research on sturgeon in the Mississippi, St. Croix and Chippewa Rivers. After implanting 47 fish with transmitters between 2013 and 2017, Joel Stiras and John Hoxmeier tracked their movement and observed that the sturgeon swim freely between the three rivers, traveling distances of 50-100 miles. This discovery has led DNR staff to wonder if Minnesota and Wisconsin should change sturgeon fishing regulations, which are quite restrictive, to be the same for all three rivers.

A fish tale from the University of Wisconsin, River Falls provided a final piece of piscatorial science for this year’s Research Rendezvous. Student Claire Simmerman wondered if microplastics – bits of fiber that break off of synthetic fabrics in the wash, miniature beads from face washes, and other tiny particles of plastic – were being ingested by fish downstream of the city’s wastewater treatment plant. So, she collecting brook and brown trout from 12 different locations in the Kinnickinnic River above and below River Falls and used precise laboratory methods to dissect and examine the contents of their stomachs. Every fish caught had a belly full of microplastic.

Researchers use microscopes to find microplastics in water and animal tissues. In her research, Simmerman would often count 150-200 particles of plastic in a single microscope slide. Photo from the Chesapeake Bay Program.

The St. Croix Research Rendezvous brings together staff and scientists from a wide array of local, state and federal agencies; nearby schools and universities; and the St. Croix Watershed Research Station. It provides a venue for the people studying our lakes, rivers, wetlands and streams to interface with the people who manage these resources, and is also an opportunity for scientists to share new methods and technologies. It’s a place for learning about the many varied elements – from diatoms, to mussels, to sturgeon – that make up the intricate network of life we call the St. Croix Basin.

Want to learn more about fish and wildlife in the St. Croix Watershed?


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Mooneye, sturgeon and microplastics in trout