“There’s a lot at stake in the St. Croix. It’s a refuge for the upper Mississippi River watershed. The diversity and abundance is unparalleled.” – Mark Hove, University of Minnesota

Under their bulky waders, the high school students wore teenager clothes. They were dressed for a day in the classroom, not a rushing river. In fact, after a few passes with a net through the Willow River near Clear Lake, Wisconsin, they would jump back in their van and return for afternoon classes. But, in their short time in the river, they would forget about their outfits and the walls of the school, and they would make a real contribution to research about a rare species of mussels.

Creek heelsplitters were once found in rivers throughout Minnesota and Wisconsin, including the St. Croix and the rivers that flow into it, like the Willow. Today, they are much harder to find and are designated as a species of special concern in both Wisconsin and Minnesota. Their habitat has been degraded by sediment and dams, and interruptions to the close links to fish species they depend on to reproduce.

That dependency on fish for the future of their species is one of the most important yet least understood parts of their biology. That is where the students, from Grantsburg High School teacher Matt Berg’s biology class, c0me in. They were assisting mussel researcher Mark Hove, of the University of Minnesota, who runs a small lab filled with aquariums in St. Paul, buried in the basement of the College of Biological Sciences. 

Between that dark room filled with glass tanks, which were occupied by fish and mussels, water steadily burbling through pumps, and the rushing Willow River and its fresh green banks one morning in early May, Hove sought answers about the creek heelsplitter and its dependence on certain fish.

Berg and his students met Hove at a bridge over the Willow near Clear Lake, Wisconsin, helping collect fish specimens with a seine net. It took at least four people to drag down the river, and another five to stand downstream and block fish trying to swim away.

“I literally could not have done this today without their help,” said Hove. The students will even be listed as co-authors on published scientific papers.

Solving mussel mysteries

Hove and Berg were capturing the fish to determine what species the creek heelsplitter prefers for raising its young. Mussels use fish as hosts for their larvae, attaching as many as possible to the fish’s gills, where the hitchhikers grow and eventually disembark as juveniles. Lab experiments had indicated the creek heelsplitter prefers particular minnows and suckers, but it needed to be confirmed in wild fish.

The varied and complex ways that mussels fool fish into picking up their eggs illustrates the intensity of the relationship. For example, anglers wanting to know what kind of walleye lure is produced by millions of years of evolution can look at the black sandshell. Its lure was basically designed by the fish themselves. They chose the lure that most looked like lunch, and that propagated mussels with that version.

“Mussels have three neurons,” Hove says, “so the lure is the product of walleye selection.”

Another species looks like a rock, and when its preferred host fish, the log perch, comes along nosing into the bottom, turning over rocks to find food, it bumps into the mussel, which ejects its larvae.

The relationship goes both ways. Mussels help fish by filtering water, and can be an important food source.

These relationships between many of the 41 species of mussels found in the St. Croix River watershed, are poorly understood.

“We know the infestation strategy for maybe a third of mussel species,” Hove says.

That makes it difficult to know which species are doing well and which species are not. If it’s found that a species is doing better than previously known, it could even mean removing it from the endangered species list. Or if the relationship with an endangered species’ favored fish species is learned, it could inform a plan to restore its population and get the mussel off the list eventually.

Heelsplitter hot-spot

The class is part of this research because of its teacher. Berg is not only a classroom educator, he owns and operates a business called Endangered Resources Services, where he contracts with public agencies and others to survey plants, animals, and water. He previously worked for the Saint Croix National Scenic Riverway. He has a deep knowledge of the biology of the St. Croix River and its landscape.

This means he knows where creek heelsplitters live. By collecting the fish also living there and later seeing what species of mussel larvae they were carrying, the scientists would find clues to the puzzling connections between fish and mussels.

They would do this the hard way. The fish would be transported back to Berg’s classroom, kept alive in aquariums, and the larvae would be reared so the species of mussel could be identified. It was a hit-or-miss strategy, hoping the fish were carrying a species of interest, but since the larvae are nearly impossible to identify, it is really the only way. And it works.

The school’s participation is also essential because Hove can’t even bring the fish in his lab, due to laws about transporting live fish across state lines. Berg needed to take them back to the high school, and he said he had plenty of aquariums to fill.

Casting a wide net

The students weren’t in the river for long before they seemed to forget any concerns about fashion. They were kneeling in mud to retrieve fish from the net, or peering on intently as Berg showed them freshwater sponges and caddisfly cases on the bottom of rocks.

Hove marshalled a team of net draggers, who pulled the net through three sections of the river, capturing writhing pile of silvery minnows, with a couple larger suckers. While they dragged, the rest of the group stood in a line across the river, kicking our legs, flushing the fish toward the net.

The wriggling catch was just what Hove had to have to verify other results.

“Heelsplitters seem to prefer minnows and suckers, but that’s from the lab, and we need to test it in the field,” he said.

The students needed to be back for afternoon classes, but after two passes, they had 15 more minutes before they needed to hit the road.

“How about one more pass?” Hove said.

The whole group hiked up another 50 yards and once more corralled the fish into the net. As soon as they brought it to the surface: jackpot. They had caught white suckers, which Hove had seen hosting heelsplitters in his lab, but never in the wild.

The fish were dumped in buckets, the river researchers splashed back to the bridge, lugged everything back to the van, and took off back up the highway. The fish would later be identified and sorted by species, put into separate aquariums, and the team would wait to see what mussels came along for the ride.