Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

Gretchen Hansen and Laura Conner working on Sparkling Lake. © Katie Hein

Gretchen Hansen and Laura Conner working on Sparkling Lake.
© Katie Hein

October 2014

Flipping lakes

An experiment turns students into scientists and moves one lake from an invasive to a native state.

Adam Hinterthuer

In the summer of 2002, Gretchen Hansen and Katie Hein were spending long days together reaching into buckets full of writhing rusty crayfish and carefully pinching the carapace of the captives right at their identifying rusty spots — the precise place where human fingers are safe from crustacean claws. It was their first season together in the field, and they spent it trolling the shoreline of Sparkling Lake, a 150–acre lake in Vilas County, in a rickety old Army boat. Their job was to retrieve the more than 100 minnow traps packed with invasive rusty crayfish that had ventured into the cages overnight, then set the traps back in the water to do it all again the next day. By the end of their workdays counting and measuring crayfish, their shoulders would be baked by the summer sun and their hands pinched, poked and smelling of the dead fish used to bait the traps.

Hansen couldn't have been happier.

"I couldn't believe people got paid to do jobs like this," she remembers. "I had been a pre–med student, but once I realized I could do science out on the lake, I changed my mind."

What's more, their daily catch often ended up as that night's dinner. The two women weren't about to toss the invasive species back in the lake, and besides, they'd found a few delectable recipes to add into the rotation along with the more traditional crayfish boil.

Hansen was then a junior at the University of Wisconsin–Madison, while Hein had just started work on her master's degree. They both hoped that all of this trapping and sorting and steaming and shelling would not only restore Sparkling Lake, but also help them get their degrees and launch their careers. What they didn't know, is how similar those arcs would be.

Rusty crayfish are just one of a long list of aquatic invasive species that have made their mark on Wisconsin waters. They first appeared in the state in the 1960s, and are now widespread in lakes and rivers across the state and Midwest. In some lakes, rusty crayfish reach exceptionally high densities, where their voracious foraging decimates aquatic plant populations. This loss of plant cover then leads to declines in aquatic invertebrates and panfish like bluegills and pumpkinseed.

Sparkling Lake saw this effect firsthand. During Hansen and Hein's summers, the bottom was crawling with rusty crayfish, while aquatic plants, snails and panfish were sparse. Lake residents complained that their grandchildren would not swim for fear of their toes encountering crayfish claws.

As is the case in many lakes and rivers, Sparkling Lake's future seemed uncertain. The harsh reality for many of Wisconsin's waterbodies is that management options are limited and often focused on preventing the spread of invasive species, not removing them once they are there. That's because eradication, or completely eliminating the species, is possible only in select situations and generally involves tremendous effort and cost while controlling, or reducing invasive abundance, rarely reaches a point where invasives can't bounce back once management pressure is gone.

It was a depressing prognosis, but Sparkling Lake did have one thing going for it — it was part of the National Science Foundation–funded Long–Term Ecological Research (LTER) program. The North Temperate Lakes LTER site was established in 1981 by the University of Wisconsin–Madison Center for Limnology (CFL). Sparkling Lake is one of the LTER study lakes, meaning that the explosive population growth of rusties and the resulting decline of aquatic plants, snails and fish had been well–documented in Sparkling Lake for two decades. And that data provided the perfect backdrop for testing whether rusty crayfish could be controlled and their undesirable impacts minimized, or even reversed.

In 2001, the CFL and Department of Natural Resources embarked on a long–term experimental removal of invasive rusty crayfish in Sparkling Lake, allowing them to test an emerging ecological idea in real life. Some mathematical models of lakes show the possibility for a lake to be permanently "flipped" from one state to another as long as an ecological "feedback loop" is in place.

"We wanted to test whether we could lock Sparkling Lake into a new state where plants, snails and panfish thrive and rusty crayfish are rare," says Jake Vander Zanden, a professor at the CFL and one of the experiment's principal investigators. "In other words, we wanted to replace the ‘vicious cycle' with a virtuous one."

In Sparkling Lake, the feedback loop looked like this — panfish like bluegills and pumpkinseed can eat enough young rusty crayfish to keep crayfish numbers low, but those fish don't thrive if rusties destroy the aquatic plant beds where they live and spawn. If researchers could remove rusty crayfish to the point where aquatic plants returned, panfish populations might be able to rebound and take it from there.

UW–Madison researchers and the Department of Natural Resources put their hypothesis to the test from 2001 to 2008, changing fishing regulations to keep larger predators of crayfish in the lake and intensively trapping rusties each summer.

"We would pull up several hundred crayfish traps daily," Hansen remembers. "We released the few native ‘virile crayfish' we caught back into the lake and removed the rusty crayfish — sometimes up to hundreds per trap."

As a result, crayfish boils became a popular pastime at nearby Trout Lake Research Station, the researchers' home base.

UW–Madison researchers returned to Sparkling Lake each summer for the next seven years to continue their blitz on rusty crayfish. In that time, Hein received a master's degree from UW–Madison, a Ph.D. from Utah State, and headed to Sweden for a post–doctoral position, while Hansen earned her bachelor's degree from UW–Madison, a master's degree from Michigan State University, and made it back to Madison to pursue her Ph.D. and get back out to Sparkling Lake.

In 2008, trapping drew to a close and the lake–wide effects of the rusty crayfish removal were, as Vander Zanden says, "stunning."

Not only had rusty crayfish declined 100–fold, but populations of virile crayfish rebounded 100–fold, resulting in approximately equal abundances of invasive and native crayfish. Furthermore, as predicted, aquatic plants, snails and panfish began to bounce back. Over the short term, at least, the project was a success.

The follow up question, however, was the biggie — what would happen next? Would Sparkling Lake stay in this low rusty crayfish state without intense trapping, or would it be overrun again?

In summer 2013, 12 years after she first encountered Sparkling Lake, Hansen dove back in. Suited up in her SCUBA gear and working with Ali Mikulyuk, a CFL Ph.D. student and research scientist with the Department of Natural Resources, Hansen helped lay white PVC grids on the bottom of the lake to survey plants growing in each transect. It was obvious that Sparkling Lake was still flipped to a more native–friendly state.

"There were plants everywhere," Hansen says. "What had been basically mud flats eight years earlier were now filled with diverse species of plants and all kinds of fish."

Sparkling Lake was clearly still in the low rusty crayfish state and fish like bluegill and smallmouth bass appeared to be eating enough rusties to prevent them from taking over again. But only time and continued monitoring will tell how long this will last.

Still, the good news raises a question dear to lake residents and resource managers alike — could other rusty–infested lakes in Wisconsin be "fixed" like Sparkling Lake? While it took a dedicated team of students and scientists eight years to remove more than 91,000 rusty crayfish from the lake, the idea of replicating the effort isn't outside the realm of possibility. It depends on resources available to support such a high level of trapping over several years. But the benefits in terms of lake restoration are potentially large, and would provide valuable scientific insight as to whether the "flipped lake" theory can transfer to other lakes.

For today, at least, Sparkling Lake is as beautiful below the surface as any lake in Wisconsin, a product of an experiment that provided insights into how invasive species can be managed in the future, and demonstrated the connectedness of natural systems and the value of understanding links between habitat, native species and healthy ecosystems.

But the experiment wasn't just a success for Sparkling Lake. Hansen and Hein "flipped" states as well. Both women are still working to understand and protect Wisconsin's waters, but now they're professional scientists instead of students. Hansen is a DNR fisheries research scientist and Hein is DNR's statewide water quality monitoring coordinator —careers made possible, at least in part, by that first summer job with the UW–Madison Center for Limnology, an old Army boat and buckets full of rusty crayfish.

Adam Hinterthuer is an outreach/communications specialist for the University of Wisconsin–Madison Center for Limnology.