by Sydney Widell – Holly Embke’s Subaru is nearly packed and, besides a few boxes, her desk is vacant. In a few hours, she’ll be leaving Trout Lake Station and driving home to Madison – another field season behind her and a summer’s worth of data to process.
Which means that Holly’s job is just getting started, because that dataset is the result of the 80,000 fish she and her team pulled out of a single lake over the last four months.
Holly is a PhD student working on freshwater lake ecosystems at the UW-Madison’s Center for Limnology. She’s spent the last two summers in northern Wisconsin leading an intense study on the region’s declining walleye populations.
Over the last 15 years, studies have shown that fewer and fewer young walleye are surviving to adulthood and reproducing — something people in the field call “poor recruitment.” At the same time, warm water centrarchid species, like bass and sunfish, have become more abundant.
Limnologists attribute the decline to a number of factors, like northern lakes warming due to climate change, tipping the scales in favor of warmer water fish species. Holly is curious if interactions between walleye and other fishes could be another part of the story.
“Bass and panfish are thought to be competing with walleye in their really early life stages — either competing directly with them or even predating, or eating, them.” Holly said. “There is potentially an interaction between those very tiny walleye and sunfish and bass that stops walleye from getting to that next juvenile stage.”
With that possibility in mind, researchers at UW-Madison, UW-Stevens Point and the Wisconsin Department of Natural Resources wondered how walleye would fare in a lake with hardly any bass or panfish at all. So, this summer, Holly and her team of undergraduate researchers embarked on the extraordinary task of creating such a lake – by physically removing the bass and panfish.
They chose McDermott Lake in southern Iron County for their experiment. McDermott is a small lake — only 100 acres — but Holly has calculated that her team will have to remove up to 80 thousand bass and panfish from the water, or roughly 80 percent of their current population.
And eighty thousand fish later, I’m sitting across the table from Holly in the airy Trout Lake Station library. A breeze that smells like pine sap and motor oil drifts in the open windows, and shifting patterns of late summer sunlight dapple the floor.
“Eighty thousand is a lot of fish, ” Holly said. “Like what does that number even mean? To think that we’ve surpassed eighty thousand is really wild to me in a great way.”
That’s more fish than there are people in Eau Claire, Holly’s hometown.
The fish Holly is removing are mostly “young of the year” or in their first season of life. She freezes every fish she takes from the lake and donates them to wildlife refuges across the state.
“Over the course of the last year, when I thought about this project and the size of the lake, I thought, ‘Oh, this is manageable. It’s a big undertaking but it’s manageable and we can do this,’” Holly said. “Then I remember getting to McDermott this summer and looking at the lake and thinking, ‘How are we ever going to do this?’”
But Holly pushed her worries aside, and little by little, her team’s efforts added up.
Holly and her undergraduate assistants have been removing fish since early May, when the ice had just melted off their test lake. Day in and day out, they made the hour-long drive to McDermott, set their nets and traps and processed their catches.
“We created our own little world, where stuff like getting a bass electrofishing, which was kind of rare, became really exciting. People would be so excited and come back and be like ‘wow, we collected thirty bass this run!”’ Holly laughed. “It was just pulling fish out of a lake nonstop and it became a really cool thing.”
Between the fish she observed during rounds of baseline sampling this summer and the summer before and the fish she removed over the last few months, Holly will be able to generate an estimate of how many bass and panfish were in the lake to begin with, and what percent of those original populations she has removed.
“That’s the thing I need to do this fall,” Holly said. “We’ll get together as a team and see where we are relative to those targets and then plan for the future. That might mean more removals next summer or it might mean just monitoring from here on out.”
So will adult walleye now be able to produce eggs in McDermott that will become fish that make it to adulthood? Only time will tell. And that means at least another three years of field work monitoring McDermott. Holly will keep her eye on the walleye population, but she plans to monitor other species — from game fish down to invertebrates — as well as the lake’s physical characteristics, like water chemistry.
At this point, she said she doesn’t really know what to expect, and that the summer has already been full of surprises.
“At the start of the summer, we thought we had a good idea of the species composition of the lake and abundances of different individual species. but we’ve added at least 5 additional species to that list, just because we have been out there putting in so much effort and handling so many fish,” Holly said.
For example, Holly removed over 50,000 bluegill this summer alone, but only caught three white suckers. Disparities like that open the door to future studies, but only become visible when researchers invest enormous amounts of time and effort into their sampling.
“When we’re out there all the time, we’re gaining an understanding of the other dynamics on the lake,” Holly said. “We have so much data and so much information that I think it presents some new opportunities that I hadn’t really anticipated when I was thinking about answering a food web question or a walleye question.”
That information will help researchers begin to trace the perimeters of what Holly calls a “safe operating space” for walleye, or the conditions that allow them to survive despite combined impacts of different environmental stressors.
“There are factors you can’t control as a fisheries manager, and those would include climate change or even habitat changes,” Holly explained. “But there are smaller things within your control, like harvest of walleye or other species.”
For ecologists and recreational walleye enthusiasts alike, understanding what those pressure points are is crucial to maintaining healthy walleye populations in a warming Northwoods.