Over the course of the last four months, PhD student Holly Embke and her team of undergraduate researchers have systematically removed more than 80,000 bass and panfish from McDermott Lake in southern Iron County as part of a whole-lake experiment studying the way those species interact with the lake’s declining walleye population.
But in a lake with nearly 20 species of fish alone, not to mention rich invertebrate and plant communities, Holly’s walleye experiment will inevitably create changes that will ripple across the entire food web.
“Since you’re doing an experiment on such a large scale, you can answer a lot of questions at once,” Holly said. “It is so dynamic, there is so much going on, and you don’t necessarily know what is going to happen because it is a natural system.”
So how will removing a group of fish that essentially compose the middle of the lake’s food web affect the rest of the McDermott ecosystem? Holly’s undergraduate researchers took advantage of an opportunity to conduct their own independent research projects to find out.
Before they left Trout Lake Station to head back to to school for fall semester, I sat down with Holly’s summer team of undergraduate field techs – Levi Feucht, Abbie Dalton, Keegan Eeping and Matt Chotlos, They told me about their projects and gave me a glimpse inside McDermott Lake’s changing food web.
Approaching his research with the intent to answer questions about competition for resources, Matt started near the bottom of the food web and analyzed zooplankton abundance. Matt describes these miniature aquatic species as “shrimp, but almost microscopically small, moving up and down in the water column.” They are a primary food resource for young fish and their abundance in a lake can indicate the health of species much higher in the trophic structure.
This is Matt’s second year working for Holly. During the summer of 2017, he helped her with the initial monitoring phase of her project. I was friends with Matt long before coming to station, and the enthusiastic way he talked about his work is one of the things that drew me to Trout Lake.
He compared zooplankton abundance to catch rates of larval fish over the course of the summer and found that those catch rates spiked following increases in zooplankton, and that as catch rates increased, the availability of zooplankton decreased. After his team removes most of the bass and panfish from the ecosystem, Matt predicts that there will be more resources available for young walleye.
“Moving forward, if we remove a lot of these young fish from the system, that will free up this food resource,” Matt explained. “Potentially fish like walleye, that we haven’t removed from the lake, will have a more abundant food resource, as there are less of these young of year panfish chewing up all of these plankton.”
Minnows are among the fish that feed on those zooplankton. Keegan, a rising sophomore from the University of Minnesota, explained that he studied bluntnose and central mud minnows, and how their catch rates changed during the summer.
“It’s not actually a minnow,” Abbie interjected. “The name implies that it’s a minnow but it’s not a minnow. Minnows fall into a certain family and mud minnows are in their own family, umberdae. They’re not classified as minnows.”
“Ok, thank you for that,” Keegan jokes. “The central mud fish. Both species enjoy littoral habitats, or very shallow waters.”
Keegan wanted to model the correlation between temperature and catch rates for those fish. He found that as the summer progressed, more and more of them wound up in the traps his team set around McDermott Lake. But instead of temperature, Keegan thinks the trend may be more related to ongoing spawning in the lake.
“The water temperature didn’t change that drastically, which is why I didn’t really find a strong relationship between the two,” Keegan said. “It seemed that toward later July, there was a large spike in bluntnose minnows. But rather than temperature, I linked that to the fact that they were spawning at that time, so there were a lot more young fish in the lake that we were catching.”
Yellow perch feed on those minnows and other juvenile fish. Their young, in turn, are prey for fish like bass and walleye. Levi wanted to see if removing the perch’s predators would result in an increasing perch population over the summer.
Levi’s the type of person who will spend his day sorting fish, get off work and go fishing again. His perpetually sunburnt shoulders are a testament to the long hours he’s been pulling on the lake this summer.
He found that catch rates for yellow perch did in fact rise gradually as the summer progressed, confirming his hypothesis.
“My best theory is that the spike was from increased resources and spatial pressure,” Levi said. “There was still a gradual increase over the amount of catches we were getting through the year, showing that as we were removing a lot of other competitors that they had, the yellow perch was able to take off.”
Largemouth and rock bass, two species targeted by Holly’s removal, feed on juveniles of all species. Abbie wanted to know how the removal of so many young fish would affect these predators’ diets.
A rising fifth year at UW-Madison, Abbie is the oldest student working on Holly’s team. When she talks about fish, you get a feeling she is talking about close friends.
She found that the fish in her study ate more young fish this year than they did last year, which she attributes to the removal.
“I figured that that was due to the removal because schooling would be much less likely,” Abbie explained. “It’s much easier for a predator to target one individual fish than to try to distinguish multiple fish in a school.”
Of course these species will not be the only ones impacted by Holly’s removal. But as Abbie pointed out, these individual studies are like puzzle pieces that, when assembled, will reveal the full image of a lake undergoing intense ecological change.
Holly will continue to monitor the entire McDermott system for at least another three years, and she said that at this point she doesn’t fully know what to expect.
“We don’t know what’s going to happen, so by looking at all of these different things, we might catch something we weren’t expecting at all, and that opens up a whole new realm of questions,” Matt said. “Because we don’t really understand what’s going to happen, we don’t want to miss these changes. We are throwing a wide net to catch as much as possible.”