Zebra mussels


The people behind the projects: Holly Kundel — impacts of zebra mussels on walleye


Most of you have heard of scientific researchers, you may even know one personally, but what exactly do they do? As a scientific researcher myself, I am excited to share a behind-the-scenes look at my work as a MAISRC Graduate Fellow.

But, before I start explaining my project and work at MAISRC, let me tell you a little bit about myself. My name is Holly Kundel and I just completed my first year in the Conservation Sciences graduate program at the University of Minnesota. I’m from Forest Lake, Minnesota

and I’ve always enjoyed the outdoors. It wasn’t until I went to Augsburg University in Minneapolis that I realized that I could make a career for myself that wove together my favorite topics of biology and the outdoors. While at Augsburg, I learned how much I enjoy doing fieldwork while sampling for dragonflies in ponds in the St. Croix River Valley. My research experiences as an undergraduate and my passion for freshwater ecology lead me to Dr. Gretchen Hansen’s lab at the University of Minnesota where I joined MAISRC. 

MAISRC Project Background and Goals

My project with MAISRC hopes to quantify the impact that invasive zebra mussels have on walleye recruitment. Recruitment refers to when walleye enter the fishery, meaning they are now large enough to be caught by anglers. 

Zebra mussels, an aquatic invasive species, are not native to Minnesota waters. However, once they are introduced, they rapidly multiply, coating hard surfaces on the lake bottom and equipment such as docks, boat lifts, and even intake pipes. Although they are tiny, zebra mussels can have profound impacts on lakes because they filter the very small plants which are called “phytoplankton” out of the water. Due to zebra mussels removing phytoplankton from the water, lakes that are invaded may begin to appear clearer as phytoplankton numbers diminish. Although we may enjoy clearer lakes, less phytoplankton can mean that organisms higher on the food chain have less to eat.

The tiny animals that feed directly on the phytoplankton are called zooplankton. When there is less phytoplankton, zooplankton numbers can also drop due to not having enough food. For smaller fish that feed upon zooplankton, including very young and small walleye, this means that they also may not get enough food as well. It is easier to see how the tiny phytoplankton and zooplankton can be impacted by zebra mussels, but the impacts that zebra mussels have on predators in the lakes, such as adult walleye aren’t as well understood. In the Hansen lab, we hope to learn how zebra mussels impact the ecologically, culturally, and economically important state fish, the walleye. 

A Day in the Life: Research Tasks

In order to learn about the impacts of zebra mussels on walleye, I am using data from lakes throughout Minnesota that are both invaded and non-invaded. In addition to whether or not a lake has zebra mussels, we also use data on walleye at all different ages and sizes. This data comes from surveys done at the Minnesota Department of Natural Resources. To learn about the state of a walleye population within a lake, there are two main surveys that researchers use in Minnesota: for the younger walleye, we typically use electrofishing, and for the older walleye, we use data from gill nets.

Electrofishing is carried out in a specialized fishing boat with anodes that can be lowered into the water. The anodes run an electric current into the water which causes the fish in the vicinity to be temporarily stunned and float to the surface of the water. Researchers can then easily net the stunned fish from the bow of the boat. Electrofishing surveys that are done in the fall often try to target “young of the year” walleye, meaning walleye that hatched that spring. The surveys can inform managers if walleye are successfully reproducing in a lake and if their young are surviving their first summer. 

Once walleye are older, they are sampled in gill net surveys. Gill nets are large nets with varying sizes of clear mesh. The nets sit vertically in the water so that the fish swim into it and become caught. Walleye typically aren’t caught in this gear until they are three years old. Data from electrofishing and gill nets can also be used together to track cohorts of walleye over time if the fish are aged.

Have you ever wondered how to age a fish? Well, it isn’t easy. Hard structures on fish such as their scales or otoliths (their ear bones) develop rings as they age, sort of like how trees develop rings each year. These rings are then counted by an experienced professional under a microscope to estimate a fish’s age. Once enough fish of varying sizes are aged, we can then use statistics to estimate their ages based on their length instead of the bony structures, which are more time consuming to process. 

When we combine 1.) the information we have on catch rates of walleye at different ages with 2.) information of when zebra mussels became established in a lake, we can use statistics to learn if changes have occurred in walleye catch rates at different ages/sizes once zebra mussels invaded. We can also use information from uninvaded lakes as a baseline to compare invaded lakes. For example, if we find that both invaded and uninvaded lakes have declining walleye catch rates, then the declines are likely caused by something other than zebra mussels and could be due to factors such as increasing water temperatures. However, if uninvaded lakes’ catch rates remain relatively stable over time, then zebra mussels may be playing a role in declining walleye populations. But, as you know, Minnesota has a wide variety of lakes, and not all lakes respond to a stressor such as zebra mussels equally. While we carry out our analysis, we hope to find lakes with zebra mussels that show resilience and continue to do well. Once resilient lakes are identified, we can hopefully learn from them, and possibly uncover strategies that can be applied to other invaded lakes, to help them persist over time. 

Whether you are passionate about walleye fishing, keeping Minnesota’s lakes healthy and free of invasive species, or helping lakes that have become invaded, I hope that my project with MAISRC will help provide some answers. Learn more about this project on the MAISRC website

Funding for this project was provided by the Environment and Natural Resources Trust Fund as recommended by the Legislative and Citizen Commission on Minnesota Resources.

Holly Kundel

Holly Kundel, Conservation Sciences Graduate Student and MAISRC researcher.


Gill net

The research team sorts through a gill net for young fish. (Photo taken Aug. 14, 2019)


Zebra mussels

Zebra mussels attached to a native mussel.



A zooplankton sample in a small vial.