Fisheries research

Effective management of Minnesota’s fisheries relies on access to the best available science to inform decisions. The fisheries research unit consists of researchers and support staff who work in area fisheries offices statewide.

Staff conduct applied research that evaluates management practices; develops new management tools and sampling techniques; and anticipates responses of fisheries to new and existing stressors.

Highlights

Sturgeon movement in Lake of the Woods

DNR in cooperation with the Ontario Ministry of Natural Resources are exploring where and when lake sturgeon move in the Lake of the Woods-Rainy River system.

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Southern walleye strain persists

A genetically distinct walleye strain found in southern Minnesota persisted despite stocking walleye of other strains in southern Minnesota lakes for decades.

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What are predator fish eating?

A project looking at the diets of muskellunge, walleye, northern pike and largemouth bass in Minnesota continues.

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Big cats inhabit the Minnesota

A three-year study of Minnesota River flathead catfish between Mankato and Shakopee showed an abundant population of large fish topping 20 inches.

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Project archive

• Are perch getting smaller?
• Invasive species: unexpected impacts
• Monitoring shows healthier, happier trout

Publications

Investigational reports: Minnesota DNR fisheries section publications for research studies that have more local or regional management implications.

Special publications: DNR manuals, lake management plans, guidelines or reviews.

Peer-reviewed publications: (PDF) Research studies that are published in scientific journals and have gone through a rigorous peer review process. Results of these studies influence fisheries management beyond Minnesota. Copies of these reports can usually be received by contacting the author.

Staff

Fisheries research unit staff have a broad range of skills and expertise including population and fish community dynamics; bioenergetics and food web dynamics; genetics; stock assessment; statistics; fish habitat; assessment methodologies and hydroacoustics; watershed and landscape ecology; and long term monitoring.

Name	Position	Location
Tyler Ahrenstorff	Research scientist	Brainerd
Hadley Boehm	Research supervisor	Duluth
Doug Dieterman	Research scientist	Lake City
Will French	Long-term monitoring biologist	St. Paul
Brian Herwig	Research scientist	Bemidji
Beth Holbrook	Research scientist	Brainerd
John Hoxmeier	Research supervisor	St. Paul
Tim Martin	Long-term monitoring biologist	St. Paul
Kevin McDonnell	Research scientist	Duluth
Loren Miller	Research scientist	St. Paul
Devon Oliver	Research scientist	Lake City
Kristen Patterson	Research scientist	Bemidji
Heidi Rantala	Research scientist	Duluth
Jeff Reed	Research scientist	Glenwood
Patrick Schmalz	Fisheries research manager	St. Paul
Casey Schoenebeck	Research scientist	Glenwood
Steve Shroyer	Research scientist	Waterville
Chris Smith	Research scientist	Glenwood
Kiira Siitari	Fisheries Permit Administrator	Duluth
David Staples	Biometrician	St. Paul

Contact

Patrick Schmalz, fisheries research supervisor, St. Paul
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tel. 651-259-5231

Big cats inhabit the MinnesotaX

Minnesota DNR fisheries research biologists hold large flathead catfish netted for a three-year study to determine population and size structure in a portion of the Minneosta River.

Just how big are those cats in the Minnesota River?

We’re talking flathead catfish here, not the felines of the house variety or reported mountain lion sightings in this southern Minnesota river valley.

Increased angling pressure on this population of flathead catfish prompted Minnesota Department of Natural Resources fisheries research biologists to conduct a three-year study to answer that question and identify ways to protect and maintain this trophy fishery.

At four sites along the river between Mankato and Shakopee each August, biologists netted fish, measured them, implanted microchips to identify them before returning each fish to the water. The capture and recapture of these fish over those three years allowed researchers to estimate the population and determine size structure, growth and survival rates.

The median size of sampled flathead catfish was estimated at 29.8 inches and most were 5-15 years old. Population estimates determined that there are 144 fish per river mile longer than 20 inches, 80 fish per mile longer than 28 inches and 48 fish per mile longer than 34 inches. These results indicated a very abundant flathead catfish population relative to others nationwide, with a high percentage of large fish.

But the existence of trophy-size fish depended on a 91% annual survival rate that allowed unusually fast-growing individuals to reach old ages Any substantial decrease in that survival rate would put the population’s size structure at risk so conservative regulations are necessary and remain in place to maintain the opportunities these big cats offer river anglers.

See these large fish opens in a new browser tab being netted, measured and tagged for this study.

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Yellow perch: disappearing or getting smaller?X

Map showing lakes included in the yellow perch research project

Yellow perch are more than great table fare. They're an important Minnesota fish species as forage for predators such as walleye and as a targeted sport fish.

The DNR has surveyed yellow perch using standard methods for decades, and the data shows a statewide decrease in catch over time.

Perch are very adaptable to different environmental conditions and populations can look very different from lake to lake, with some containing mostly smaller fish and others having destination trophy fisheries.

The problem is that standard nets used during population sampling only collect yellow perch larger than five inches so it is unknown if overall populations are decreasing or if populations are shifting to smaller individuals. Interestingly, research has shown that in some Minnesota lakes, yellow perch can live out their entire life cycle without reaching five inches in length.

A collaborative research project is currently underway to address gaps in knowledge about yellow perch populations in Minnesota lakes. Additional methods such as small mesh gillnets and boat electrofishing are being used to sample all sizes of yellow perch in more than 25 lakes statewide.

The objectives of this project are to develop methods for more effectively sampling of yellow perch two inches long and larger and describe the range of population types that exist in lakes throughout Minnesota. These results will be used in future research to understand the fish community and environmental factors that shape yellow perch populations.

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Better habitat, catch-and-release pay offX

The Minnesota DNR’s long-term stream monitoring program was initiated to identify population trends, evaluate management actions and test ecological theories regulating trout populations.

Significant increases in brown trout abundance and biomass occurred between 1970 and 2018 in southeastern Minnesota. Sites managed with habitat improvement projects had an additional 30% higher abundance for trout greater than 12 inches and 57% higher for trout greater than 14 inches. Sites with a catch-and-release regulation had nearly 130% higher abundance of trout greater than 12 inches.

Increasing abundance trends represent a fisheries management success and suggest that these populations were largely regulated by in-stream habitat improvement projects as well as improved land-use practices that facilitated greater water infiltration and increased stream flows.

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Understanding change to prepare for the futureX

Minnesota's designated sentinel lakes.

The Sentinel Lakes Program is an intensive, long-term lake ecosystem monitoring program created to detect and understand the physical, chemical and biological changes occurring in Minnesota's lakes.

Along with our partners, we monitor the water quality, aquatic plants, phytoplankton, zooplankton and several aspects of the fisheries community.

Ultimately, the data gathered from monitoring will be used to help us understand and develop management approaches that can mitigate or minimize impacts caused by large-scale stressors such as climate change, land use patterns, shoreline development and invasive species.

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What are predator fish eating … and how much?X

Map shows the location of study lakes for 2019-2023. Blue indicates lake with stable isotope sampling only. Orange indicates lakes with both stable isotope sampling and stomach content sampling of top predator fish. Yellow indicates lakes with stable isotope and stomach content sampling as well as estimates of population-level consumption by top predators.

Muskellunge, walleye, northern pike, and largemouth bass are top predator fish in many Minnesota lakes. What and how much those predators eat, and how their diets compare are the focus of an ongoing fisheries research project. The DNR’s Fisheries Research Unit, in partnership with the University of St. Thomas and Bemidji State University, have been sampling lakes since 2019 for the project.

We are using three approaches to examine consumption patterns of these top predators in Minnesota lakes. First, we are directly examining stomach contents of muskellunge, walleye, northern pike, and largemouth bass during the spring, summer, and fall in 12 lakes. It is as simple as catching the fish, flushing the stomach out with lake water, and releasing the fish unharmed. Second, we are analyzing stable isotope ratios of tissue samples from fish and invertebrates in 20 lakes. This technique provides additional evidence of who is eating who in each lake because predator isotope signatures reflect those of their prey in a ‘you are what you eat’ fashion. Lastly, in three lakes, we are combining diet information with estimates of predator population size to examine how much of each prey species is consumed by each predator population.

Preliminary results indicate that muskellunge populations consume less food than the other predators in a lake, because they are typically much less abundant. They also consume different prey resources such as white sucker, northern pike, and bullheads. While not consistent in every lake, walleye and northern pike appear to have the highest amount of diet overlap, with yellow perch and sunfish being their favorite prey in many lakes. Lastly, largemouth bass love to eat crayfish and other invertebrates in most lakes when they are available, but sunfish were also important in some lakes.

The results of this work will better inform the public, fisheries staff, and policy makers about the ecological role and prey demands of muskellunge, walleye, northern pike, and largemouth bass. It will also be useful for fisheries managers when considering stocking plans or possible regulation changes for any of these top predators in lakes throughout Minnesota.

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Multiple invasive species: unexpected impactsX

A spiny water flea perched on a human fingertip

Water clarity is often used as an indicator of water quality and is an important component of fish habitat.

Zebra mussels are an invasive species well known for their ability to increase the clarity of lakes they invade by filtering algae out of the water column. Another invasive species, spiny water flea, have been shown to indirectly decrease water clarity in some lakes by eating native zooplankton that feed on algae.

Given these potentially interacting traits, we were interested in their combined impacts on water clarity. To investigate, we partnered with the University of Minnesota-Duluth to study of Lake Mille Lacs, a lake that has been invaded by both invasive invertebrates.

After zebra mussels were found in Mille Lacs in 2005, DNR fisheries biologists added frequent monitoring of both the zebra mussel population and the zooplankton community to their survey activities. We used these data, along with water chemistry and clarity data (including data collected by community scientists), to understand the effects of zebra mussels and spiny water flea on water clarity.

We found that while late spring water clarity is greater in the presence of zebra mussels and spiny water flea than before these invaders got into Mille Lacs, increased water clarity does not continue later into the summer. This lack of change in summer water clarity from zebra mussels was a result of the predation effect of the spiny waterfleas on the native zooplankton.

We found that total consumption of algae by the lower native zooplankton population was approximately equal to the amount of algae that were filtered from the water column by zebra mussels.

Although the water clarity impact of these two invasives was offset, both of these invasive species have large impacts on lake food webs, including in Lake Mille Lacs, these relationships can be complicated and produce unexpected outcomes.

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Sturgeon movement in Lake of the WoodsX

A spiny water flea perched on a human fingertip

The DNR in cooperation with Ontario Ministry of Natural Resources is tracking Lake sturgeon movements throughout the Rainy River-Lake of the Woods system.

Seventy lake sturgeon 17-70 inches long have been implanted with internal transmitters and their movements are being tracked by 28 receivers. The receivers will detect and record when a tagged fish swims near it.

The line on the map shows the movements of a 53-inch sturgeon that was tagged near Garden Island in June 2024 and moved up the Rainy River one week later.

Yellow buoys like the photo at right can be seen throughout Lake of the Woods during the open water season. They are marking the location of a receiver and should not be moved or disturbed.

These buoys are in approximately 10 feet of water near the Lighthouse Gap, Morris Point, Rocky Point, Stony Point and Garden Island.

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Southern walleye strain persistsX

Electrofishing for walleye in a snowstorm

A genetically distinct strain of walleye persists in southern Minnesota lakes despite decades of stocking more easily obtained strains from outside the region.

Because of the regional performance advantage inferred by this persistence, Minnesota DNR fisheries researchers conducted experiments to compare performance and stocking cost of the southern Minnesota walleye fry against a frequently stocked northern Minnesota strain. Both southern and northern strain walleye were marked with oxytetracycline (OTC) and stocked into six southern Minnesota lakes.

Sampling in subsequent years was conducted to compare growth, maturity and survival between the two walleye strains. The cost of producing each strain was also estimated and compared. The southern Minnesota strain had a higher production cost but also survived better to their first fall, making them more cost effective compared to the northern Minnesota strain.

These results show that a local strain can outperform non-local strains at a level that can overcome additional costs of developing a new egg source. The research also showed that natural reproduction occurred in southern Minnesota and that it may reduce the need for future stocking.

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