Fisheries research

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 being netted, measured and tagged for this study.

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.

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.

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.

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.

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.