Climate change and freshwater fish

Lakes in Wisconsin are getting warmer, and fish communities are changing as a result.




Understanding recent trends and anticipating future changes can help decision-makers protect resilient populations, adapt to new conditions, and effectively communicate realistic expectations.



Walleye populations in many Wisconsin lakes are declining,

while largemouth bass populations in many lakes are increasing

Walleye reproduction is declining, while largemouth bass relative abundance is increasing from 1990 to 2015. Walleye reproduction is declining, while largemouth bass relative abundance is increasing from 1990 to 2015.

Statewide trends for walleye natural reproduction (blue; indexed as young walleye surviving their first summer caught per mile of electrofishing) and largemouth bass relative abundance (red; indexed as largemouth bass greater than 8 inches caught per mile of electrofishing, presented on a standardized scale). Dashed lines show overall directional trend over time.

Walleye and largemouth bass thrive in different types of lakes

Water temperature influences the suitability of a lake for walleye and largemouth bass


Can you draw the relationship between these two fish and lake temperature?


Lake temperature in this study is measured as growing degree days (GDD).


GDD are a measure of cumulative temperature over the entire open water season of a lake (from ice-out to ice-on). Higher GDD means warmer water, lower GDD means cooler water. In this case, we used a base temperature of 5 degrees C (41 degrees F). We calculated GDD for each lake and each year: for each day that the water temperature was above 5 degrees C, subtract 5 from the water temperature (in C) for that day. GDD is the sum of all of those numbers for the entire year.

What are growing degree days?

We're giving you a hint to help you fill out the pattern for Walleye and Largemouth Bass. Walleye are most likely to reproduce in cooler lakes. Largemouth Bass are more likely to reach high abundance in warmer lakes, so their line looks like this: . It's up to you to figure out what happens to walleye in warmer lakes.

Probability of walleye is higher than largemouth bass in cooler lakes and lower in warmer lakes.

Predicted probability of a lake supporting successful walleye reproduction and high largemouth bass abundance as a function of water temperature (growing degree days). Probabilities are based on a statistical model that incorporates other lake characteristics such as water clarity and lake size.

Lakes in Wisconsin have gotten warmer over the past 30 years

Map showing lake warming as change in degrees Celcius for 2361 Wisconsin lakes from 1980 to 2014.

Lake warming since 1980 across the studied lakes of Wisconsin. The rate of warming is variable across lakes and depends on geography and the type of lake (clear or dark, large or small). Warming shown here is based on modeled lake temperature for 1980 to 2014.

Lakes are expected to get even warmer in the future

From 2000 to 2090, global average air temperatures are predicted to rise about 2.5 degrees Farenheit in a low emissions scenario and over 5 degrees Farenheit in a high emissions scenario.

Global average air temperatures predicted under two emission scenarios, A2 and B1. The A2 (greater greenhouse gas emissions) was used for modeling future lake habitat for this project. Variability among Global Climate Models for each scenario is shown as a band around the line.

Water temperature growing degree days are predicted to increase for Trout Lake, Green Lake, and Lake Mendota for 2040 to 2064 and again for 2065 to 2089.

Predicted water temperature growing degree days in three example Wisconsin lakes from current conditions to mid-century and late-century conditions based on projected climate change. Lakes will respond differently to climate change depending on their size, depth, water clarity, and starting temperatures, as demonstrated by the different trajectories of Lake Mendota, Green Lake, and Trout Lake.

In the future, many lakes in Wisconsin are expected to become

less suitable for walleye and more suitable for largemouth bass

Lake Count

Total Lake Area

Present and future count of lakes for each type of species dominance: walleye dominant, bass dominant, co-existance, or neither. Current distribution is 184 walleye lakes, 41 co-existance, 1236 bass, and 687 neither. Predicted distribution for 2040 to 2064 is 25 walleye lakes, 58 co-existance, 1857 bass, and 208 neither. Predicted distribution for 2065 to 2089 is 17 walleye lakes, 59 co-existance, 1961 bass, and 111 neither. Present and future area of lakes for each type of species dominance: walleye dominant, bass dominant, co-existance, or neither. Current distribution is 326,183 acres walleye, 20,025 co-existance, 187,975 bass, and 217,207 neither. Predicted distribution for 2040 to 2064 is 47,813 acres walleye, 226,134 co-existance, 414,400 bass, and 63,043 neither. Predicted distribution for 2065 to 2089 is 41,342 acres walleye, 233,207 co-existance, 444,916 bass, and 31,923 neither.

Number or acreage of lakes classified by their ability to support walleye and largemouth bass predicted species dominance under contemporary (1989-2014), mid-century future 2040-2064), and late-century future (2065-2089) conditions. Toggle the button to switch between lake count and total lake area. Hover over the figure to see the number of lakes or acreage moving between categories. Lakes are classified based on their predicted probabilities of walleye recruitment and largemouth bass relative abundance. Classes are defined as: walleye lakes (blue; walleye recruitment success and low largemouth bass relative abundance), coexistence (purple; walleye recruitment success and high largemouth bass relative abundance), largemouth bass lakes (orange; walleye recruitment failure and high largemouth bass relative abundance), or neither (grey; walleye recruitment failure and low largemouth bass relative abundance). Colored lines show projected movements among lake classes, and line width is proportional to the number or area of lakes moving between each class. Values are based on median projections across multiple global climate models.


Explore lake-specific predictions of supporting walleye and largemouth bass!

Link and preview of map showing distributions of species dominance in Wisconsin lakes

To explore a map of predicted game fish for Wisconsin lakes, click on the button below.

We can manage lakes to protect and enhance walleye where they are likely to thrive

Fishing boats out on a lake.