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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.
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.
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.
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.
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.
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.
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.
Lake Count
Total Lake Area
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.
Importantly, the relationships between water temperature and fish identified in this study are based on correlations. We know that walleye natural reproduction is more frequent in cooler lakes, and we know largemouth bass reach high densities more often in warmer lakes. However, we do not know the reasons behind these relationships. Water temperatures in most Wisconsin lakes are not predicted to get too warm for walleye to survive in the foreseeable future. We can only speculate about the true cause for failed walleye reproduction in warmer lakes. We believe a number of factors may be at play, including potential mismatches between young walleye’s need for food and the availability of that food, increased competition and/or predation, increased need for food due to warmer temperatures, or a combination of these and other factors.
