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Shifts in fish habitat under climate change
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
Recreational fishing in Wisconsin is valued at over $2 billion annually.
Anglers target both walleye and largemouth bass in Wisconsin lakes, but walleye are often preferred and are important to Wisconsin’s economy and culture.
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.
There are many possible explanations for the observed changes in Wisconsin lake fish communities, and much interest exists in understanding these trends and whether they can be reversed.
Some believe that largemouth bass are directly responsible for walleye declines. However, evidence to date does not support this claim.
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.
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.
In general, walleye reproduce more successfully in cooler lakes, and largemouth bass populations are most abundant in warm lakes.
Both species exhibit a threshold response to temperature, where small changes in temperature can produce big changes in the fish population. Lakes with the best habitat for natural walleye reproduction generally do not have the habitat to support high largemouth bass abundance, and vice versa.
These relationships are averages. Other lake characteristics influence the suitability of the lake for each species. For example, walleye are likely to reproduce successfully in large lakes even when they are warm.
Lakes in Wisconsin have gotten warmer over the past 30 years
Some lakes have warmed more than others, and some lakes have always been warmer than others - for example, lakes in southern Wisconsin are generally warmer than lakes in northern Wisconsin.
Lake size and depth, water clarity, and surrounding tree cover all influence how lakes respond to climate.
Differences in water temperature affect the suitability of a lake for supporting different fish species.
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
Scientists agree that the earth will get warmer through the end of the 21st century.
However, the amount of warming depends on actual greenhouse gas emissions and how physical processes such as precipitation and wind speed will change.
We estimated how water temperatures of Wisconsin lakes would change assuming a relatively large increase in air temperature. These predictions can be considered a "worst case scenario" in terms of the amount of warming.
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.
As air temperatures increase, we expect lake temperatures to increase as well. However, not all lakes will respond equally to climate change.
The incredible diversity of lake types in Wisconsin means that we can expect different warming rates in big vs. small lakes, deep vs. shallow lakes, clear vs. turbid lakes.
Understanding differences among lakes in their responses to climate change is a high priority for managing fish populations that are sensitive to warming temperatures.
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
Water temperature is a critical component of fish habitat.
As water temperatures get warmer, many lakes that currently can support natural walleye reproduction are unlikely to continue to have the thermal habitat conditions to do so.
The majority of Wisconsin lakes and total lake area are predicted to have habitat conditions suitable for high largemouth bass densities by the mid-21st century due to warming.
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.
The lakes where walleye are expected to persist are for the most part some of the state's largest and most popular lakes. Therefore, when considered in terms of total lake area, walleye are predicted to persist in a large amount of Wisconsin's total lake area.
Most of the surface area of lakes that support natural walleye reproduction will also support high largemouth bass densities, and largemouth bass are predicted to be supported in a high proportion of the state's total lake area.
The predictions shown here are based only on projected changes in water temperature, and do not account for changes in the fish community, water quality, fish management, or angler behavior that may influence walleye and largemouth bass populations.
Explore lake-specific predictions of supporting walleye and largemouth bass!
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
Nearly 100 lakes in Wisconsin are predicted to support naturally reproducing walleye populations even under extreme warming conditions.
These lakes are resilient to climate change, and should be protected from other stressors such as habitat loss, invasive species, or overfishing to maximize the potential for continued walleye production.
Some lakes that are unlikely to support natural walleye reproduction will continue to be suitable for adult walleye, and fishable walleye populations could potentially be maintained in these lakes via stocking.
Quality fishing opportunities for largemouth bass and other warm water species will improve in many lakes that become unsuitable for walleye.
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.