Could Wisconsin study solve Mille Lacs walleye decline mystery?

Climate change appears to be be hitting home for walleye fishermen in the Upper Midwest.

A new study in the journal Global Change Biology finds that warming lakes due to climate change are much less likely to support walleye habitat in Wisconsin and the Upper Midwest. Meanwhile, warmer water loving largemouth bass habitat is forecast to increase as lakes continue to warm.

Since about the year 2000, Wisconsin walleye numbers have crashed as largemouth bass numbers rose dramatically.

United States Geological Survey

The study examined 2,148 lakes in Wisconsin, and forecasts a 50% drop in the percentage of lakes likely to support natural reproduction of walleye by mid-century. Currently about 10% of Wisconsin's lakes support natural walleye reproduction. That number is forecast to drop to around 4% by mid-century. Meanwhile, the percentage of lakes with conditions conducive to high largemouth bass abundance was predicted to increase from 60 to 89 percent.

Here's a clip from the United States Geological Survey's release on the new study.

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Release Date: SEPTEMBER 12, 2016

A recent study looks at the impact of climate change on certain fish in Wisconsin lakes.

MADISON, WISCONSIN – Climate change is predicted to alter sport fish communities in Midwestern lakes, according to a new study that related water temperature to suitability for walleye and largemouth bass in more than 2,100 Wisconsin lakes.

For the past 30 years, walleye populations have been declining and largemouth bass populations have been increasing in lakes across Wisconsin. These changes are cause for concern for many anglers and policy makers since freshwater fishing in Wisconsin is valued at more than $1.5 billion, and walleye are the preferred species for many anglers.

According to the study, this downward trend in walleye populations is likely to continue as climate change will cause lakes to get warmer overtime. Researchers identified characteristics of lakes where walleye or largemouth bass were most likely to thrive and found that both species were strongly influenced by water temperature. While walleye populations thrived in cooler, larger lakes, largemouth bass were more abundant in warmer lakes.

“Generally this means that lakes that are best for walleye are not the best for largemouth bass, and vice versa” said study author Gretchen Hansen, former Wisconsin Department of Natural Resources research scientist currently with the Minnesota DNR. “Going forward, we predict that many Wisconsin lakes are going to become more suitable for largemouth bass, and less suitable for walleye.”

The suitability of individual lakes for supporting walleye and largemouth bass was based on a computer model that estimated daily water temperatures from 1979-2014 for thousands of lakes using information on lake size, depth, water clarity, and historical weather. “We don’t measure water temperature in many lakes,” said co-author Jordan Read, U.S. Geological Survey civil engineer. “Modeled lake temperatures can fill the gaps and provide fisheries managers and researchers with a full historical record of water temperature for each of these lakes.”

Future lake temperatures were also forecasted for the 2,100 lakes using mid- and late-21st century climate projections and the study authors emphasized that the diversity of lakes will determine how they will respond to climate change.

“Wisconsin’s lakes are going to get warmer in the future, but how much warmer they will get varies among lakes” said study co-author Luke Winslow, USGS research hydrologist.

By accounting for variability among lakes in how they respond to climate change, researchers estimated how individual lakes and their fish communities were expected to respond to future warming.

The percentage of lakes likely to support natural reproduction of walleye was predicted to decline from 10 percent to less than 4 percent of Wisconsin lakes by the middle of the century. At the same time, the percentage of lakes with conditions conducive to high largemouth bass abundance was predicted to increase from 60 to 89 percent of lakes within the same timeframe.

Fewer 'walleye lakes' ahead   

As lakes continue to warm they are less likely to remain suitable as walleye habitat. Shallower lakes are warming more quickly than deeper lakes. Here are some key findings from the USGS Wisconsin study.

  • 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.

  • 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.

  • 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.


Solving the Mille Lacs walleye mystery?

The Wisconsin study may be a major step forward in solving the mystery of the significant walleye decline on Lake Mille Lacs. Those of us who follow climate change trends closely have long suspected that climate change is affecting shallow lakes like Mille Lacs more quickly than deeper Minnesota lakes.

Lake Mille Lacs near Garrison, Minn. MPR News | File 2013

In my analysis, the Wisconsin study now provides significant evidence that climate change is likely the primary reason for the abrupt decline in Mille Lacs walleye populations over the past 15 years.

It is interesting to note that two of the authors of this study now work for the Minnesota DNR. Expect to hear more data on how Minnesota's lakes and fisheries are reacting to climate change in the near future.