Warming winters and waters will encourage largemouth bass to expand their range in northern states, while walleye populations will diminish and retreat, according the U.S. Geological Survey (USGS) researchers and fisheries scientists in the Upper Midwest.
In a recent study about the effects of climate change, USGS revealed that walleye numbers have been declining and bass numbers increasing in Wisconsin during the past 30 years. It added, "This downward trend in walleye populations is likely to continue as climate change will cause lakes to get warmer over time. 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."
In other words, what's best for bass is not best for walleye, added Gretchen Hansen, an author of the study and research scientists for the Minnesota Department of Natural Resources."Going forward, we predict that many Wisconsin lakes are going to become more suitable for largemouth bass, and less suitable for walleye."
The trend will be most notable, she added, in smaller inland lakes.
Larger fisheries are not immune, however, with Minnesota's Mille Lacs a notable example. Biologists believe that warming water is a primary reason for walleye decline in this 132,516-acre lake.
But it's not just that the walleye don't thrive in warmer water. Neither do forage, such as cisco, which they rely on to stay fat and healthy. Also known as tullibee, these baitfish seek refuge in the coldest parts of lakes during summer.
"I liken them to sticks of butter swimming around, a pretty fatty fish with high energetic content," said Paul Venturelli, an assistant professor in fisheries at the University of Minnesota.
In looking at 2,100 lakes in Wisconsin, USGS scientists considered not just water temperatures from 1979 to2014, but acreage, depth, water clarity, and historical weather. "Wisconsin's lakes are going to get warmer in the future, but how much warmer they will get varies among lakes," said Luke Winslow, a research hydrologist.
Scientists estimated that the percentage of lakes with conditions conductive to high largemouth bass abundance will increase from 60 to 89 percent by mid century. In contrast, the percentage of lakes likely to support natural reproduction of walleye is predicted to decline from 10 to less than 4 percent.
On a positive note for walleye, the percentage of lake acreage likely to support natural reproduction should decline by a much smaller amount, from 46 to 36 percent. "Walleye populations in large lakes appear to be more tolerant of warming than walleye populations in small lakes," said Hansen, who added increasing stocking rates could help mitigate the decline.