June 17, 2020
By Scott Mackenthun
What first appeared to regional fisheries biologists as a small dip in walleye abundance in isolated locations across the Upper Midwest has been confirmed as a range-wide issue across the fish’s native range. Habitat shifts in Midwestern lakes show a similar trend of declining walleye abundance and increasing bass and sunfish abundance (bluegills, crappies, largemouth and smallmouth bass), according to several published studies and empirical data observations across Minnesota, Wisconsin and Michigan.
"There had been these grumblings about this [bass-walleye] shift," said Jon Hansen, Fisheries Management Program Consultant for the Minnesota Department of Natural Resources (DNR) and a former biologist for Wisconsin DNR. The grumblings were coming from anglers as well as fisheries managers, possibly as early as the late 1990s, but certainly during the first decade of the new millennium.
Wisconsin might have noticed the issue first, but in time it was apparent in some lakes in Minnesota and Michigan, according to Hansen. At a local level in Wisconsin, the administrative response was often to make small changes in walleye length or bag limits or stocking strategies. Wisconsin DNR’s Central Office staff and research unit pitched a large-scale study, which was funded in cooperation with the United States Geological Survey (USGS) and the University of Wisconsin-Madison, and the results corroborated the observations of field staff. Walleye populations in many Wisconsin lakes were and are declining, while largemouth bass populations in many lakes were and are increasing.
"Temperature and water clarity set the stage for what is possible in a lake," says Dr. Gretchen Hansen, assistant professor of fisheries ecology at the University of Minnesota. Dr. Hansen worked as a post-doc on the Wisconsin USGS study, co-authoring a number of papers that examine landscape-level change within lakes and use predictive modeling tools to examine the future of some Midwestern walleye fisheries.
”If a lake is getting warmer and clearer, and walleye habitat is not suitable anymore, changing bag limits or stocking more fish is not going to bring the results anglers or managers are looking for," she says. "The expectation that every lake can support a walleye fishery is not likely to be realistic anymore."
The shifts in habitat are shifts in water quality parameters. Wisconsin’s lakes have warmed by as much as two degrees Fahrenheit since 1980. Although that might not seem like much to people, to fish it is a tremendous amount, as water temperature is a critical component of habitat. The trend for the foreseeable future is continued warming, and as water temperatures get warmer, many lakes that currently support natural walleye reproduction are unlikely to continue to have the thermal habitat conditions to do so. While transparency was not part of the Wisconsin/USGS study, it, too, is changing in some Midwest lakes.
Dr. Nigel Lester, one of the world’s foremost walleye researchers, conceptualized walleye habitat in his 2004 co-authored paper "Light and Temperature: Key Factors Affecting Walleye Abundance and Production." As lead author on the paper, Lester determined optimal light and temperature ranges for walleyes and coined the term "thermal optical habitat area," or TOHA. When light penetration and/or temperature changes, TOHA changes and so, too, does the critical habitat. Across much of the walleye’s native, naturally reproducing range, the area of optimal habitat is shrinking. With changes in the habitat come changes in the fish community.
Pinpointing the origins of system change is complex and challenging. Climate change is the main driver on temperature. Light penetration, or the transparency of a lake, can shift from changes to land use in the surrounding watershed. Nutrients being tied up on the landscape or in-lake might prevent algal blooms, for example. Removal and replacement of failed septic systems or hooking up to municipal wastewater treatment could have unintended consequences. Even artifacts of the 1972 Clean Water Act and its role in cleaning up point source-pollution could be impacting lakes over time.
Manmade changes happen slowly, but effects do eventually become apparent. More drastic change comes from ecosystem invaders. Invasive quagga and zebra mussels, once introduced, feed on phytoplankton and zooplankton, thus increasing water transparency and, in turn, speeding up warming by increased sunlight penetration.
The future is not hopeful for many formerly marginal walleye fisheries. It’s bleaker on some major walleye fisheries that are seeing walleye abundance declines. Meanwhile, some lakes will be resilient and largely unaffected.
"There are going to be places in the Midwest that are going to be these great walleye fisheries for a long time; however, we may not be able to support the lesser walleye fisheries unless we want to spend an exorbitant amount of money propping them up with stocking," says Hansen. In an era of tightened budgets and fewer fishing license sales, efficient walleye stocking is going to be a premium. "Anglers need to promote lake and watershed resilience," he says. "They can engage in local watershed planning. Change is happening right now, and we have to think beyond the lake. You can’t fix a lake by just working in the lake."
Zebra and quagga mussels, Dr. Hansen adds, are a direct driver of lake ecosystem change that isn’t good for walleye. Anglers following the Clean-Drain-Dry prevention steps for watercraft and gear used in infested waters is essential.
Adaptation to system change and reasonable expectations must come for anglers. Some walleye anglers might be advised to pick up a bass rod or drive to lakes with better walleye habitat. For some anglers, Hansen points out, that could just mean a short drive down the road if you live in a lake-rich region.
"Responsible management is needed," Hansen stresses. "Resource managers need to be science-based, even if the result is something you as an angler don’t like.”