Walleye production in Wisconsin lakes has declined considerably since 1990, according to a new study that says it now takes 1.5 times longer to produce the same amount of walleye as it did a couple of decades ago.

The study of fish population data from 473 Wisconsin lakes builds on similar findings at a time when scientists in Minnesota, other northern states and Ontario are trying to better understand the erosion of walleye productivity.

The Wisconsin study, published last week in the Canadian Journal of Fisheries and Aquatic Species, said the state’s walleye fishery is dominated by “low-production” populations.

No causes were pinpointed, but the authors correlate the erosion in walleye productivity with a combination of factors. Those include warming water, other habitat degradation, changes in aquatic food webs and fish harvest rates that might outpace production levels.

“It’s an alarming trend … because we know how big of a deal walleye are to the region,” said lead author Andrew Rypel, an ecologist at the University of Wisconsin and the Wisconsin Department of Natural Resources during the time of the study.

Rypel, who now works as an associate professor at the University of California-Davis, said in an interview Tuesday that Wisconsin has one of the most comprehensive walleye data sets in the world for such a large band of lakes. His research calculated population trends based on annual fish counts, including growth rates and size structure.

“The replacement ratio has gotten slower through time,” he said. “It takes longer to produce the same amount of walleye biomass as it used to.”

The study examined walleye production in three types of lakes: those where walleyes reproduce on their own, those totally reliant on stocking and “combination” lakes where natural reproduction is augmented by stocking.

Walleyes fare best in naturally reproducing lakes, the study said, but the proportion of those lakes has declined while “combination” lakes have increased. Combination lakes experienced walleye productivity declines of 47 percent over the 1990-2012 study period while productivity in lakes with only stocking and no natural reproduction declined by 63 percent.

The study’s authors noted that Wisconsin’s fisheries managers recently switched to large-scale stocking of extended-growth fingerlings during the fall to combat a perception that spring fingerling walleye were not surviving at the same rate as they were during the 1980s and 1990s.

Weakened “recruitment” of walleyes — sustaining baby fish into maturity — is central to the problem.

Walleye/bass paradox

Minnesota’s anglers have heard other scientists say that while walleye populations are struggling, bass numbers are on the upswing. Authors of the Wisconsin study noted the dichotomy, citing 2016 research by Minnesota DNR scientist Gretchen Hansen that found largemouth bass abundance to be one of the strongest predictors of walleye declines.

Hansen is now studying walleye diets and what’s going on in the food chains below and around the prized game fish in nine of Minnesota’s 10 largest walleye lakes.

The Wisconsin study said bass like warmer waters and that the decline of walleyes has corresponded with a rise in lake temperatures. “This points to climate change as one factor in the loss of walleye,” according to a written summary of the study.

Yet many lakes still retain quality thermal habitat for walleye growth and survival, suggesting that effects of temperature change are cumulative and “additive with other climate- and human-mediated effects,” the study said.

Rypel said one such effect now under study in Wisconsin is the human removal of wood from lake edges. Coarse woody habitat is thought to be beneficial for populations of perch — an important forage fish for walleye. Various “fish sticks” programs are reintroducing that habitat.

He also said angling pressure on walleyes — coveted as table fare — is tougher than it is on bass, a species enjoyed more often on a catch-and-release basis.

The study said invasive zebra mussels are “notably problematic” for walleyes, but the vast majority of the lakes in the study were not hosts to the lake-changing mollusks.

More study

Walleye are native to large rivers and cool-water glacial lakes in Wisconsin. Escanaba Lake, a 300-acre body of water north of Rhinelander in Vilas County, is singled out in the study for hosting what might be the state’s most productive, naturally recurring walleye population. Escanaba’s walleyes have been closely tracked since 1946, and the lake’s productivity fits a reassuring pattern.

According to the study, many of the most intensively managed lakes in Wisconsin tend to possess high production.

The findings suggest that Wisconsin’s fish management system “is capturing key empirical dynamics of production,” the study said.

Still, Rypel said there are new implications for fisheries managers to figure out. He hopes his study will trigger more research to understand what declining production means for future walleye harvests.