MADISON, Wis. -- Most of the walleyes Wisconsin fisheries biologists are netting in some northern lakes, and 80% of those tested last fall from two state hatcheries, are turning up females, and the experts don’t know why the sex ratio is so skewed.

In a natural environment under normal conditions, about half of all walleyes should be male and half female. But Wisconsin Department of Natural Resources fisheries biologists in recent years noticed that many of the walleyes they were catching in lake survey nets were female.

The DNR circled back and checked small walleyes being raised in two fish hatcheries just before they were released into lakes. The problem was confirmed in fish sampled last fall at the Governor Tommy G. Thompson Fish Hatchery outside Spooner and the Art Oehmcke Fish Hatchery near Woodruff, where up to 80% of the walleyes were female.

“We’re going to continue to do some sampling to see if we can get a handle on that. We don’t know why it was 80/20 skewing to females,’’ said Dave Giehtbrock, fish culture section chief for the DNR.

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“We do have more questions than answers,’’ said Lori Tate, fisheries management section chief for the DNR.

It’s not clear if the problem is entirely related to the two fish hatcheries, or if more hatcheries are involved, or if the issue may be a broader, environmental issue. The Duluth News Tribune first reported in 2001 that endocrine-disrupting hormones in some Northland waters, including estrogen, were skewing fish to be female. The problem has occurred worldwide.

But because the two hatcheries affected get their water supplies from different sources, it's not likely an environmental issue but possibly a "fish husbandry" issue, Giehtbrock said, noting walleye eggs fertilized at water temperatures too cold or too warm could skew the sex ratio.

It’s even less clear what’s causing the problem in lakes, although some of the lakes were stocked with walleyes from the two hatcheries in previous years. It takes about four years for stocked walleyes to grow big enough to be caught in survey nets, hinting that whatever is at play has been happening for several years.

Hatchery-raised walleyes before they were released into a northern Wisconsin lake. The DNR is concerned that 80% of the walleyes sampled before being released last fall turned out to be females. The ratio should be about 50-50. (Photo courtesy of Wisconsin DNR)
Hatchery-raised walleyes before they were released into a northern Wisconsin lake. The DNR is concerned that 80% of the walleyes sampled before being released last fall turned out to be females. The ratio should be about 50-50. (Photo courtesy of Wisconsin DNR)

“The sex ratios from field sampling (lakes) are more complex. We noticed female-dominated sex ratios in adult populations from stocked lakes that were surveyed in 2019 and 2020. Some stocked lakes were about 50-50 male to female, while other lakes were as high as three-fourths females,’’ Tate said. “We don’t have a clear picture of how widespread this issue might be. We will be doing much more detailed sampling and analysis on this in future.”

Tate said the unusually high number of females has been seen more in stocked lakes, but also in some lakes with naturally reproducing populations of walleye.

“We do see skewed sex ratios, one way or the other, in naturally reproduced populations. However, we were seeing these skewed ratios toward females more in stocked lakes,’’ she said.

The high percentage of females in stocked lakes likely won’t impact fishing too much.

“The fact that females grow bigger and faster is probably a good thing” in lakes where all walleyes are stocked, Tate noted. “But it’s not something you want to see in a natural population.”

As part of its walleye stocking program the Wisconsin DNR collects wild walleye eggs each spring when the fish spawn, fertilizes and hatches them in hatcheries and then feeds and grows the little walleyes to 6 inches or longer by October, when the fish are stocked in lakes across the state. Since 2013, the DNR has intensified its walleye stocking under the Wisconsin Walleye Initiative, increasing stocking levels from about 200,000 per year to more than 800,000 fish per year.