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Research: CWD Still Bigger Deer Threat than Predators

At least one thing’s increasingly clear as the DNR ends Phase 1 of its long-term study of disease and predator impacts on southwestern Wisconsin’s deer: Whitetails have more to fear from CWD than bobcats or coyotes.


Of course, we assumed as much before the study began 3½ years ago. Likewise, we assume some folks won’t believe that even after the data get analyzed, published and peer-reviewed in the years ahead. But at least our top DNR and university biologists will have documented the objective truth, while learning some cool stuff, too.


Just to remind you, the Department of Natural Resources launched this $5 million study in late 2016. Since then its teams have live-captured 47 bobcats, 69 coyotes, 323 fawns, 319 adult bucks, and 491 adult does at trap sites in Iowa County west of Madison, and collared most of them with GPS transmitters to track their movements.


By studying the animals’ activities — or lack thereof after they die — researchers will better understand their survival rates, habitat preferences, and population sizes; as well as the impacts of predators and chronic wasting disease on the deer herd.


Of the 810 deer captured the past four winters, 10% had CWD when they were caught and collared. Only 2 percent of the 8-month-old male and female deer had CWD. In contrast, the CWD infection rate for deer at age 20 months was 22% for bucks and 12% for does, while at age 2 years and older it was 16% for bucks and 19% for does.


Dan Storm, the DNR’s research-ecologist overseeing the study, said they’re done capturing and collaring animals, but they’ll continue compiling data from the collars for four to five years. Collars activated this past winter could transmit the animal’s locations until 2025, but few deer, bobcats or coyotes will live that long. Storm said three staffers remain afield this summer finding animals that die, and then collecting samples and retrieving the collar. By winter, only one person will remain afield.


When monitoring the whitetail’s spring dispersals and fall rutting activities, researchers set the animals’ GPS units to transmit hourly locations. In summer and winter, when deer are less active, the units transmit data every six hours. With hundreds of deer “on the air,” researchers are downloading 10s of thousands of GPS coordinates daily.


“We’ve hardly looked at all the movement data, relatively speaking, because we’ve been focused on survival rates,” Storm said. “The more deer we monitor and the longer we do it, the more questions we’ll start asking. It’s clear that CWD prevalence is moving and worsening more north and west than south and east. As we analyze the activity data, maybe we’ll learn which landscapes hinder or facilitate movements that increase CWD transmission.”


Storm said the GPS data should also help them compare movements by healthy deer and CWD-infected deer to learn how the disease affects their activities, habitat choices, and interactions with other deer.


Previous analysis found that deer testing positive for CWD when collared in January or February were at least twice as likely to be dead by December than were healthy deer. Storm said the DNR didn’t run that analysis this year, but planned to nail it down after the 2021 deer season.


Storm said another interesting aspect of the study is that female deer sometimes make unexplainable “excursions” that weren’t possible to detect before GPS collars were developed. One yearling doe, for example, took off for Illinois in June, and didn’t turn around until it was south of Galena, roughly 80 miles away. The doe eventually returned but continued on to the Pheasant Branch Conservancy on Middleton’s north side, just west of Madison.


“We expect yearling bucks to disperse from their home area, but they usually go 2, 3, 5 or 10 miles before settling down,” Storm said. “Young bucks are actually kind of predictable. But does do weird things we never documented before. With the old radio-transmitting collars, we would have needed an airplane to follow that doe to Galena. We probably never would have known she left.”


Storm said the study is also revealing insights into coyotes and bobcats. Of the 70 coyotes caught and collared each winter from 2017 to 2019, roughly half died within the same year. “A 50% mortality rate is pretty high, and most of it was from trapping and hunting, as well as a few vehicle collisions,” Storm said. “It never seems like there’s many people hunting and trapping coyotes, but they’re getting about half of the population every year.”


Storm said his team hasn’t looked as closely at death rates for bobcats, but most deaths they’ve detected were “human-caused,” too. “Our plan is to finalize the bobcat survival results in spring 2021, a year after we collared the last batch,” he said. “They have a much higher survival rate than coyotes, but I don’t know the rate.”


Meanwhile, the DNR recently updated its CWD prevalence rates for parts of Dane, Iowa, Sauk, Richland, Grant and Columbia counties. As expected, the news keeps worsening with the state’s passive approach. Iowa County easily leads the state with 3,083 CWD cases, with Dane trailing at 1,003; Sauk, 967; and Richland, 689.


Disease rates, however, are climbing fast in southeastern Richland and southwestern Sauk counties; roughly north of Highway 14 from Gotham to Spring Green.


• SW Sauk County: adult bucks, 57% infection rate; adult does, 38%; yearling bucks, 28%; yearling does, 28%.


• SE Richland County: adult bucks, 48% infection rate; adult does, 30%; yearling bucks, 18%; yearling does, 21%.


• NC Iowa County: adult bucks, 42% infection rate; adult does, 37%; yearling bucks, 18%; yearling does, 18%.


• Devil’s Lake (Baraboo) area: adult bucks, 39% infection rate; adult does, 20%; yearling bucks, 16%; yearling does, 22%.


You might recall CWD rates stayed flat in southeastern Richland and southwestern Sauk until 2006, and likewise around Devil’s Lake until about 2012. As Storm said, the disease has spiked since. Maybe the GPS research will help scientists learn why, but here’s a sobering thought from Wyoming research:


Once CWD prevalence rates exceed 30% in adult does, population declines will likely follow.

The DNR has captured and collared 810 white-tailed deer the past four winters to learn more about the impacts of CWD and predators on the herd. — Patrick Durkin photo

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