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Southwest Wisconsin CWD, Deer & Predator Study Newsletter

Issue 4: February 28, 2018

Winter adult trapping is well underway. Our crew began collaring in late December and will continue through March, or as long as the freezing temperatures hold.

Many of you have already gotten in touch with us to express your interest in volunteering with our late spring fawn collaring effort in Grant, Iowa and Dane Counties. Thank you to everyone who has expressed interest! If you’d like to join the list for future volunteer opportunities, fill our quick online form.

Learn more about our study design by visiting our website.

Click the titles below to read this issue’s full articles and view our videos.

Deer Survival: Digging into Year-One Numbers

In January 2017, the Office of Applied Science began a comprehensive study of deer mortality in southwestern Wisconsin as part of the Governor’s Initiative on Chronic Wasting Disease (CWD). In this article, we report the results of our first year of monitoring deer survival. Since these are early results, we’re going to focus on broad differences between CWD positive and negative deer. We’ll report the percentage of deer that survived over the course of the year and how that differed between CWD-infected deer and uninfected deer. We’ll also break down the causes of mortality. More focused analysis, including sex, age and geographic differences will become possible as we gather more data.

Katie Luukonen prepares to collar a whitetail
doe captured in a clover net.

The primary way CWD could influence deer populations is by reducing survival at the population level. This leads to the question ‘are CWD-infected deer less likely to survive over the course of the year than uninfected deer and if so, by how much?’ It is important to note that there is more to population impacts than just this survival difference - which we will explain in a future article - but for now, let’s focus on the question of CWD and deer survival.

One way CWD can reduce survival, of course, is for the disease itself to kill the deer. To the best of our knowledge, CWD is always fatal. Thus, it is not unusual or unexpected to observe deer dying from CWD. But what does it actually mean to say a deer has died from CWD? Like many diseases, dying from CWD means dying from complications related to CWD infection. As the prions accumulate and damage the nervous system, they disrupt the deer’s natural physical and physiological processes. In addition to what we normally expect when we think of deer with end-stage CWD (emaciation, drooling, drooping head and ears), infected deer often have difficulty swallowing, leading to aspiration pneumonia. These complications may be the direct cause of death of an infected deer.

Study crew prepare to collar a doe restrained in a clover net.

While individual deer deaths directly from CWD are noteworthy and interesting, they are only a part of the story of how CWD may reduce deer survival. CWD may significantly negatively impact deer populations whether the final mortality event is directly or indirectly related to the disease. It could do this by making deer more vulnerable to other sources of mortality (predation, vehicle collisions, hunting etc.), because infected deer become less aware of their surroundings, responsive to stimuli, and physically coordinated. The ultimate cause of death is less important to understanding the impact of CWD than the difference in survival between infected and uninfected deer, regardless of cause. Therefore, we shouldn’t focus directly on CWD-specific mortality but on the more basic question of whether CWD is leading to an overall reduction of deer survival. It is this question that is most important to evaluating how best to manage deer in the face of this disease.

Findings by the numbers

We collared 138 deer during the 2016-2017 winter, and we successfully tested 122 of these for CWD. Twelve of the 122 (9.8%) returned positive CWD test results.

The plot below shows survival of CWD-positive and CWD-negative deer through time. The vertical axis shows the percentage of animals alive (referred to scientifically as survival probability) and the horizontal axis is time. We can use this plot to see what survival is at any point in time, and to compare survival between infected and uninfected deer. (Note the vertical dotted line that denotes the beginning of the archery season and the 2 vertical shaded areas that denote the 9-day firearm and Holiday hunting seasons).

The two trend lines show the survival estimates and the shaded areas around the lines show the amount of uncertainty around those estimates. We can see that almost 90% of deer that tested negative for CWD at capture survived to the beginning of archery season, while only about 50% of CWD-positive deer survived over that period.

As expected, survival drops more sharply later in the year for both CWD-positive and -negative deer as hunting begins, and by the end of 2017, less than 25% of CWD-positive deer were still alive and about 75% of deer that didn’t test positive were still alive.

Year-One Survival Graph

This graph shows the difference in rate of survival between CWD-negative (blue) and CWD-positive deer (orange) over one year. It’s important to remember that this graph does not show mortality causes, only whether the deer survived.

The shaded areas around the two lines represent how certain we are that our line is correct. Certainty is determined by the size of our sample. This year, we observed many more CWD-negative deer than CWD-positive deer, so our sample size for negative deer is much larger than our sample size for positive deer. The larger our sample, the more certain we can be in our analysis, and the narrower the shaded area becomes around the line.

Right now, our estimation of CWD-negative deer survival is more certain than our estimation for CWD-positive deer. As our sample size continues to grow over the course of the study, the shaded regions will shrink around the solid lines, which may move up or down some as we collect more data.

Significantly, the shaded regions for CWD-negative deer survival and CWD-positive deer survival do not overlap. This makes us confident that the survival difference between positive and negative deer is statistically significant. We will need many more years of data to understand how big that difference actually is.]

Causes of mortality

Of CWD-positive deer that have died so far, at least 4 died directly from CWD, meaning they were found dead and were obviously emaciated and were unconsumed or with small amounts of scavenging. These deer were taken in for laboratory necropsy, except for 1 deer that was too decomposed. Laboratory necropsies confirmed CWD-infection and found substantial emaciation, muscle wasting, and 2 had pneumonia.

Three CWD-positive deer had been significantly consumed by coyotes, though it was not clear whether these were the result of scavenging or predation. One of these was captured on a trail-camera just prior to death looking extremely emaciated and with a bloated abdomen, suggesting late stage CWD symptoms.

Another CWD-positive deer was harvested by a hunter during the 9-day firearm season. For those deer that were not CWD-positive at capture, most mortality was due to hunting. A handful of additional deer died from other causes (vehicle collisions, coyote predation, etc.).

The survival difference between CWD-positive and CWD-negative deer is notable, but we must keep in mind that 12 CWD-positive animals is a relatively small number. Referring back to the survival plot, you’ll see the shaded area around the line is relatively wide. What this means is that there is a lot of uncertainty in the survival estimates of the CWD-positive deer, final survival estimates of CWD-positive deer could be higher or lower but likely fall within the shaded band. This level of uncertainty is a direct consequence of the relatively small number of CWD-positive deer we collared so far.

Despite this level of uncertainty, the estimated survival rates are already significantly different from each other by statistical standards. Therefore, we can now say with confidence that the survival rate of CWD positive deer is lower than for negative animals, which was expected.

A whitetail doe looks out from the woods.She was collared in Winter 2018.

As the study continues, we will gather much more information from both positive and negative deer. which will allow for more conclusive statements about the magnitude of difference in survival rates and the overall impact of CWD on the population. This preliminary analysis suggests CWD may be negatively impacting deer survival in Wisconsin and it certainly warrants paying attention to as the study moves forward.

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In the Field: How We Trap Adult Deer

We’ve set out to collar 200 adult deer in our winter trapping season. It’s an ambitious goal.

We divided our team into two crews, one for each study area. Each crew has a crew leader, assistant crew leader and technicians who work together to coordinate with landowners, set and check traps and, of course, collar deer.

Katie Luukkonen and Wes Ellerson collar and ear-tag an adult whitetail doe.

Deer safety is paramount, as is the safety of our crew. To trap as many deer as possible as safely and efficiently as we can, our work starts long before we trigger the first trap. Crew members go through an extensive orientation and training program to prepare them for deer handling. Some are new to the job, but many on our team are veterans of this and other deer projects.

By late December, our team is ready to operate traps and safely handle captured deer.

We use a variety of techniques to capture deer, but first, we have to find them. It often goes like this: We’ll see a bunch of deer in a field, and we’ll seek permission from the landowner to capture deer on their land. (Thanks to landowners, we’ve had good success accessing properties.) Then we’ll scout the property to get a sense for how many deer are around and where they are. In this, the landowner is often extremely helpful. Many landowners are very tuned into their deer and know where the deer bed and travel. The intel that landowners provide really helps.

Lindsay Martinez waits in a blind watching for deer to come
under the drop net.

All our methods require deer coming to a specific spot where we’ve set a net trap or box trap. To do this, we place a small volume of shelled corn in aluminum pans. Every day or two, we change out the pans and uneaten corn to reduce the chances of our activities being a source of CWD transmission for deer. It is important to note that our intent is not to bring deer in from far and wide. It is to get deer that are already using an area to stand in an exact place for us to capture them.

Once we find a spot, we’ll place corn in a suitable location for a trap. If the deer come to that place consistently for a couple of days, we’ll set up the trapping equipment. We’ll then give the deer another couple of days to get used to the trapping equipment. Once the deer become comfortable with all the new things we’ve added, it’s go time!

Waiting on the cold weather

To trap deer efficiently, cold and snow cover are critical. There is a certain amount of fear that deer need to overcome to come to our trapping equipment, which are obviously foreign objects. Snow and cold make deer much more attracted to the corn we provide. Another benefit of cold weather is solid ground. When the ground thaws, we risk rutting our volunteer landowners’ roads (which we avoid at all costs), muddying the deer or getting trapped ourselves. There are times when we have to shut down trapping because the ground is just too muddy.

We obsessively check the weather forecasts, crossing our fingers for the polar vortex or a big dump of snow. The winter weather most people dread is what we love. The colder and snowier the better!

Clover net traps

Clover nets are our most portable traps. They’re relatively easy to haul up hillsides, set in underbrush or stake along a trail.

We space clover traps a couple hundred yards apart on a property, forming a trap line. Typically, a line would have five to 20 clover traps on it. With the help of a UTV, we’ve been able to expand our efforts. One line currently has 17 traps set!

We check all our live traps daily to collar deer, release any non-target animals that may have wandered in and make sure our equipment is safe and ready to deploy when the next deer comes by.

Rabbits and squirrels come to bait just like deer, but instead of hopping back out the door, they prefer to chew their way through the net walls. Crew members make sure to repair any holes they find in the clover netting.

Drop nets

If we find large numbers of deer feeding in open areas, we may set up a drop net. We hoist a 40’ by 40’ net to a flagpole and suspend the edges about four feet off the ground with stakes.

Field crew reset a drop net after a successful deer capture.

Drop nets are most effective later in the day when we can take advantage of their evening feeding pattern. At dusk, a lookout waits in a blind, watching for deer to come to bait at the foot of the flagpole. Once the deer are safely under the net, the lookout fires the trigger, and in seconds the deer safely become entangled and immobilized in the mesh.

The lookout radios to the rest of crew to come to the net. They’ve been waiting farther off—in a warm truck cab if they’re lucky—to avoid spooking the deer. When they get the signal, comes to the net with the capture drugs and collaring equipment.

Box traps

Box traps are by far the heaviest and most cumbersome traps in our arsenal. Despite being difficult to transport and set up, they’re perfect for trapping deer in areas with higher human traffic. For instance, if a landowner reports frequent deer sightings in their yard we can safely catch them with the assurance that the dark and quiet confines of the box trap keep them from getting too nervous.

That tranquil environment comes at a cost—weight. Weighing in at 400 pounds, box traps are impossible to move from property to property without a truck. The lack of portability means that we use box traps only in limited situations.


A tranquilizer dart.

While darting is available to us, we almost never go this route. As much as we can, we rely on clover traps, box traps and drop nets to get the job done. Even though our crew has dedicated time to training with dart guns, we have only darted a couple of animals for this study so far.

What makes darting a last resort? Compared to drop nets, the deer is more likely to escape (darts miss, nets don’t) and the dart hitting the target is not the end of the capture. Unlike in the movies, a darted animal doesn’t drop to the ground as soon as it’s hit; a darted deer may move several hundred yards before the sedation drug takes effect. In short, darting simply has a lower capture yield than our other methods. The one time when it can be more effective than our trapping methods is when weather warms up and deer are less willing to go to drop nets or traps.

Stay tuned for more

Are you interested in learning more about our field work? In future editions of Field Notes, we’ll look at animal handling, CWD testing and all the techniques we use to study deer in Southwest Wisconsin. You can also follow our progress on the DNR’s Facebook page @WIDNR.

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UTV Donation from SCI Boosts Trapping Effort

OAS crew maneuver a UTV along a trap line in Southwest Wisconsin.

We spend a long time sorting through the logistics of trapping season, coordinating with landowners, managing crew schedules, stocking and restocking supplies. The list goes on, but finally winter arrives and it’s time to get outside and get the job done.

Weather permitting, our crew is in the field every day of the week from late December through March trapping for deer, bobcats and coyotes. With all our trapping and collaring equipment, we don’t travel light. “We need to be where the deer are,” said Mike Watt, deer coordinator for the study, “and that often means getting way back on these properties.” There are a lot of properties which are inaccessible by pickup. Walking in all our gear and heavy trapping equipment just isn’t practical.

That’s why SCI’s donation of two UTVs is such a boon to the project. UTVs are great because they can haul 3 people each, all the gear, and they can navigate the narrow, winding trails often found on the properties where we work.

When Dan Trawicki and the members of SCI’s four Wisconsin chapters heard about the study, they asked how they could contribute. “I talked with Dan Storm, [DNR’s Deer and Elk Ecologist],” says Trawicki, “and he said you know, the biggest obstacle we’re having is getting back into these trapping areas. And he said if you guys could do UTVs that would be cool. I said that works for us.”

Wisconsin boasts four SCI chapters: the Wisconsin chapter out of Milwaukee; the Badgerland chapter in Madison; Green Bay’s Northeast chapter; and the Southeast Wisconsin Bow chapter from Waukesha.

Trawicki says a lot of folks have a misconception about SCI, thinking it’s mostly for international big game hunters: “The majority of our members are regular people who just like to hunt, you know, ducks, bear, deer, you know Wisconsin turkeys, etcetera. It’s not a safari club. It’s SCI: First for Hunters.”

Our crew arrive to a clover trap in their Arctic Cat UTV. Hanna Manninen
records field notes while the rest of the team handles the deer.

Trawicki wants the DNR and sportsmen across the state to realize that while SCI is a national organization, the local chapters support Wisconsin. “Of the funds we raise,” says Trawicki, “30% goes back to national, and 70% stays here to support hunters, local chapters of the National Archery in Schools Program, Boy Scouts, the DNR.” Every year, the four chapters give away between $300,000 to $350,000.

The study crew is putting the Arctic Cats to use every day, and we’re grateful to have them. Check out the video below to see them in the field!

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Last revised: Thursday May 16 2019