More Hunting Equals Less CWD: New Research from Colorado

September 17, 2020 | by Kristen A. Schmitt

With deer season already here in some states – and steadily approaching in others – new research published in the Journal of Wildlife Diseases discovered a positive correlation between increased hunting pressure and decreased chronic wasting disease (CWD) in Colorado mule deer. The study, “Hunting Pressure Modulates Prion Infection Risk in Mule Deer Herds,” analyzes 16 years of data spanning 12 key areas where mule deer herds roam. The suggestion? Hunters may be the key tool to manage herd health.

Historically, mule deer management strategies called for decreased hunting to help deer recover from severe winters or to boost older age-class bucks for trophy harvest; however, doing so may have actually increased the prevalence of CWD in mule deer herds, particularly because of the disease’s persistence within male deer.

“There aren’t too many absolutes in biology, but there is a remarkably consistent pattern that males have about twice the prevalence of infection compared to females in a particular area,” says Dr. Michael Miller, senior wildlife veterinarian for Colorado Parks & Wildlife (CPW) and lead author of the study. “Because we actually harvest way more bucks than we do does in most of our herds here, if we can get good samples off the bucks, then we have a pretty good guess of what prevalence is going to be in does.”

“If we keep an eye on the bucks and start managing to keep prevalence low in bucks,” adds Miller, “then we’ll automatically be intervening before things get too bad in does.”

Prior to 1999, Colorado’s mule deer management allowed for an unlimited number of tags for male deer. However, in the 2000s, tags became limited and quotas were assigned based upon specific geographic areas, resulting in a nearly 50 percent reduction in the amount of annual hunting pressure on male deer. Around the same time, a rise in CWD cases occurred, leaving scientists to deduce that limiting the amount of hunting pressure actually allowed CWD to bloom within herds. This spike of CWD in lower-pressured herds directly contrasted with herds where tags were increased to meet certain population objectives (or were at least maintained) – those herds reported a lower occurrence of CWD.

“Hunting pressure is one of our best tools to manage deer,” says Andrew Holland, CPW’s statewide big game coordinator and co-author of the study. “Not only from a population management standpoint, but from a deer health standpoint. I like that hunters are part of the solution to this.”

But is more hunting actually sustainable?

Miller warns that one of the initial tradeoffs with a CWD management strategy that calls for increased hunting means fewer older age-class bucks, but he adds that “if you want to get after the disease in a hurry, that should not be your highest priority.”

Like most states, Colorado manages deer conservatively based upon a specific buck to doe ratio objective – a ratio that looks at the “long-term sustainability of that herd,” says Holland. To determine those ratios, biologists consider a variety of factors that include winter survival/severity, fawn recruitment, migration and mating seasons. Regardless of how each factor independently impacts deer herds, Holland says that biologists often manage population objectives “for lower population sizes,” which means switching to a CWD management strategy that calls for more hunting is sustainable and it actually works.

For example, in northern Larimer County after CPW biologists increased hunting pressure back in 2000, biologists noticed that CWD prevalence dropped by nearly 50 percent.

“In the Poudre-Red Feather herd, we have a lot more buck licenses,” says Holland. “Essentially, we’ve reduced the prevalence in that herd from 15 percent to 5 percent.”

The Middle Park and Big Thompson herds have also experienced declines in CWD after an upswing in hunting pressure. Current data shows that sustained hunting pressure helped flatten the epidemic curve in the Middle Park herd over the past 15 years – even while CWD increased in the White River and Bear’s Ears herds.

“If you look at our Middle Park herd here in Colorado or the Poudre-Red Feather, they're doing fine,” says Miller. “You can always back off a little bit for one season if you need to. There is some flexibility with the annual prescriptions if you have a bad winter or if you are concerned, but the advantage of using harvest is that it is something that we control and we can turn on and off or modify.

“The disease will also have suppression affects if infection rates get high enough. And so the problem there is that if it gets too high that we can't do a whole lot about it.”

Miller adds, “So, personally, I would rather have something that I was able to control than something that I couldn’t.”

Beyond Colorado

CWD is found across the U.S. with state wildlife agencies actively trying to contain the disease and curb the spread. While every state differs in how seasons are structured, this new research allows wildlife managers another factor to consider when setting licenses, especially in areas where CWD already exists and biologists want to keep it from increasing. Montana, for instance, has moved towards an increase in hunting to help areas hit hard by the disease.

“Those of us who’ve been working together were aware of some of this data,” says Emily Almberg, lead ecologist for Montana Fish, Wildlife and Parks (MFWP). “Increasing harvest, particularly of bucks; lengthening your hunting season, specifically if you have a short season so that you’re harvesting later in the season, targeting maybe more susceptible or infected animals…are definitely in line with what we’re doing in Montana as well.”

While Miller and his team had 16 years of data to work with, Almberg says that it will take several years for MFWP to gather enough data to see how the prevalence changes over time. As Miller noted before, CWD trends are similar to watching paint dry; it takes many years to see if changes in herd management strategy actually work. And, despite efforts to stop its spread, Miller cautions that the disease will likely linger in infected areas for years to come.

 As for the application of the research to whitetail deer, Miller notes that herd dynamics may differ and how hunting pressure affects whitetail herds is still to be determined.