MILES CITY – For Montana Fish, Wildlife & Parks, keeping track of mule deer populations has no season; data collection and analysis happen all year long amidst a constantly changing backdrop.
And in FWP's Region 7, each year's data is also compared to 40 years worth of aerial survey and hunter check station data to determine population trends, and to inform future management decisions.
This wealth of data exists because southeast Montana hosts one of the largest, most established mule deer populations in the state, so the status of herds here is closely watched.
The past three years of aerial surveys show that the region is pulling out of a drought-driven downward cycle. Observations of mule deer are increasing in nearly every area, and fawn recruitment is strong.
While hunters tend to look at one season at a time, wildlife biologists pinpoint periods of growth and decline over decades and study what drives these shifts.
"Weather and habitat are the main drivers of mule deer populations throughout their range, so wildlife staff are constantly keeping an eye on range conditions and weather patterns to understand what impacts they may have on mule deer numbers," said Brett Dorak, wildlife manager in Region 7.
"Mule deer numbers in southeast Montana are cyclical," Dorak said. "That cycle usually operates within a 10-year pattern. Numbers are typically suppressed by a moisture pattern in that cycle, whether it be too much moisture from excessive snow accumulation during severe winters, or not enough moisture from drought conditions that negatively impact habitat."
Mule deer have been heavily impacted by a lack of moisture in recent years, which is to be expected, but strong recruitment numbers are a step in the right direction.
"While biologists have no control over weather, they can track weather patterns and their impacts and compare them to population trends from aerial surveys to help determine potential future impacts from current weather conditions," Dorak said.
Aerial surveys assess make-up of herds
Aerial surveys occur during two specific times of the year: After hunting season and during spring green-up. During those flights, the data collected provide FWP with a wealth of information about what is going on with herds in the region.
In regions with enough mule deer density, wildlife staff take to the air in fixed-wing aircraft immediately after general season ends and start conducting surveys across established deer trend areas. Southeast Montana, for example, has 15 deer trend areas (DTAs) spread out across the region. These DTAs were created to represent the varying habitat types, landownership types and access opportunities. Post-hunting season surveys provide staff with not only total counts in the DTAs, but from those counts, wildlife staff can understand ratios of bucks per 100 does as well as fawns per 100 does.
Once winter has receded and the landscape begins to turn green, wildlife biologists hop back in the planes and conduct their spring "green-up" or recruitment surveys. It is at this point in the annual cycle where the fawns on the landscape have survived their first winter, the most challenging part of the year for them, and are now considered recruited into the population as mortality rates are much lower for them. The data collected during these flights includes total counts within DTAs and ratios of fawns per 100 adults, as the bucks have shed their antlers.
Aerial surveys are the most efficient way to determine populations from season to season. Counting every deer in a region is not possible either financially or physically due to time, weather and pilot availability. As it is, biologists log hundreds of hours each year to conduct surveys.
Flights made during the same time window every season in the same areas allow for the best comparison of herd numbers and make-up from year to year. This data equips FWP to make adjustments in its management.
"Starting about five or six years ago, as mule deer numbers started to decrease here in response to drought conditions, FWP started cutting back on their B (antlerless) licenses," Dorak said. "Over time, when we watched our mule deer numbers go from above our long-term average down to about 52 percent below long-term average, we decreased our B licenses about 91 percent, so we're at a pretty low number of B licenses out there on the landscape (1,000 since 2023)."
Dorak stressed that it's important to maintain some B licenses for management because higher mule deer numbers in some areas can lead to game damage and habitat degradation, and hunter harvest is a critical tool to manage impacts from chronic wasting disease and other illnesses.
Aerial surveys keep a pulse on the health of the deer herd by including trends in population size, fawn recruitment rates, mortality events of deer, hunter harvest rates and the age structure of harvested deer.
Aerial surveys also account for the multitude of mortality events that occur throughout any given year. And recruitment ratios in spring take into account any loss from predation, disease, malnutrition or other potential causes of loss in the population.
When data from all these efforts in a single season are added into the long-term dataset, FWP can see what the long-term trend looks like with the herd. "Through 40 years of data, looking at those deer trend areas, it pieces together a long story so we can understand what's occurring," Dorak explained.
Aerial surveys are extremely informative, but they are just one of several tools that FWP utilizes to gather data. Other methods are biological check stations, hunter harvest surveys and disease monitoring. Checking in with hunters at check stations
Biological hunter check stations play a critical role in data collection. Wildlife staff collect an array of information on harvested wildlife as well as information from hunters and their experiences. Typical data collected include success rates, age estimates from harvested mule deer, antler measurements, location and landownership where the deer was harvested, satisfaction rates and disease testing.
Region 7 wildlife staff have been operating hunter check stations for more than 40 years, and every year staff visit with hundreds and sometimes thousands of hunters from across the region and beyond.
What we can learn from mule deer teeth
At check stations, wildlife staff collect age data on harvested deer by looking at eruption and wear on the teeth on the lower jaw of hunter-harvested deer. This typically requires cutting the cheek of the deer to see the row of premolars and molars along the jawbone. The first three teeth are technically premolars and help with aging deer that are typically 1 to 1 ½ years old. Deer older than that require looking at the fourth tooth in the row, the first true molar, for the amount of wear and how much dentin is showing beneath the enamel.
As part of increasing sampling efforts for chronic wasting disease surveillance, FWP has been aging incisors collected from hunter-harvested mule deer using microscopic growth rings to provide a more accurate age estimate than eruption-wear techniques. Every year hundreds of incisor samples are sent in from Region 7 for lab analysis, and that information provides staff with critical age ratio and structure data about the population.
Hunter harvest surveys track effort, success
Harvest and effort rates are estimated through the annual hunter harvest survey phone calls completed by FWP staff. These randomized survey sampling efforts reach out to many hunters, both resident and nonresident, throughout the fall and winter to gather specific data used to provide localized and statewide information on deer hunting efforts and success rates.
Harvest information collected typically includes residency, species, license or permit type, sex and age of harvested animal, method of harvest, antler points, and location of harvest (typically hunting district).
Effort data collected include residency, species sought, location (hunting district or region), and number of days spent hunting. These data help wildlife staff look at trends in harvest and effort rates as they relate to the long-term data as well as whether there is some shift occurring over time or spatially across the region.
How disease impacts mule deer populations
Disease outbreaks and their impacts play a large role in mule deer populations and their management. Two of the main diseases that wildlife staff monitor in southeast Montana are chronic wasting disease (CWD) and hemorrhagic disease (epizootic hemorrhagic disease [EHD) and bluetongue virus [BTV]). Throughout the year, wildlife staff collect lymph node samples to test for CWD from hunter-harvested mule deer, roadkill, symptomatic deer that need to be euthanized, and dead deer where samples are still viable. Information from these test results helps determine prevalence rates for CWD within mule deer herds across the region.
During late summer and fall, wildlife staff keep their eyes open for EHD or BTV outbreaks. These outbreaks happen every year, but typically there is a more severe outbreak roughly every 7 to 10 years, and it occurs more in white-tailed deer but also impacts mule deer. When it appears like an outbreak is occurring, staff collect lung and spleen samples from fresh mule deer to send to the lab for testing. When lab results confirm EHD or BTV, staff work with landowners and members of the public to keep track of the number and location of reported sick or dead deer to have an understanding of the localized impact.
While adding to and analyzing decades of aerial survey data and lab-based age analysis, FWP is always looking for ways to better understand what is currently going on with mule deer herds in Region 7. These data collection efforts and future research projects help guide mule deer management so that FWP can continue to fulfill our mission to steward the fish, wildlife, parks and recreational resources for the public, now and into the future.
For those interested in more on mule deer management, FWP will host a series of regional townhall-style meetings this fall to discuss the state of mule deer and CWD management as a whole. The Miles City meeting is set for Sept. 29 at Miles Community College from 6 to 9 p.m. More details will be provided closer to the meeting. For other meeting dates and locations, visit FWP's website. This article is also part of a series of articles, videos and other outreach FWP has planned on mule deer in Montana.