 Each winter, the Idaho Department of Fish and Game and volunteers capture and radio-collar hundreds of mule deer throughout Idaho as part of a statewide effort designed to monitor and better understand mule deer populations. |
Mule Deer Wrestling Rodeo
January 4, 2006
By Pete Zager and Mark Hurley - IDFG
Wrestling a frantic, untranquilized mule deer to the ground makes for a good Hollywood action scene, but each winter, dozens of Idaho Department of Fish and Game employees, volunteers, and any other warm body that can be rounded up wrestle deer as part of a statewide effort designed to monitor and better understand mule deer populations.
These camo-clad soles hide amongst the snow and sagebrush while a helicopter herds small groups of mule deer into a "drive net" - a strategically placed system of tall nets -- where the deer become entangled.
Enter the wrestling rodeo. The biologists quickly subdue and disentangle the animals. Adults are ear-tagged with a few fitted with radio-collars and then released. Fawns are sexed, weighed, measured, ear-tagged, radio-collared, and also released. All of this is done as quietly and as quickly as possible to minimize stress. If all goes well, all deer are processed and released within five minutes of capture.
Mule deer are frustrating to manage because their populations are so volatile - and seemingly unpredictable. A myriad of factors influence mule deer populations. Weather, including a severe winter or an exceptionally dry summer, can cause dramatic year-to-year variation in survival and recruitment.
Habitat quality also varies across the state, and from year to year. Habitats are continually changing, too. Wildfire alters habitats literally overnight, while invasive species such as cheatgrass take a stealthier approach. Predation is certainly an issue, but determining the extent to which predators limit populations is very difficult.
Mule deer across the West have declined recently. Biologists do not understand why populations declined or why they are not recovering in many areas. However, they agree that most of the factors that affect deer survival have the greatest impact on fawns.
Because fawns accumulate less fat reserves than adults during summer and fall, they are more susceptible to weather severity, poor quality habitat, predators, and other factors. Therefore, fawns typically have higher, and more variable, mortality rate than adult deer. Annual variation in fawn survival can significantly impact mule deer populations.
Each year, eight to ten study areas are selected throughout Idaho that represents a variety of mule deer habitats. Twenty to thirty, six-month-old mule deer fawns are captured and radio-collared in each study area. After a few radio failures, deer that immediately disappear, etc., biologists end up monitoring 200 to 250 mule deer fawns in a typical winter.
The fawns are monitored about three times each week from time of capture, between mid-December and mid-January, until mid-May. When a "mortality signal" is detected, a biologist investigates immediately to determine cause of death.
Fawn survival varies from study area to study area, and from year to year. For example, 90 percent of the fawns survived the winter of 2000-01 along the Boise Front, but survival was less than 50 percent during winter of 2001-02, which was a difficult winter in many areas.
Furthermore, survival is typically low in some study areas and high in others. Fawn survival in game management unit 36B, near Challis, is typically lower than elsewhere, often less than 40 percent, while survival along the Boise Front is usually good.
Among the fawns that die, malnutrition and predation by coyotes and cougars are the most important mortality factors. Expectedly, malnutrition takes an especially heavy toll during difficult winters.
Using the data accumulated over the last several years, Mark Hurley, senior wildlife research biologist and project coordinator, developed a model to predict mule deer fawn overwinter survival across central and southern Idaho.
Significant variables in the survival model include mass of the fawn, winter precipitation, and summer precipitation. Heavier fawns survive better than lighter ones, survival declines as winter precipitation increases, and survival increases with summer precipitation. Predation did not factor into the model because the other variables were better predictors of mule deer fawn overwinter survival.
Obviously there is not much IDFG can do about the weather, but there may be some possibilities on the habitat front, which may translate into larger fawns that survive better. The Department is exploring those possibilities through its Mule Deer Initiative.
One result of this work is the realization that there are fundamental differences in mule deer population dynamics across the state. To account for that, IDFG has incorporated these local estimates of fawn overwinter survival into their mule deer monitoring program. This data helps IDFG meet population goals by adjusting doe harvests according to annual changes in survival, recruitment, and population size.
So if you are looking for an IDFG wildlife biologist during December or January, don't look to Hollywood. You may find him or her hiding nearby, under some sagebrush, ready to pounce on an unsuspecting mule deer fawn.
Pete Zager is a principle wildlife research biologist stationed in Lewiston. Mark Hurley is a senior research biologist stationed in Salmon