On the La Sal Mountains, mule deer (Odocoileus hemionus) fawn production from birth to 6 months of age has been significantly lower during the past 20 years than values observed during the 1970s and 1980s. Recent extreme drought conditions have certainly played a role in poor neonate survival and recruitment, but even during wet years with light to moderate winter conditions, fawn production has been lower than historic values. These historic values were similar to the neighboring San Juan, Abajo Mountains Management Unit; however, the La Sal's values have decreased through the years, despite having greater perennial water sources and higher quality and quantity summer range habitats. The La Sal Mountains have one of the highest densities of black bears (Ursus americanus) in Utah along with a healthy cougar (Puma concolor) population; however, the role of these predators in limiting mule deer populations on this unit is unclear. Better understanding the causes of neonate mortality and the possible impacts of predation whether it be additive or compensatory, will enable more effective management of these populations. Potential explanations for limited growth of ungulate populations include intraspecific competition, winter severity, drought and associated poor nutrition, predation, and degradation or fragmentation of habitat (Pojar and Bowden 2004, Bishop et al. 2005, Harrington and Conover 2007, Lomas and Bender 2007). Adult animals typically have relatively high survival (Unsworth et al. 1999). Two years of survival data on this herd have yielded an average of 81% apparent survival for adult female deer, with the majority of mortality being cougar predation. One year of survival data for fawns (6 months to 1.5 years) in 2019-20 yielded an 82% survival estimate, with no clear trend in a single mortality factor. Even with these average to high recent survival estimates, this herd remains well below population objective and consistently has a relatively low fawn:doe ratio, despite historically being one of the top units in the region for deer production (Pederson 1970). There has been little evidence for intraspecific competition in previous studies suggesting this is likely not the cause of the current population trend (Hansen and Reid 1975, Torstenson et al. 2006). Except for winter 2016-2017, Utah has experienced relatively mild winters since 2012 also suggesting that severe weather is likely not the cause of limited population growth. Variables that that have the greatest likelihood of explaining the pattern of limited population growth include habitat quality and predation. An expanse of summer range habitat exists on the La Sal Mountains and is not thought to be a limiting factor. However, nutritional components of that habitat may be lacking, which can limit growth (Bishop et al. 2009). Additionally, the La Sal Mountains have a complex predator community at relatively high densities. It has been well documented that complex, high-density communities of predators can limit population growth, especially when populations are well below carrying capacity (Ballard et al. 2001, Laundre et al. 2006, Griffin et al. 2011). Predation is most likely to manifest an effect on juveniles and is consistent with the pattern of low fawn:doe ratios. However, predation could also influence adult populations in this complex predator community (Griffin et al. 2011).

The objective of this project is to determine the factors limiting mule deer fawn production and recruitment on the La Sal Mountains and San Juan WMUs. More specifically, we will try to determine the relative importance of habitat, climatic conditions and predation on adult and juvenile mule deer. To accomplish this objective we will 1) capture adult and neonate mule deer, 2) determine nutritional condition of adult animals, and 3) determine survival and cause-specific mortality of all animals.

La Sal and San Juan units

The mule deer management has a strategy to implement research projects on populations that are chronically below objectively. The mule deer management plan also calls for DWR to evaluate fawn production.

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To determine the effects of habitat on nutritional condition of adult mule deer, we will randomly capture adult deer (stratified among 5 winter range locations) in December and March of each year to assess body condition coming into winter (a function of summer habitat quality) and condition leaving winter (a function of winter habitat quality). Body condition will be determined by estimating percent body fat based on a proven combination of palpation and ultrasonography measures (Cook et al. 2007). Body condition data will be compared with data collected on units from around Utah. We will fit each captured individual with a GPS collars to determine space use patterns, estimate survival, and determine probable cause of death. In addition, by knowing the exact location of deer during the entire year, we anticipate using these locations in a resource selection framework (RSF) to evaluate relative seasonal and annual selection for habitat types across the entire study area at both 2nd and 3rd order selection scales (Johnson 1980). By recovering deceased animals within 48 hours of death, these collars also will enable the determination of survival and cause-specific mortality. We will use model selection and known-fate models in Program MARK (White and Burnham 1999) to estimate seasonal and annual survival rates and examine the influence of body condition (see below), habitat use, movement patterns, and environmental conditions on these rates. To examine herd production and recruitment, we will determine pregnancy rates and survival of neonate animals. During the February and March captures, all pregnant adults will be fitted with vaginal implant transmitters (VITs; Bishop et al. 2007). These VITs will allow us to capture and collar neonates in spring to estimate survival, recruitment, and cause-specific mortality of this under-studied age class. Neonate deer collars will last for 6-8 months and will be supplemented by capturing 6-month old fawns in December to estimate recruitment to 1 year old. This project was be initiated in January 2023 and will be completed in December 2026. Captures will be conducted by the DWR contracted capture company using standard net-gunning techniques. Once a deer is captured, the capture company will transport it back to a staging area for processing by UDWR employees/volunteers. Data collected from the animal will include age, sex, various body measurements, blood samples, reproductive condition, and body condition (Cook et al. 2010). All animals will be processed as quickly as possible and released at the staging area.

Deer will be monitored using satellite GPS collars.

This project is a partnership with DWR, BYU, and conservation organizations.

By understanding what factors are limiting fawn production and recruitment on the La Sal Mountains and San Juan WMUs, we can target management actions to address the issues.

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