Comparing fecal DNA mark-recapture to mark-resight for estimating mule deer (Odocoileus hemionus) abundance on winter range in the eastern Sierra Nevada

Andi Stewart and Alisa Ellsworth, California Department of Fish and Wildlife, Bishop, CA, Andi Stewart and Mary Conner, Natural Resources, Utah State University, Logan, UT, Jane McKeever, California Department of Fish and Wildlife, Ontario, CA

Background/Question/Methods
Monitoring big game populations is vital for making well-informed management decisions. In the eastern Sierra Nevada, the Round Valley mule deer (Odocoileus hemionus) herd is monitored using traditional mark-resight methods on the winter range. Although mark-resight methods work well for estimating population size, animal capture for marking is expensive, invasive, and has the potential of injury or death for animals and researchers. These drawbacks motivated us to evaluate fecal DNA-based mark-recapture, a non-invasive method for estimating population abundance. This method has been successful for estimating abundance in areas where it’s difficult to use resighting methods but hasn’t been evaluated on winter ranges where traditional methods work well. Our primary goal was to compare precision and cost of fecal DNA mark-recapture surveys to aerial mark-resight surveys for estimating population abundance. We used GPS data to define the Round Valley winter range, over which we laid a hexagonal grid of 1km2 cells. We walked 300m transects in randomly selected cells to collect fecal samples. After samples were individually genotyped, we constructed capture histories for four sessions conducted each winter. We used closed-capture models to estimate population abundance (N) and precision (CV), which we compared to estimates from concurrent aerial mark-resight surveys in 2019 and 2020.
Results/Conclusions
In 2019, we sampled 20% of the Round Valley study area. Based on fecal DNA mark-recapture analysis, the abundance estimate was 4,814 deer with a CV of 22%. In 2020, we sampled 25% of the Round Valley study area and the abundance estimate was 3,303 deer with a CV of 9.4%. For 2019, the abundance estimate from aerial mark-resight surveys was 2,779 deer with a CV = 4.6% and the 2020 results will be available by August 2021. Based on these preliminary results, if an adequate proportion of the study area is surveyed, fecal DNA mark-recapture can produce similar estimates of N with reasonable precision compared to aerial mark-resight methods. An economic analysis of the cost per unit of precision for fecal DNA mark-recapture compared to aerial mark-resight will be completed in May 2021 and results will be available by August 2021. The economic analysis will evaluate whether fecal DNA mark-recapture has potential as a cost-effective and non-invasive method for estimating population abundance. Results from this study will advance methods for management and conservation of big game species.