Investigation into Possible Factors Affecting the Recruitment of Rocky Mountain Elk on the Valles Caldera National Preserve



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The Valles Caldera National Preserve (VCNP) is a relatively new public land holding located in the Jemez Mountains in north-central New Mexico. For the past decade, low calf:cow ratios of elk (Cervus elaphus) have been recorded on the VCNP prompting concern by both the Valles Caldera Trust, the managing body for the VCNP, and the New Mexico Department of Game and Fish (NMDGF) managers who are responsible for ensuring a sustainable elk population. These studies were done to look at possible factors that may be contributing to the low recruitment of elk on the VCNP as well as simulate how the population may respond to different management actions.

A serological survey was conducted on hunter-harvested elk from the VCNP during the 2010 and 2011 fall hunting seasons. We tested 119 (2010, n =74; 2011, n = 45) for brucellosis, bovine viral diarrhea virus (BVDV), infectious bovine rhinotracheitis (IBR), bluetongue (BT), epizootic hemorrhagic disease (EHD), parainfluenza-3 (PI-3) and 5 serovars of Leptospirosis. Exposure rates were highest for PI-3 (34.5%) and IBR (10.1%) for total samples. Exposure for BVDV was found in 2.5% of the total samples. Exposure to EHD and BT was in 1.7% and 0.8% of the total samples. Exposure to the Leptospirosis serovars hardjo (n=2) and canicola (n=1) were found in the samples tested. All samples tested negative for brucellosis. The low to medium positive exposure to all the diseases except brucellosis indicate these diseases are present within the VCNP elk population but are not likely causing the low recruitment rates.

A secondary study was done to address the hypothesis that the observed low calf:cow ratios on the VCNP may be the result of low calf survival rates. To determine cause-specific mortality and estimate survival, we radio marked elk calves (n =140) born within the VCNP from 2009-2011. Two approaches were done to estimate 14 day survival, overall summer survival (t = 13 weeks), and annual survival. First, biological covariates were used to model survival for 14 day post capture and weekly summer survival for all calves. This was done in order to compare survival estimates to other studies experiencing low calf:cow ratios in the western United States. Then, the data were re-analyzed by censoring calves (n = 9) which died the day after tagging. This provided a more conservative survival estimate reduced possible biases due to handling calves and its potential effect on survival. Fourteen day survival was 0.57 (SE = 0.05, 95%CI: 0.48-0.66) when using uncensored data; survival was 0.64 (SE = 0.05, 95%CI 0.54-0.72) using censored data. Summer survival was estimated to be 0.37 (SE = 0.05, 95%CI 0.28-0.47) using uncensored data; summer survival was estimated 0.43 (SE = 0.05, 95%CI: 0.33-0.53) using censored data. Fall/winter survival (mid-August-April) ranged from 0.76-0.95 based on study year. Annual survival ranged from 0.32-0.55 with a mean of 0.42 using uncensored data; annual survival ranged from 0.32-0.59 with a mean of 0.46 using censored data. Predation accounted for 94.8% of the known deaths. Black bears (Ursus americanus) were the highest source of predation (47.3%, n = 26) and overall mortality (40.6%). Coyotes (Canis latrans) were the second highest source of predation (41.8%, n = 23) and overall mortality (35.9%). The results using both uncensored and censored data are comparable to other studies which were experiencing low calf:cow ratios. Therefore predation of elk calves is likely additive and causing suppressed recruitment rates in the elk population on the VCNP. However, it is highly recommended to estimate the large predator population prior to any large predator management to ensure the population of large predators is not reduced to a level from which they cannot recover.

For the third part of this study the overall objective was to incorporate data that were available into the population simulation program POP-II, to model current population trends based the current data and then run simulated population trends based on possible management decisions. Data that were used included: summer survey data which used distance sampling to estimate population size and herd ratios; fall harvest numbers for adult male and adult female elk; and estimated summer survival rates of elk calves born on the VCNP. Models were developed based on current preseason mortality of neonatal elk with a reduction in harvest of adult males and females by 10%, both separately and collectively. Current harvest numbers were maintained while preseason mortality of neonates was reduced by 25% and 50%. Comparisons of the model output were then made for both the reduction of preseason mortality along with a reduction in harvests. Models showed that decreasing preseason mortality while maintaining current harvest rates would either stabilize or begin a positive increase in the population trend. The models also showed that making a 10% sex-specific (males only or females only) harvest reduction while maintaining current preseason mortality would maintain the current population trend. Making a 10% harvest reduction of either sexes or both sexes along with a reduction in preseason mortality should lead to positive population trends.

The results of this study might be used to help guide adaptive management decisions regarding population trends of the elk herd in the Valles Caldera National Preserve.