Research Article
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Article first published online: 8 SEP 2015
DOI: 10.1002/jwmg.985
Published 2015. This article is a U.S. Government work and is in the public domain in the USA.
ABSTRACT
Harvest indices are used by state wildlife management agencies to monitor population trends and set harvest quotas for furbearer species. Although harvest indices may be readily collected from hunters, the reliability of harvest indices for monitoring demography and abundance of the harvested species is rarely examined, particularly amongst large carnivores. The overall objective of this study was to assess whether cougar (Puma concolor) harvest statistics collected by wildlife managers were correlated with changes in cougar demography, mainly survival rates and abundance. We estimated key demographic parameters for 2 cougar populations in Utah over 17 years during which we monitored 235 radio-collared cougars. We then compared these demographic parameters to harvest statistics provided by the Utah Division of Wildlife Resources over the same time period for the Oquirrh-Stansbury (lightly harvested population) and Monroe (heavily harvested population) harvest management units. In the Oquirrh-Stansbury unit, the percent of harvested cougars >6 years old was positively correlated with annual survival, indicative of a population experiencing several years of high survival resulting in an older age structure. Percent of permits filled and cougar abundance were also significantly correlated, suggesting higher hunting success with increased density. In the Monroe management unit, the annual percent of permits filled was correlated with changes in overall annual survival and male and female annual survival. Of utmost importance, pursuit success (cougars treed/day) increased with the number of cougars on the unit suggesting that pursuit indices may be an informative metric for wildlife managers to determine cougar population trends. Because both management units were subjected to contrasting mortality regimes, results provided by this assessment could potentially be applied to additional management areas sharing similar ecological characteristics and harvest metrics. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.
Knowledge of the status of a carnivore population is essential for the development and implementation of an effective management plan (Ginsberg 2001, Pollock et al. 2012). Carnivores are often managed through regulated sport hunting to maintain viable populations (Sillero-Zubiri and Laurenson 2001, Keefover-Ring 2005), and reduce impacts of predation on their principal prey species and domestic livestock (Treves and Karanth 2003, Anderson et al. 2010, Loveridge et al. 2010). Management agencies often face the difficulty of opposing demands for more effective carnivore control to protect human safety, big game populations, and domestic livestock, and the demand for additional carnivore-hunting opportunities by sportsmen and outfitters and even societal demands for protection from exploitation (Sillero-Zubiri and Laurenson 2001, Anderson et al. 2010, Funston et al. 2013).
Given their large spatial requirements, low densities, and elusiveness, the management of large carnivores is often challenging because of the difficulties in estimating vital rates and population abundance (Gese 2001, Pollock et al. 2012). Cougar (Puma concolor) management nevertheless depends on the ability to monitor demographic responses to changing policies and management actions (Anderson et al.2010). Unfortunately, state and provincial wildlife agencies are often required to make management decisions without the demographic information needed to monitor and maintain sustainable cougar population levels from one harvest season to the next (i.e., adaptive harvest management) because this information is often unavailable. Frequently, harvest composition statistics (e.g., age structure and sex composition) are used in lieu of measured demographic variables of population performance and abundance (Whittaker and Wolfe 2011). Harvest data alone is not sufficient for estimation of population size but rather should be used in conjunction with additional demographic data such as annual survival rates (Erickson 1982, Kolenosky and Strathearn 1987, Lindzey 1987, Rolley 1987, Chilelli et al. 1996). The question arises as to whether harvest statistics and harvest composition are reasonable approximations of changes in demographic performance (e.g., survival) and population abundance over time.
Of all demographic estimates, wildlife managers are most interested in monitoring animal abundance because annual changes in abundance measure the net balance among births, immigrants, deaths, and emigrants (BIDE), and indicate whether there is a surplus that can be sustainably harvested from year to year. Because a complete census is never possible, abundance must be estimated using appropriate methods that can account for imperfect detection and even multiple counting of individuals. Indeed, a number of approaches have been proposed for estimating cougar abundance and associated densities (Van Dyke et al. 1986, Smallwood and Fitzhugh 1995, Choate et al. 2006), but all have logistic limitations and statistical assumptions that are difficult to meet in a field setting.
When abundance becomes too difficult to accurately estimate, attention is sometimes transferred to the BIDE vital rates that determine abundance to monitor population trends rather than abundance per se. Immigration and emigration may play a large role in the change of male cougar abundance (Robinson et al. 2008), but in the female-limiting component of the population attention should be focused on reproductive success and survival (Lambert et al. 2006). Regardless of whether the focus is on the male or female component, cause-specific mortality analyses can provide deeper insight into the factors underlying management-relevant changes in survival and population dynamics (e.g., hunting vs. vehicle collisions).
The Utah Division of Wildlife Resources (UDWR) currently uses harvest rate, percent females in the harvest, and number of cougars treed per day to set the following years harvest quotas (Utah Cougar Advisory Group 2011). The cougars treed per day can be thought of a catch-per-unit-effort estimator (Choate et al. 2006). Although there was no significant relationship between cougars treed/day and the size of 2 cougar populations monitored for 6 years (Choate et al. 2006), the UDWR incorporates this index in their formula to determine harvest levels. We calculated estimates of key demographic parameters from 2 cougar populations that were intensively monitored in Utah for 17 years, and compared these estimates to harvest statistics provided by the UDWR over the length of the study period. Cougars in the Oquirrh-Stansbury cougar management unit (OSCMU) were primarily exposed to non-hunting anthropogenic sources of mortality and cougars in the Monroe cougar management unit (MCMU) were mostly influenced by hunting mortality. Our objective was to assess the correlations between currently used harvest statistics and independently derived population parameters within the OSCMU and MCMU.
STUDY AREA
We examined cougar populations on the OSCMU and MCMU, located in the Great Basin and Colorado Plateau ecoregions, respectively, in Utah. Mountain ranges in these ecoregions were surrounded by desert basins and formed a basin and range landscape. Annual precipitation ranged from 60 cm to 120 cm in the higher elevations to 15–20 cm in the desert basin regions with most of the precipitation arriving as snow in January and February (Moller and Gillies 2008). The Oquirrh-Traverse Mountains were dominated by Gambel oak (Quercus gambelii), sagebrush (Artemisia spp.), and Utah juniper (Juniperus osteosperma), whereas Monroe Mountain was dominated by pinyon (Pinus edulis)-juniper (Juniperus spp.) woodlands.
The OSCMU was located in north-central Utah on the eastern edge of the Great Basin (40.5°N, 112.2°W). The Oquirrh Mountains measured >950 km2, but the study area was focused on a 500-km2 area encompassing the northeastern slope on properties owned and managed by the Utah Army National Guard (Camp Williams) and the Kennecott Utah Copper Corporation. The site was bounded on the north by the Great Salt Lake and on the east by the Salt Lake Valley. Approximately, 55% of the study area was under the jurisdiction of the Bureau of Land Management (BLM), with the remainder held by individuals, grazing associations, mining companies, and the military. The study area was situated within the larger OSCMU, but both properties (Camp Williams and Kennecott) were closed to the public and cougar hunting was prohibited. Although radio-collared cougars leaving those properties were legally protected within the management unit, they were susceptible to poaching, depredation control, trapping, and road kill. Thus, this population was considered to be semi-protected.
Monroe Mountain comprised part of the Sevier Plateau in south-central Utah (38.5°N, 112°W). The study area measured approximately 1,300 km2, and formed the central part of the Fishlake National Forest. Additional landholders included the BLM, the State, and various private interests. The study area was within the MCMU, where cougars were managed for sustainable hunting opportunities. Other carnivores present included bobcats (Lynx rufus) and coyotes (Canis latrans), which were both subject to trapping pressure. Resource use included livestock grazing (cattle, sheep), logging, fossil fuel exploration, and off highway vehicle recreation (e.g., all terrain vehicles). Stoner et al. (2006) provide a more detailed description of the study areas.
METHODS
Cougar Harvest in Utah
Nearly all cougars harvested in Utah are taken with the aid of dogs (Utah Cougar Advisory Group 2011). An individual hunter is restricted to holding either a limited entry permit or a harvest objective permit per season, and must wait 3 years to reapply once they acquire a limited-entry permit. The bag limit is 1 cougar/season, and kittens and females accompanied by young are generally protected from harvest. Currently, the cougar hunting season runs from late November through late May on both limited entry and most harvest objective units. Some units are open year-round and some have earlier or later opening dates. Pursuit (chase or no-kill) seasons provide additional recreational opportunities over most of the state. The pursuit season generally follows the hunting season, but specific units have year-round pursuit and a few units are closed to pursuit (Utah Cougar Advisory Group 2011).
We used information covering 1996–2012 that was published in the most recent Utah Cougar Annual Report (Utah Division of Wildlife Resources 2012), which collated information for a number of harvest and pursuit statistics used by UDWR managers from the OSCMU and MCMU; reporting of each cougar harvested is legally mandated. We first focused on the 3 indices used to monitor cougar population trends and guide management in Utah: percent females in harvest, number of cougars treed per day, and number of cougars harvested annually. We examined additional harvest indicators that were specific to each sex (i.e., annual no. harvested males, % of males in the harvest) and harvest indicators that pertained to age (i.e., proportion of cougars that were ≥6 years of age in the harvest, the mean age of harvested animals each year). Finally, we examined statistics related directly to harvest regulations (i.e., % of hunting permits filled each year, no. sport-harvested cougars, no. harvest permits allotted, including all limited entry, conservation, and conventional permits; Utah Division of Wildlife Resources 2012).
