Effect of Net‐Gun Capture on Survival of Mule Deer

Abstract

Capture techniques to deploy radio-collars often risk mortality and injury to the animal. Capture-induced mortality can affect population sizes but also introduces bias in survival estimates based on data from captured animals. In recent years, a large-scale research and monitoring project in Utah, USA, has involved capturing and radio-collaring hundreds of mule deer (Odocoileus hemionus), a species of great interest in large parts of North America. Our objective was to investigate how the survival rates of these mule deer were affected by capture and handling. During winters of 2014-2018, an experienced capture crew net-gunned and fitted 1,805 animals with global positioning system (GPS)-collars. We estimated survival rates during the first 6 weeks after capture using Cox proportional hazard regression, and compared the survival rates of animals that were captured in a particular year to those of animals that were not captured but fitted with a GPS-collar in a previous year. We used a model selection framework to evaluate how long survival rates of captured animals were different from those of animals that were not captured. Our results indicated that weekly survival rates of captured animals were 0.985 ± 0.003 (SE), 0.988 ± 0.002, and 0.990 ± 0.001 in weeks 1, 2 and 3, respectively. Weekly survival rates of captured deer during weeks 4-6 were 0.993 ± 0.001, the same as those of deer that were not captured at the same time. Furthermore, post-capture survival rates were positively influenced by body size and negatively influenced by age. We conclude that the mortality resulting from helicopter capture was low but recommend comparing newly captured and previously captured individuals to examine what proportion of observed mortality is likely capture-related.

Evaluation of helicopter net-gunning to capture wild fallow deer (

“…Jacques et al 2009;Latham et al 2020;Ortega et al 2020) may also have contributed to the favourable outcomes, despite the high ambient temperatures (cf. Webb et al 2008;Jacques et al 2009;Latham et al 2020;Van de Kerk et al 2020). Our chase times were comparable to those in other helicopter net-gunning or chemical immobilisation operations (Latham et al 2020;Hampton et al 2021).…”

“…Previous work has shown that high initial costs of helicopter net-gunning for capturing bighorn sheep (Ovis canadensis nelsoni) were offset by labour savings when distributed over $20 animals (Jessup et al 1988). Animal mortalities in helicopter net-gun capture programs typically occur because of trauma during pursuit and capture, and capture myopathy up to 30 days after release (Webb et al 2008;Jacques et al 2009;Van de Kerk et al 2020). Two deer in our study could have suffered major trauma or death during pursuit and capture; one deer became entangled in a fence and another fell into a creek after being netted.…”

“…Animals are captured using a weighted net fired by a shooter in a helicopter and are usually physically restrained and handled without chemical immobilisation. Helicopter netgunning is now established as standard practice for capturing several ungulate species in North America (Ortega et al 2020;Van de Kerk et al 2020), and it has also been used to capture deer for research in New Zealand (Nugent and Yockney 2004;Latham et al 2020) and South America (Flueck et al 2005). It can provide an efficient and selective method for capturing many animals in a short time over a large geographic area (Webb et al 2008).…”

“…As for any large-herbivore capture method, netgunning can pose considerable animal welfare risks, including mortality, morbidity and traumatic injuries. Large-scale studies of helicopter net-gunning for ungulate capture have shown that capture-related mortality and serious injury rates are commonly #2% (Webb et al 2008;Jacques et al 2009;Van de Kerk et al 2020). However, mortality rates $8% have been reported in some species and landscapes that have subsequently been associated with inherently high risk of injury (e.g.…”

Evaluating risks associated with capture and handling of mule deer for individual‐based, long‐term research

“…Censoring periods have ranged between 0 and 26 days (Beringer et al 1996; DelGuidice et al 2005, Hebblewhite et al 2018) for large herbivores and between 1 and 21 days for avian species (Blomberg et al 2018), with the majority of studies implementing censorship periods that are largely arbitrary (Norton et al 2016, Blomberg et al 2018). Despite the need for identifying the timeframe within which to appropriately censor survival data, the time at which capture and handling appears to have a measurable effect on survival is unknown for most species; however, it has been evaluated in some systems (Van de Kerk et al 2020, Wagler et al 2022).…”