Effects of helicopter net gunning on the survival and movement behaviour of nilgai antelope

Abstract

Context. Research on large, terrestrial mammals often requires physical captures to attach tags or collars, collect morphological data, and collect biological samples. Choice of capture method should minimise pain and distress to the animal, minimise risk to personnel, and consider whether the method can achieve study objectives without biasing results. Aims. We studied how capture via helicopter net-gunning affected survival, post-capture movement patterns, and space use of exotic nilgai (Boselaphus tragocamelus) in southern Texas, USA. Methods. We estimated daily survival rates for 101 collared nilgai over 28 days, following 125 captures. We calculated mean daily movement rates and net-squared displacement for 21 recaptured nilgai for 60 days, starting 30 days before capture. Key results. The survival probability of 125 nilgai individuals was 0.97 (95% CI = 0.92-0.99) over the 28 days following capture, with the lowest daily survival for the day after capture (x ¯ = 0.99; 95% CI = 0.96-1.00). We observed an increase of ~65% in the mean daily movement rate of 134 m/h on the first 2 days since capture, followed by a period of reduced movement out to the 5th day before returning to pre-capture levels. Analysis of netsquared displacement for 21 nilgai showed that 17 resumed pre-capture space-use patterns within a week, whereas four individuals did not return to the pre-capture range for ≥1 month. Conclusions. Capture-related mortality rates for nilgai using helicopter net-gunning in our study (3%) were similar or lower than those reported for similar species captured using the same method. While we were able to detect a period of elevated movement rates, followed by a recovery period of diminished movement as a result of capture, nilgai appeared to return to typical behaviour ~6 days post-capture. Most nilgai in our study also resumed typical space-use patterns within a week of capture; however, our results suggest high individual variability in their response. Implications. We recommend using net-gunning from a helicopter as a method for capturing nilgai when conditions and where vegetation and topography allow. We suggest censoring data for a minimum of 7 days following capture for analyses related to survival and movement rates. For analyses relating to space use, we suggest inspecting net-squared displacement or some similar displacement analysis for each animal separately to account for individual variation in response and exclude data accordingly.

Evaluating aerial net gunning and chemical immobilisation for capture of invasive sambar deer (Rusa unicolor) and red deer (Cervus elaphus) in alpine Australia

“…4). Short-term increases in movements for the period following capture have previously been reported for many Cervidae species, including moose (Alces alces) and male roe deer, which exhibit flight behaviour and show greater displacement from their home ranges following capture (Morellet et al 2009;Neumann et al 2011), as well as for an antelope species, nilgai (Boselaphus tragocamelus; Baumgardt et al 2023). Like sambar deer, male roe deer and moose are largely solitary and tend not to exhibit strong social bonds (Geist 1963;Hewison et al 1998), so may be more likely to move large distances following capture.…”

“…Comparing movement and activity rates pre-and postcapture would have allowed the delineation of movement changes due to capture (e.g. Morellet et al 2009;Neumann et al 2011;Northrup et al 2014;Baumgardt et al 2023) but was not possible in the present study. Elevated movement rates for the period immediately following collaring did not occur in red deer (Fig.…”