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 ¯ WR22049_IE1.gif = 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 net-squared 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.