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

Authors: BAUMGARDT, JEREMY - Texas A&M University; FOLEY, AARON - Texas A&M University; SLIWA, KATHRYN - Texas A&M University; DEYOUNG, RANDY - Texas A&M University; ORTEGA S., ALFONSO - Texas A&M University; HEWITT, DAVID - Texas A&M University; CAMPBELL, TYLER - Texas A&M University; Goolsby, John; Lohmeyer, Kimberly - Kim

Submitted to: Wildlife Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/29/2022
Publication Date: 1/12/2023
Citation: Baumgardt, J.A., Foley, A.M., Sliwa, K.M., Deyoung, R.W., Ortega S., A.J., Hewitt, D.G., Campbell, T.A., Goolsby, J., Lohmeyer, K.H. 2023. Effect of helicopter net gunning on the survival and movement behaviour of nilgai antelope. Wildlife Research. DOI: 10.1071/WR22049.
DOI: https://doi.org/10.1071/WR22049

Interpretive Summary: Net-gunning from a helicopter is a common method of capture for many ungulates; however, impacts of this method on survival and movement patterns have not been assessed for nilgai antelope. This study estimated survival following capture using data from 3 projects and compared movement patterns 1 month before capture to 1 month after capture for a subset of collared nilgai. Results from this study indicate that captures utilizing net-gunning from a helicopter are a viable option for capturing nilgai. The authors also make recommendations on censoring post-capture movement data according to the type of analysis.

Technical 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 evaluate whether the method can efficiently achieve study objectives. For studies involving space use and movement behaviour, it is also important to understand the influence of capture on animal movements. Aims. We studied how capture via helicopter net-gunning affected survival, post-capture movement patterns, and space use of exotic nilgai antelope (Boselaphus tragocamelus) in South Texas, USA. Methods. We estimated daily survival rates for 125 collared nilgai over 28 days following capture. We calculated mean daily movement rates and net squared displacement for 21 recaptured nilgai for 60 days starting 30 days before capture. Key results. Survival of 125 nilgai averaged 0.968 (95% CI = 0.918–0.0988) over the 28 days following capture, with the lowest daily survival for the day after capture (x¯ = 0.988; 95% CI = 0.960–0.997). We observed an increase of ~65% in the mean daily movement rate of 134 m/hr on the day of and the day following 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 revealed that 17 resumed pre-capture space use patterns within a few days, while 4 individuals did not return to the pre-capture range for = 1 month. Conclusions. We conclude that net-gunning from a helicopter is a safe method for capture of nilgai antelope, and that most individuals resume pre-capture behaviour within 1 week. Implications. We recommend using net-gunning from a helicopter as a method for capturing nilgai antelope 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.