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RESEARCH ARTICLE
Contents Vol 36(5)

Estimating the density of free-ranging wild horses in rugged gorges using a photographic mark–recapture technique

Karl Vernes A C , Melissa Freeman A and Brad Nesbitt B
+ Author Affiliations
- Author Affiliations

A Ecosystem Management, University of New England, Armidale, NSW 2351, Australia.

B Parks and Wildlife Group, Department of Environment and Climate Change NSW, PO Box 120, Dorrigo, NSW 2453, Australia.

C Corresponding author. Email: kvernes@une.edu.au

Wildlife Research 36(5) 361-367 https://doi.org/10.1071/WR07126
Submitted: 3 September 2007  Accepted: 21 April 2009   Published: 21 July 2009

Abstract

Estimating the density of large, feral species such as wild horses at landscape scales can present a logistical hurdle for wildlife managers attempting to set density-based management targets. We undertook aerial surveys of wild horses by using a helicopter in Guy Fawkes River National Park in north-eastern New South Wales across 3 years to determine whether meaningful density estimates could be obtained efficiently by a mark–recapture technique based on recognition of individual horses. Horse groups photographed from the air on the first of two surveys conducted each year were ‘marked’ on the basis of a unique combination of colours and natural markings, and ‘recaptured’ if they were photographed and identified on the second survey. Population size was estimated with the program MARK using a range of population estimators; however, because horses appeared to be evading detection on the second survey of each year, we chose a final estimation model that accounted for detection shyness in the study species. In 2005, the density estimate was 3.8 horses per km2 (upper and lower 95% CL = 3.5–5.7 horses per km2). Following horse control in these catchments, the estimate in 2007 was 2.3 horses per km2 (upper and lower 95% CL = 2.1–3.4 horses per km2), and this change in density can be accounted for by the known number of horses removed from the survey area between survey periods. Overall, the technique proved useful for estimating densities of wild horses in deeply dissected gorge country where other estimation techniques (such as line transects) are not practical; however, low recapture rates in one of the years of the study shows that the technique may not always be applicable. Our technique should also be suitable for surveying other large mammals with broad ranges in open environments, provided recognition of individuals from unique marks is possible.


Acknowledgements

We thank Tony Prior (DECC, Ranger for Guy Fawkes), Sean Leathers (DECC) and Lockie Onslow (Fleet Helicopters) for helping to collect the survey data, Joanne Lenehan for help with some of the analyses, and Peter Jarman (UNE) for his contributions to discussions of horse identification and project methodology. We also thank two anonymous referees who provided valuable advice that improved earlier drafts of the manuscript. The research was supported under Australian Research Council’s Linkage Projects funding scheme (project number LP0560490), and by funding provided by the DECC.


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