The application of catch–effort models to estimate the efficacy of aerial shooting operations on sambar deer (Cervus unicolor)

Abstract

Context Aerial shooting from a helicopter targeting introduced sambar deer (Cervus unicolor) is a key activity being undertaken on public land in the North East and East Gippsland regions of Victoria. However, there is currently little published information on the efficacy of aerial shooting for reducing sambar deer populations in Australia. Aims The aims of this study were to analyse the operational data collected during an aerial shooting program in eastern Victoria, to assess the efficacy of aerial shooting at reducing sambar deer density and to inform management decisions on the required intensity of aerial shooting to achieve target densities. Methods Operational data (locations of all shot animals as well as aerial search effort) were analysed from 10 sites using a Bayesian generalised catch–effort model, which allowed for population changes between five periods of intensive control. The model allowed estimates of initial and residual abundance for each site to be made from the catch–effort data, which were used to estimate the efficacy of aerial shooting. Estimates of the detection rate of deer, which were allowed to vary with removal occasion and site, were then used to estimate the amount of aerial search effort required to reduce population densities by various proportional amounts. Key results Aerial shooting resulted in population reductions of 50–70% of sambar deer at four sites where aerial search intensities per unit area were highest. However, results at the remaining sites suggest that sambar deer densities have either remained static or increased over the five periods of aerial control. Recruitment of sambar deer between control periods, which was strongly influenced by study site elevation and season, was largely responsible for eroding reductions achieved by aerial shooting. Conclusions Catch–effort models applied to operational data collected during aerial shooting programs can be used to estimate control efficacy without the need for additional monitoring. Our analysis suggests that sufficiently high search intensities, around 1.4 km of search effort per km2 of habitat in each of five removal occasions, would need to be applied to achieve at least a 50% reduction in sambar deer densities.