Aerial survey methodology and the cost of control for feral goats in Western Queensland

Abstract

Feral goats (Capra hircus) were shot from a helicopter on four sites in central-western Queensland. These sites centred on Idalia National Park (660 km²) and three nearby properties: Lissoy (360 km²), Mt Calder (260 km²) and Ravensbourne (320 km²), the order reflecting increasing goat density. On Idalia, 134 goats were shot in 8 h of flying time. On Lissoy, 28 goats were shot in 3 h, 1038 goats were shot on Mt Calder in 15 h, while 2307 goats were shot on Ravensbourne in 21 h. The reduction was monitored by aerial surveys using fixed-wing aircraft. No goats were recorded on either Idalia or Lissoy immediately following shooting and substantial reductions in goat numbers were achieved on Mt Calder (75%) and Ravensbourne (49%). Surveys conducted six months after this reduction indicated population increases on all sites following shooting. This was particularly marked on Ravensbourne, where goat numbers had increased by 31% despite further substantial removals by commercial mustering. These population increases were largely the result of immigration. The cooperation of landowners over a large area is therefore essential for effective control of goats. The per capita cost of shooting goats from a helicopter was essentially constant down to a threshold density of <1 goat km^-2 , below which the cost increased substantially.

The accuracy of fixed-wing aerial surveys was improved by using double-counting and these corrected estimates compared favourably with index–manipulation–index estimates. Estimates of densities <1 goat km^-2 were imprecise and at these densities goats were difficult to detect. The extent of vegetation cover did not affect the sightability of goats, whereas group size was positively correlated with the sightability of goats. To estimate true density, correction factors of 1–2 were applied to goat group sizes using survey parameters of 100 m strip width, a survey height of 76 m and a speed of 187 km h^-1.