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

In: Wildlife research, Band 25, Heft 4, S. 393

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.