Suchergebnisse

The performance of helicopter surveys for estimating population densities of
red kangaroos (Macropus rufus), eastern grey kangaroos
(Macropus giganteus) and common wallaroos
(Macropus robustus) was investigated by comparing
line-transect density estimates obtained from helicopter surveys with those
from ground (walked) surveys. Comparisons were made at four sites in central
western Queensland (areas with high densities of red kangaroos and common
wallaroos) that were surveyed during winter and summer from December 1991
until February 1995, and one site in south-eastern Queensland (an area of high
eastern grey kangaroo density) surveyed annually during autumn from March 1991
until March 1994. Helicopter surveys generally recorded lower sample sizes
than did ground surveys (means ± s.e = 34 ± 6%, 33
± 9% and 76 ± 2% lower for red kangaroos, eastern
grey kangaroos and wallaroos, respectively). Density estimates obtained from
the helicopter surveys were not significantly different from those obtained
from ground surveys for both red and eastern grey kangaroos as assessed by
repeated-measures ANOVA and regression analysis. However, helicopter surveys
of common wallaroos consistently returned density estimates about half those
of ground surveys. The relationships between the two methods did not differ
between winter and summer for any species. The conventional aerial survey
method for kangaroos of strip transects from fixed-wing aircraft has limited
ability to adjust for varying sightability conditions. Therefore, helicopter
surveys with line-transect sampling are an attractive alternative.

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.

The use of line-transect methodology, on foot or from a helicopter, is likely
to return the most repeatable, least biased estimates of kangaroo density.
However, the associated costs make both methods impractical for broad-scale
surveys. For these, a fixed-wing aircraft remains the most cost-effective
platform. Limitations of the standard fixed-wing method (200-m strip
transects) are well known, but it continues to be used because it provides an
index of trends, because there are now long runs of data (almost 20 years in
some cases) collected in this standard form and an alternative method is
lacking. In this study, four variations of fixed-wing surveys of kangaroos
were investigated: two line-transect methods (involving different scanning
techniques), the standard 200-m strip transect and a 100-m strip transect.
Surveys using these methods were compared with helicopter line-transect
surveys along the same flight lines in three areas (5000–7500
km2) in western Queensland. Both fixed-wing
line-transect methods failed to produce consistently accurate estimates of
density for all three species surveyed: red kangaroos
(Macropus rufus), eastern grey kangaroos
(M. giganteus) and common wallaroos
(M. robustus). While generally more accurate than the
uncorrected strip-transect counts, they were no less variable. However, the
strip-transect counts still need to be corrected for bias for which this study
offers revised estimates of correction factors for eastern grey kangaroos
(3.7–10.2) and common wallaroos (3.8–4.1), and estimates for red
kangaroos (1.7–2.7) that support currently used values. An attractive
alternative is to survey in 100-m strip transects, which offer improved
visibility (correction factors of 1.0–1.8 for red kangaroos,
2.1–3.6 for eastern grey kangaroos and 1.7–2.1 for common
wallaroos) and are therefore likely to be more accurate and repeatable.
However, these advantages need to be assessed in relation to continuing long
runs of data using the standard 200-m strip transect. Correction factors for
wallaroos are conservative as helicopter-based density estimates are known to
be underestimates. Further work is needed to assess the generality of
correction factors, both spatially and temporally.