Accuracy and consistency in the aerial survey of kangaroos

Accuracy, or bias, is the closeness of a measured value to its true value. It can be of secondary importance in relation to repeatability and precision in a sampling programme, but where management decisions, such as the setting of kangaroo harvest quotas, are involved, accuracy assumes a greater importance. Associated with this is the requirement that the survey method used be consistent. If this is the case, as has been common practice, then the negative, but consistent bias of standard fixed-wing aerial surveys of kangaroos can be redressed through the use of correction factors. The correction factors currently used to account for the negative bias in fixed-wing aerial surveys of kangaroos, which were derived for red kangaroos, have not been found to be suitable for all species likely to be encountered in surveys. This was confirmed by comparison of standard fixed-wing aerial survey counts with those obtained using helicopter line transect surveys. While the currently used correction factors may be suitable for red kangaroos Macropus rufus, correction factors of at least 1.5 times these are probably required for eastern and western grey kangaroos M. giganteus and M. tuliginosus. The standard fixed-wing aerial survey method was found to be spatially density independent and therefore consistent over seven survey blocks in Queensland. If desired, this would allow the derivation of broad-scale correction factors for both red and eastern grey kangaroos of 2.25 and 4.37, respectively. Currently used correction factors range from 2.29 to 2.57, depending on vegetation cover.