Kangaroo population surveys do not provide real numbers but rather estimates that are derived, in part, by using correction factors (Pople 2004). Correction factors vary and are dependent on a number of survey conditions including habitat type, vegetation density, canopy cover and kangaroo species (Pople and Grigg 1999). At least in some cases historical resetting of corrections factors has not included adjustments of earlier surveys, thereby precluding meaningful comparison of population estimates. Some examples are provided below.
A case study of harvested kangaroo populations that focused on the New South Wales Management Program indicated substantial population spikes of Eastern and Western Grey Kangaroos in 1993. The population spikes had been influenced by an upwards shift in correction factors (Olsen and Low 2006). Red Kangaroo populations spiked after adjustments to the correction factor in 1992 (Pople and Grigg 1999) only to subsequently drop.
The jump in the Red Kangaroo quota in 1992 from 450,000 to 600,000 represented a change in methodology for determining the state-wide population size in Queensland (Pople and Grigg 1999). In 1995 the population of Western Grey Kangaroos in South Australia was calculated using a revised correction factor (increased by a factor of 2) for aerial survey estimates (Pople and Grigg 1999). The result was a continuation of a dramatic population increase from the previous year, only to be followed by a sharp decline the following year. Correction factors were revised upwards again in 2001 only to be followed by population spikes in 2002 (Olsen and Low 2006) and sharp declines thereafter. Alarmingly, if historical data were reworked to accommodate upward variations in correction factors, populations may now be substantially lower than they were in the 1970s and 1980s.
Olsen, P. and T. Low (2006). Update on Current State of Scientific Knowledge on Kangaroos in the Environment, Including Ecological and Economic Impact and Effect of Culling, Kangaroo Management Advisory Panel.
Pople, A. R. (2004). "Population monitoring for kangaroo management." Australian Mammalogy: 37-44.
Pople, T. and G. Grigg (1999). Commercial harvesting of Kangaroos in Australia, Department of Environment, Water, Heritage and the Arts.