The fundamental reason for counting kangaroos is to ensure that harvests are demonstrably sustainable. Given this goal, it is essential that the survey method should be able to detect broad scale trends in population size. Aerial counts will detect trends only if the constant (“correction factor”) that relates the number of sightings to actual population size either remains constant, or the way in which it changes depending on environmental conditions is known. Actual counts, and not an index, are essential if harvesting policy is to be proactive, rather than simply reacting to previous changes in population size. It is evident from other papers in this volume that there is a particular problem in estimating appropriate correction factors for grey kangaroos. Several contributors suggested a research programme to produce better correction factors, which would then be used in the future to correct raw sighting data. A better solution may be to use “double sampling”, which is a well-established sampling method. Each time an aerial survey is conducted, a subsample of the transacts surveyed would be re-surveyed using helicopter counts, which are more accurate, but more expensive. The ratio of the helicopter to fixed-wing counts is then used to produce a survey-specific correction factor. Double sampling is more expensive than fixed-wing surveys alone, so it would be necessary to decrease the intensity of the fixed-wing surveys to allow for the extra cost of the helicopter surveys. The experiments necessary to see whether double sampling is worthwhile are very similar to those that are necessary to develop better correction factors. I suggest that any experiments designed to improve correction factors should also be analysed to investigate the feasibility of using double sampling to estimate kangaroo population size.
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