There are four methods which have been, or are being used by F.R.I, to measure possum populations and/or to assess the effectiveness of possum control operations.

Spot-light counts are applicable in farmland situations but become inefficient as the amount of bush or scrub cover increases. Weather markedly affects the number of animals counted.

Gin grapping, as well as contributing towards control, provides an index of animal density based on the numbers caught on successive nights. Precise estimates are dependent upon the catchability of animals remaining constant, a factor often subject to change because of climatic variations and changes in the structure of the population due to animals being removed.

Interference by possums of non-toxic baits provides an index of animal numbers and is a technique which may be applied in a wide range of habitats. The accuracy of the estimates is influenced by weather changes and changed behaviour as animals learn locations of bait stations.

Faecal pellet counting is a suitable method for sampling large areas, although dense ground vegetation limits its application. Current research is aimed at solving problems associated with variations in defecation rates and pellet decay rates.

It can be seen that there are problems associated with all techniques and that more research is required before any one of them can be unconditionally recommended for any given situation.