Proceedings of the First Symposium on Marsupials in New Zealand

Discussion

Discussion

The bait interference method was introduced to provide a simple and easily executed alternative to trapping for estimating possum densities. Faecal pellet counts and spotlight counts (Batcheler 1970) have also been tried in exotic forests. Low faecal pellet counts (less than 5% frequency) and highly variable decay rates have made pellet counts unsatisfactory. Spotlight counts are unsuitable due to physical constraints (need for open areas) resulting in an unrepresentative or biased sample (as indicated by the high kills estimated in Table 1). Thus the bait interference method seems to be a feasible alternative to the well-tried trapping method. The most time-consuming operation is the establishment of the trap or bait interference lines since this involves marking the lines and setting up traps or bait stations. However if there is an adequate sample intensity (as determined from binomial tables) a greater number of assessment visits (say 4 or 5) on a bait interference line will give a better estimate of kill at a lower cost. In indigenous forest tracts this advantage is very evident. Similarly in sampling small areas of say 200 ha, or in low density areas such as farmland, insufficient animals may be trapped to obtain a reliable estimate of density but it would still be possible to obtain a minimum of 100 bait station sets.

The bait interference method therefore offers an alternative or even first choice for assessment of possum density. However the sample design must be chosen with care to obtain consistent estimates of relative density and one must be sure that the possum is the sole animal using the baits.