Methods and ethical considerations of pitfall trapping for the western pygmy possum (Cercartetus concinnus Gould) (Marsupialia : Burramyidae), with observations on capture patterns and nest sites

In order to enhance future trapping of the western pygmy possum (Cercartetus concinnus) and minimise the capture of non-target species in the context of animal ethics, we examined the effectiveness of several pitfall-trap capture methods and described capture patterns at Innes National Park, South Australia. For 2606 trap-nights, 78 (2.99 per 100 trap-nights) captures were pygmy possums (70 individuals). They represented 69% of the 113 vertebrates captured, followed by house mice (Mus domesticus) (22%). Nocturnal captures of pygmy possums varied with season (5.21 per 100 trap-nights in spring and summer, and 1.02 in autumn and winter). Males were more commonly captured than females. Half of the pygmy possums used the artificial shelters provided in the traps. We alternated 39-cm-deep and 31-cm-deep pitfall traps along trap lines, with and without drift netting. Pitfall depth did not significantly affect the capture rate of pygmy possums, but house mice were captured significantly more often in deep pitfalls. Use of a drift net did not affect the capture of pygmy possums, but increased mouse captures. These results have significant ethical implications because the use of ‘shallow’ traps and the elimination of the drift net minimised the capture of non-target mice. We recommend reporting trapping details precisely, because a difference of only 8 cm in trap depth can have a significant impact on species selected for capture. Our study also showed that pygmy possums are able to use a range of structures for daily shelter and are not dependent on hollows.