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1080 poisoned baits (Wanganui No. 7) were aerially spread throughout the three blocks of Mapara reserve in September 1990 and again in October 1991 to control possums. Changes in possum abundance were monitored by trapping, using a modified version of Seber's "removal method". A trapping method was chosen in preference to spotlight counts, bait take and pellet counts because a suitable control area, critical to these methods, was not available; traps were already in place on permanent sets; and trapping contributes to the control objective. The removal index was considered more suitable for monitoring than the simpler catch per unit effort index (number of possums caught per 3 trap-nights) because the latter is affected by variation in possum catchability, as well as possum abundance. The removal method depends on obtaining declining catches on successive nights. There is a significant risk that the method will not give a meaningful result if trapping intensity is inadequate or if nightly variation in catchability is excessive. The catch per unit effort index is still available if an abundance index based on the removal method is unobtainable. The 1990 poison operation reduced possum numbers to 21% of pre-poison abundance (95% Confidence Limits (CL) were 13.8% and 28.2%). During the following year, possum numbers built up to 39.5% (95% CL 28.7% and 50.3%) of pre-poison abundance and 1991 poison operation caused a non-significant reduction to 32.2% (95% CL 21.8% and 42.6%). Thus the September 1991 operation probably did not reduce possum densities to the level attained in October 1990. The reason for the poor kill following the 1991 poison drop is unknown, although several possibilities are identified.