Possums (Trichosurus vulpecula) on Hawke's bay farmland: Spatial distribution and population structure before and after a control operation

Over 6.5 years, 723 possums were live-trapped and marked in swamp, willow, and farmland habitats on approx. 350 ha of two adjoining sheep farms in Hawke's Bay, New Zealand. About equal numbers of males and females were trapped in each habitat, but males were recaptured more often than females. Eighty-five percent of the possums were trapped along the margins of swamp and willow habitats that comprised only 6% of the study area. The swamp habitat contained at least 8.8 possums ha(-1), the willow habitat 16.7 possums ha(-1), and the farmland habitat 0.13 possums ha(-1). Possums generally moved less than 200 m between successive recaptures, and there were few trap-revealed movements between habitats. Possums on part of one farm comprising 23 ha of swamp and willows were then poisoned with cyanide in bait stations. Two hundred and fifty-eight possums-including 142 previously marked animals-were found dead, an estimated 90% of the population. Fifty-nine percent of the marked possums were poisoned within 100 m of their most recent trapping point, and 85% within 200 m. Proportionately more adult possums were killed on the first night's poisoning and more juveniles on subsequent nights. After the poisoning, the surviving possums showed very varied responses to the reduced population density, ranging from no change in location or range of movements to changes in location, range of movements, or both.