Poisoning Rats on Stewart Island

Poison was used at remote anchorages of southern Stewart Island in spring and summer of 1984/85 to reduce the likelihood of ship rats (Rattus rattus), Norway rats (R. norvegicus) and kiore (R. exulans) boarding fishing boats heading for the Snares Islands. Poison baits were removed at successively slower rates, probably because poisoning had reduced rat numbers. The effectiveness of poisoning was tested by (i) live-capturing and tracking marked rats at a simulated anchorage near Halfmoon Bay, (ii) poisoning there as in southern Stewart Island, and (Hi) monitoring the survival and responses of the marked population. Population density approximated 2.0-2.5 ship rats per hectare before poisoning. The minimum monthly home range of ship rats averaged 0.54 ha (mean range length 142 m), which is much larger than previously recorded for ship rats in New Zealand. Neither Norway rats nor ship rats were restricted to the shoreline or along creeks. Poisoning caused a 93% reduction in an index of rat numbers in a 0.69 ha poisoning zone over 16 days, and a 76% reduction over the larger 10.4 ha effective trapping area including the poison zone. Poisoning reduces the risk of rats boarding boats, and can protect endangered plants and animals on infested islands.