LARGE-SCALE POISONING OF SHIP RATS (RATTUS-RATTUS) IN INDIGENOUS FORESTS OF THE NORTH-ISLAND, NEW-ZEALAND

This paper describes the impact of nine poison operations on ship rats in four areas (35 ha to 3200 ha) of North Island forest. Poisoning wi th 1080, brodifacoum, or pindone killed 87-100% of rats, based on trap ping and tracking-tunnel indices. Rat populations took 4-5 months to r ecover. Operations to protect nesting birds should therefore coincide with the onset of nesting and be repeated each year, although not nece ssarily with the same methods. Population reduction declined each year at Mapara, King Country, during three annual 1080 operations which us ed the same lures and baits, but remained high at Kaharoa, Bay of Plen ty, where poison toxicity was higher, non-toxic bait was pre-fed, and poisoning methods varied each year. Mouse tracking rates increased in poisoned forests 3-6 months after poisoning if the initial kill of rat s exceeded 90%, peaked 7-9 months after poisoning, then declined to pr e-poison levels. Future research should focus on how prey and non-prey species within a forest community respond to a temporary reduction in rat numbers, and on methods to maintain low rat densities after initi al knock-down.