New Zealand Journal of Ecology (1995) 19(1): 5- 17

Large-Scale Poisoning of Ship Rats (Rattus rattus) in Indigenous Forests of the North-Island, New Zealand

Research Article
John Innes 1,5
Bruce Warburton 2
Dale Williams 1,5
Hazel Speed 3
Philip Bradfield 4
  1. Manaaki Whenua - Landcare Research New Zealand Ltd, Rotorua, New Zealand
  2. Manaaki Whenua - Landcare Research New Zealand Ltd, P.O. Box 31011, Christchurch, New Zealand
  3. Department of Conservation, P.O. Box 1146, Rotorua, New Zealand
  4. Department of Conservation, P.O. Box 38, Te Kuiti, New Zealand
  5. Present address: Manaaki Whenua - Landcare Research New Zealand Ltd, Private Bag 3127, Hamilton, 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 with 1080, brodifacoum, or pindone killed 87- 100% of rats, based on trapping and tracking-tunnel indices. Rat populations took 4-5 months to recover. Operations to protect nesting birds should therefore coincide with the onset of nesting and be rePeated each year, although not necessarily with the same methods. Population reduction declined each year at Mapara, King Country, during three annual 1080 operations which used the same lures and baits, but remained high at Kaharoa, Bay of Plenty, 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 rats exceeded 90%, Peaked 7-9 months after poisoning, then declined to pre-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 initial knock-down.