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

The ship, roof, or black rat Rattus rattus (L.) was introduced into New Zealand last century, and is now a widespread pest in indigenous forests. Ship rats eat seeds and invertebrates (Beveridge, 1964; Best, 1969; Daniel, 1973; Innes, 1979; Gales, 1982; Meads, Walker and Elliott, 1984), lizards (Whitaker, 1978), and birds and their eggs (Atkinson, 1973, 1985; Innes, 1990), and perhaps compete with some native animals for food. The impact of their predation on mainland prey populations is little understood (Moors, 1983; King, 1984), but they are contributing directly to the demise of North Island kokako, Callaeas cinerea wilsoni (Bonaparte) (J.R. Hay, 1981, unpubl. report; J. Innes, unpubl. data) and perhaps other declining endemic bird species with low reproductive rates (Moors, 1983). Previous attempts to control ship rats on the mainland have been confined to small areas of less than 5 ha, either to protect nesting birds or to eradicate animals coming ashore from fishing boats (Hickson, Moller and Garrick, 1986). Poisoning has, however, eradicated ship rats from five New Zealand islands, the biggest of them Somes Island at 32 ha (Veitch and Bell, 1990). Disincentives to more frequent or larger-scale control on the mainland have been the high cost, the rapid reinvasion (Innes and Skipworth, 1983) and a lack of clear conservation objectives for undertaking the control at all (Innes, 1990). R. rattus is an important cosmopolitan commensal rodent pest, damaging crops both in the field and in storage throughout the temperate and tropical regions of the world (Prakash, 1988a). However it is widespread in indigenous forest only in New Zealand and on other islands which lack an indigenous rodent fauna. The control operations described here are unusual in a world setting in that they target ship rats as threats to other fauna, rather than to crops. Also, New Zealand is apparently the only country in which baits have been aerially distributed to kill this species; aerial bait distribution was used against mice in crops in Australia (Saunders, 1983). This paper reports on the first large-scale control operations against ship rats in mainland forests, undertaken mainly to protect nesting North Island kokako. We also assess how aerial poisoning with 1080 (sodium monofluoroacetate) for control of brushtail possums Trichosurus vulpecula (Kerr) affects ship rat populations. This method of possum JOHN INNES, BRUCE WARBURTON, DALE WILLIAMS, HAZEL SPEED and PHILIP BRADFIELD 1 Manaaki Whenua Landcare Research New Zealand Ltd, Rotorua, New Zealand. Current address: Manaaki Whenua Landcare Research New Zealand Ltd, Private Bag 3127, Hamilton, 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.