Density of rabbits (Oryctolagus cuniculus L.) in the Mackenzie Basin, South Island, New Zealand

The density of rabbits (Oryctolagus cuniculus) in the modified tussock grasslands of the Mackenzie Basin, South Island, New Zealand, in August-September 1991 was determined within 26 I-ha quadrats spread over 1000 ha. The area was poisoned with 1080- carrot baits and dead and live rabbits counted. The overall kill rate was 93%. Wide variability in rabbit densities amongst the quadrats was correlated with burrow density, but vegetation was not a significant predictor of rabbit numbers. High density quadrats were not all spatially clumped together. Variation amongst quadrats of 0-43% of rabbits dying underground shows that searching burrows as well as the surface will provide the most accurate rabbit densities. Poisoning efficacy was greater than or equal to 90% in 77% of the quadrats, but two low- density quadrats recorded kill rates <70%. Nine per cent of carcasses had been partly eaten by predators, suggesting a potential for the predators to be killed during rabbit control operations via secondary poisoning. The average density of 19 rabbits per ha confirms the classification of this area as a moderate to high rabbit-prone zone, with the rabbit population imposing grazing pressure equivalent to at least 1-2 sheep per ha at the beginning of spring. Sustainable agriculture cannot be attained in these semi-arid regions of New Zealand without cheap, widespread and effective rabbit control.