Spotlight counts for assessing abundance of rabbits (Oryctolagus cuniculus L.)

Abstract

We determined the precision of spotlight counts of rabbits (Oryctolagus cuniculus L.) and their accuracy as estimates of density, by making counts from a motorcycle along 17 1-km transects in the Mackenzie Basin, New Zealand. Rabbits were poisoned and density per 1-ha quadrat was measured. Precision of spotlight counts would be between 5–28% and 6–39%, using impracticably large numbers of counts, even allowing for the effects of snow and heavy rain, observer bias and number of runs per night. Spatial and unexplained variance would result in even less precise counts using 25 transects and 1–5 nights. Actual rabbit densities explained only 41% of the variance in spotlight counts. Confidence intervals of absolute rabbit densities are extremely large, especially when observed counts are high. At best, spotlight counts could be used to detect differences in actual rabbit abundance spanning an order of magnitude or more. Observed reduction in spotlight counts is likely to underestimate reduction in actual density because the spotlight count index ‘saturates’ at high rabbit density. However, spotlight counts along fixed transects before and after a control operation can be used to estimate percentage kill with acceptable precision if the kill rate is at least 80%.