Kill rates by rabbit hunters before and 16 years after introduction of rabbit haemorrhagic disease in the southern South Island, New Zealand

Context European rabbits (Oryctolagus cuniculus) are serious economic and environmental pests in Australia and New Zealand. Since the illegal introduction of rabbit haemorrhagic disease virus (RHDV) in New Zealand in 1997, the disease has persisted in most rabbit populations, with major epizootics occurring usually each autumn. Aims We evaluated the efficacy of the virus as a biological control agent in the southern South Island. Methods We used an index of rabbit abundance (kills per hunter) based on a region-wide annual rabbit-hunting competition to evaluate rabbit population trends 7 years before and 16 years after the first outbreak of RHD. We also evaluated the influence of rainfall and temperature in the preceding year on post-RHD trends in the index. Key results Kill rates declined by 60% following the initial epizootic. They remained low for the following 3 years and then increased steadily to intermediate levels punctuated by occasional declines. The instantaneous rate of increase in kill rates during the increase phase was low, but above zero (0.04 per year). No relationship between kill rates and rainfall was apparent, but there was a negative relationship between kill rates and winter temperature in the preceding season. Conclusions The kill-rate data obtained from this hunting competition suggest that RHD still appears to be killing rabbits. Every 2–3 years over the past decade, kill rates have been as low as they were when government rabbit-control programs were in place before RHD arrived, but the efficacy of RHD as a biological control agent is waning compared with the first outbreaks of the disease. This concurs with findings based on spotlight counts. Implications The data collected from this hunting competition are a good example of how ‘citizen science’ can be used to capture large volumes of pest-monitoring data from a wide geographic region for very little cost. The information is a valuable addition to understanding the effects of a major wildlife disease.