Dispersal and survival of juvenile feral ferrets Mustela furo in New Zealand.
Introduced feral ferrets Mustela furo are a significant pest of both conservation and economic importance in New Zealand. Ferrets prey on indigenous wildlife and they also carry bovine tuberculosis (TB), a disease which may threaten New Zealand's international beef, dairy and venison markets. Very little is known about the role of dispersal and survival of juvenile ferrets in the recovery of ferret populations after control operations, and how these parameters might affect ferret impacts on native wildlife and their role in the spread of TB. Fifty-two juvenile ferrets were radio-collared at emergence from their natal dens on six study sites during 2 years, 1997-98 and 1998-99. On three sites, most introduced mammalian predators (including ferrets) had been removed by kill-trapping from October to January to protect critical bird nesting areas. Three sites were left as non-removal sites. Dispersal and survival of juvenile ferrets was therefore measured at low and normal ferret densities. Survival of juvenile ferrets showed a strong density-dependent response to predator removal. In year 1, survival of juvenile ferrets from emergence to the end of the dispersal period (about 4 months) was 86% (95% confidence limits 63-100%) on removal sites, whereas survival on non-removal sites was 19% (0-38%). In year 2, survival was 100% on removal sites compared with 71% (45-97%) on non-removal sites. Dispersal of juvenile ferrets was not related to population density at source. A frequency distribution of dispersal distances of juvenile ferrets showed a typical negative-exponential pattern. The median dispersal distance of juvenile ferrets was 5.0 km (range 0.5-45.0 km). There was no apparent sex bias in survival, dispersal distances or timing of dispersal in either season. Density-dependent survival of juvenile ferrets should be a key parameter of interest in determining frequency and seasonal timing of ferret control. Ferret control to conserve native wildlife and minimize TB spread should be carried out in late autumn after dispersal, to provide a longer lag time before juveniles reinvade an area. This study fills a critical gap in the understanding of introduced mustelids in island systems such as New Zealand. It has direct implications for management of this important introduced predator, and it augments literature on dispersal and survival of mammalian carnivores.