Differences in the capture rate of cage-trapped red foxes Vulpes vulpes and an evaluation of rabies control measures in Britain.
Abstract
Methods used to control canids may differ in their relative effectiveness and how they 'sample' age and sex classes. However, there is little information on such biases. We utilized data from a population of known size and structure to quantify differences in the seasonal capture rate of foxes Vulpes vulpes of different age, sex and social status. The effects of home-range utilization and trap experience were also investigated. The capture rate of juvenile (<1 years) foxes was highest in spring and declined throughout the year. Adult (≥1 years) trap rates were equivalent to those of juveniles in winter. Neither juveniles nor adults exhibited sex differences in trappability. Adults were classified as dominant or subordinate based upon patterns of dyadic interaction. Subordinate foxes were trapped more frequently than dominants. Trappability did not differ inside or outside of core areas, after controlling for trap effort. Previous trap experience did not affect the capture rate of adult foxes. Captures of non-resident foxes peaked in winter (52% of captures), associated with dispersing juveniles and adult males, and were lowest in summer (13%). In Britain, 75-90% of foxes would need to consume poison or vaccine baits to control a rabies epizooty. During simulated rabies control operations, where cage-trapping was used to measure the proportion of foxes taking dummy baits containing a biomarker, <30% of foxes consumed baits. However, non-resident foxes present during the period of baiting but not during trapping, or vice versa, may bias these calculations. We used the frequency of capture of non-resident foxes to re-examine the success rate of these trials. Revised estimates still suggest that <45% of foxes were reached during the simulated control operations. This is significantly below the levels necessary for rabies control. Further biomarker trials are required that do not rely only on trapping for sample collection and where the residency status of captured foxes has been determined.