Applying distance-sampling methods to spotlight counts of red foxes

1. Monitoring red fox Vulpes vulpes abundance is necessary to assess the status and management of this species and to understand predator-prey relationships. Spotlight counts are most often used for this purpose. However, comparisons between regions or over years may be questionable when using encounter rates, i.e. the number of foxes seen per kilometre. We evaluated whether distance-sampling methods, which take account of variation in visibility, could be applied to spotlight counts of foxes along roads and trails. 2. Distance-sampling methods were used at 12 contrasting sites in France in a systematic design with equally spaced transects or points. Line and point transects were simultaneously applied at two sites to find the more precise and efficient method. 3. The number of foxes seen near the centreline was always low, although some foxes may have been missed. A peak of sightings at subsequent intervals from the centreline suggested evasive movements. 4. Despite the low sighting frequency near the centreline, which may reflect a violation of distance-sampling assumptions, a good model fit was obtained for eight out of 12 data sets using a regular 50-m grouping of the distance data. Increasing the first interval to account for evasive movement improved model fit in the four other data sets. Density estimates ranged from 0.39 to 3.54 foxes km(-2) (range of coefficient of variation 4.5-24.6%). 5. Point and line transects resulted in similar density estimates, but point transects were more time consuming and resulted in larger coefficients of variation due to a smaller number of foxes seen by this method. Line transects may therefore produce better estimates of fox numbers. 6. There were few differences among the effective strip width estimates between the 12 sites (range 191-286 m), thus line transect estimates may have a limited advantage over encounter rates. 7. Synthesis and applications. The systematic scheme we applied in this study improved sampling design and variance estimations and should be useful for surveying terrestrial mammals with spotlight counts. However, the location of transects along roads and in open habitats probably induced biased results. Methodological improvements are necessary before spotlight distance sampling can become a routine monitoring tool for foxes.