Patterns of otter Lutra lutra road mortality in Britain
Philcox CK., Grogan AL., Macdonald DW.
1. This paper examines patterns of otter road traffic accidents and the possibilities for reducing this mortality. 2. Records of otter road casualties in Britain (673 between 1971 and 1996) were compiled and accident sites were plotted on 1:50000 scale Ordnance Survey maps. Casualty records were categorized by geographical region, road type, distance to watercourses and presence and type of river crossings. The data were analysed for demographic, seasonal, temporal and spatial trends. 3. A rapid increase in numbers of road traffic accident records has occurred since 1983. Fifty-six per cent of fatalities were males, representing a statistically significant bias. 4. The number, but not the sex ratio, of otter road casualties differed between seasons and in all areas was positively correlated with either seasonal rainfall or river flow. 5. Significant effects of geographical region, road density and year were found on the annual number of casualties recorded in mainland regions, and the regional rates of increase in numbers of road traffic accidents were significantly different. There is also evidence that road traffic accidents are influenced by otter site occupancy of a region. 6. Trunk and A-roads accounted for 57% of road traffic accident records, even though they comprise only 13% of the road network. 7. A 100-m wide zone surrounding fresh water and the coast accounted for 67% of all casualty records. Measures to reduce road mortality should target this zone. 8. The seasonal correlations of otter road traffic accidents with rainfall and river flow, and the fact that 91% of accidents occurred where a road crossed a watercourse, suggest that substantial reductions in road mortality could be achieved by improving the design of road crossings of watercourses. 9. We suggest that the optimal approach to road crossing design is to maintain a continuous, and where possible, natural bank above the level of high flows, using either wide-span bridges, over-sized culverts or artificial ledges. 10. The use of otter-proof fencing may be required to reduce mortality where roads pass close to watercourses, but care must be taken that this does not create a barrier to all movements of otters and other wildlife.