Culvert, Bridge, and Fencing Recommendations for Big Game Wildlife Crossings in Western United States Based on Utah Data

Long term monitoring of culverts and bridges with and without wildlife exclusion fencing is helping to determine the best designs for wildlife crossings for mule deer and other species. In this study 35 bridges and culverts across Utah were monitored with remote cameras to determine: if wildlife exclusion fencing (8 feet, 2.4 m high) was necessary to convince wildlife to use structures to move beneath roads; if mule deer and other wildlife use increased at these structures with the placement of wildlife exclusion fencing; what structure dimensions were most important to mule deer success rates in using structures; and to determine recommendations for structure designs that encourage the greatest wildlife use, especially by mule deer (Odocoileus hemionus), and elk (Cervus canadensis). The study was conducted from 2007 through 2013 using 44 remote cameras (camera traps) placed at 15 structures made explicitly for wildlife (wildlife crossing structures), and 20 multiple use culverts and bridges built for other purposes. Wildlife exclusion fencing was present at all wildlife crossing structures for a minimum of one mile in each direction. The multiple purpose structures did not have wildlife exclusion fencing initially. Two fencing projects were completed during the study and four culverts in those stretches were monitored pre and post fencing to help determine wildlife use. Cameras were placed at culvert and bridge entrances to examine animals that used the structures and those that approached and then repelled away. There were a total of 20 culverts, 13 bridges, and two overpasses monitored along seven highways. Camera traps produced over 2 million pictures over 40,000 plus camera days. All designated wildlife crossing bridges and culverts were used by mule deer, the target species of these structures. Individual mule deer were recorded moving successfully through these structures on over 31,000 occasions. Success rate at each structure was defined as the number of successful individual animal movements through divided by the total number of animals photographed at the entrances. Bridged wildlife crossings had a higher average success rates for mule deer (87%) compared to wildlife crossing culverts average success rate (74%). Statistical analyses found culvert length was the most important dimension relative to mule deer crossing success rates; the shorter the length, the greater the success rate. The width of the crossing was the second most important dimension, and the height was the least important. Elk were reluctant to use any structures. Moose were most often photographed using a single corrugated steel culvert. Recommendations for the design of future wildlife crossing structures include open bridges with spans that approach or exceed 100 feet (31m), that are under 100 feet in length as the animals traverse under the road, culverts well under 150 feet (46m) long as the animals traverse under the road, and the installation of wildlife exclusion fencing. Wildlife exclusion fencing does not always ensure mule deer and elk use of existing multi-purpose culverts and bridges. Future research in specific geographic regions with local animal populations is necessary to determine local and regional species’ preferences.