28th Annual Meeting and Symposium of the
Desert Tortoise Council, February 21-23, 2003
Abstracts

Evolution of Line Distance Sampling to Monitor Desert Tortoises in the Mojave Desert

P.A. Medica¹, P.S. Corn², and R.M. Marlow^3
1United States Fish and Wildlife Service,4701 North Torrey Pines, Las Vegas, NV 89130 USA
²Aldo Leopold Wilderness Research Institute, Missoula, MT 59807 USA
³Ecology, Evolution and Conservation Biology, and Biological Resources Research Center, University of Nevada, Reno, NV 89557 USA

The United States Fish and Wildlife Service approved the Desert Tortoise Recovery Plan in June 1994. The plan recommended development of a methodology to determine desert tortoise population status and trends rangewide. A series of technical meetings ensued beginning in the winter of 1995 in Reno, NV, and subsequently in 1996 at Laughlin, NV, and finally with a workshop in Las Vegas, NV in the fall of 1998 using model styrofoam tortoises to evaluate the sampling technique. In June 1999, the Desert Tortoise Management Oversight Group agreed to use the Line Distance Sampling technique as the method of choice to monitor desert tortoise populations rangewide. Beginning in the spring of 2001 Line Distance Sampling was implemented throughout the geographic range of the Mojave Population of the desert tortoise. Sampling in 2001 provided encounter rates for each of the Desert Wildlife Management Areas but not enough captures to obtain adequate sample sizes for each area. To compensate for this, the number of transect kilometers was increased rangewide from the 2901 km. sampled in 2001 to 4117 km. in 2002. Also, the transect footprint was modified from a single 400 meter square (1.6km) transect, to two square transects each 500 meters on a side (each 2km) in the shape of a bow tie, totaling 4.0km. In 2001 the mean observed rangewide encounter rate was 0.145 tortoises/km., and the encounter rate in 2002 was 0.092 tortoises/km. This reduction in activity of live tortoise observed above ground or observable within their burrows was documented with the focal animals. In 2001, the overall mean activity recorded for go was 84% rangewide and 64% in 2002. This reduction in activity and encounter rate was likely drought induced. The resulting numbers of live adult tortoises (> 180mm) MCL were approximately equal in 2001 and 2002 although over 1200 more kilometers were surveyed in 2002.