Desert Tortoise Inventory at Lake Mead National Recreation Area 2022 - Open Format Dataset (sensitive version)

The Lake Mead National Recreation Area (NRA) spans the Colorado River corridor in Arizona and Nevada and sits at an ecologically important intersection of the listed and unlisted Mojave tortoise populations. A large portion of the Lake Mead NRA contains suitable desert tortoise habitat, but no contemporary estimates of population densities exist for different areas of the recreation area. Three new Lake Mead NRA monitoring strata consisting of eight substrata were identified based on potential tortoise habitat and geography: 1) Detrital - Detrital 1 and Detrital 2, 2) Overton - Gold-Butte-Pakoon, Overton East, and Western Overton Arm, and 3) Eldorado Valley - Eldorado Mountain, Eldorado Valley, and Piute Valley. In 2022 at Lake Mead NRA, 25 teams surveyed a total of 170 12-km transects between 5 April and 2 May. Basing density estimates only on the tortoises that are visible will result in density estimates that are consistently underestimated. To account for this, we used telemetry to estimate the proportion of tortoises available for sampling. For the Lake Mead NRA surveys field technicians used high frequency receivers and directional antennas to locate 9-16 radio-equipped tortoises in two focal sites: Gold-Butte and Piute-Mid. Surveyors at Lake Mead NRA in 2022 reported five adult tortoises on transects and one additional adult, which was encountered opportunistically. Overall, the encounter rate was low, and the number of tortoises detected on transects was not sufficient to develop a detection function nor derive meaningful density estimates for any of the Lake Mead stratum.