Desert tortoise inventory at Lake Mead National Recreation Area: 2022 line distance sampling surveys

Mitchell, Corey; Allison, Linda

doi:10.36967/2304840

The Mojave desert tortoise (Gopherus agassizii) inhabits the Mojave and Colorado Deserts in the southwestern United States. 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 of the long-term range-wide monitoring strata overlap the Lake Mead NRA: Eldorado Valley, Gold Butte-Pakoon, and Piute-Valley, covering a small area of the potential tortoise habitat within the recreations area. For this project, 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, including 60 in the Detrital strata, 60 in Overton, and 50 in Eldorado Valley. 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. Encounter rates ranged from 0.0020-0.0075 tortoises encountered per km surveyed. The lack of detections is likely due to multiple factors, with a principal cause related to the drought in the southwestern United States. During the spring of 2022, fewer tortoises were detected during range-wide line distance sampling across the entirety of species’ range than in years prior, resulting in decreased encounter rates in most monitoring strata. Further studies on population dynamics in this area may be warranted to determine advantageous management actions needed to maintain this population going forward.

Desert tortoise inventory at Lake Mead National Recreation Area: 2022 line distance sampling surveys

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