25th Annual Meeting and Symposium of the
Desert Tortoise Council, April 21-24, 2000
Abstracts

Results of Line Distance Transects Conducted on the Chocolate Mountains Aerial Gunnery Range, Riverside County, California

Peter Woodman
Kiva Biological Consulting, P.O. Box 1210, Inyokern, CA 93527

The approximately 387,200 acre Chocolate Mountain Aerial Gunnery Range (CMAGR) is located in the central portion of the Colorado Desert, in Riverside County, California. It is a live-fire air to ground and air to air range. Most of the base is a buffer for approximately 30 target sites that are on the CMAGR. The target sites range from 200 to 1000 acres, are heavily impacted, and portions of many are virtually denuded. The surrounding habitat is impacted very little, if at all. Only two ground-based activities occur on the base: a Navy SEAL training area is on the west edge, and EOD clears the target areas of unexploded ordinance. Two utility corridors traverse the CMAGR: a Gas Company natural gas pipeline and an electrical transmission line.

That portion of the CMAGR north and east of the Chocolate Mountains is within the Chuckwalla Bench Desert Wildlife Management Area (DWMA) and within Critical Habitat (the boundaries for both the DWMA and Critical Habitat are the same). Two strata were determined to be on the CMAGR, the Chuckwalla Bench Strata on the north end and the Milpitas Wash strata in the eastern portion. It was estimated that approximately half of the Chuckwalla Bench strata is on the CMAGR and that approximately 15% of the Milpitas Wash strata is on the CMAGR.

In 1997, forty-one line distance transects were established within Critical Habitat on the CMAGR. The transects are four kilometers long, one kilometer on a side, and in the shape of a square or diamond. Thirty-one transects were placed in the Chuckwalla Bench strata and 10 transects were placed in the Milpitas habitat. The locations were not randomly selected but were spaced throughout the two strata.

The transects were first read between April 21 and May 6, 1999. Three person crews were used to read each transect. The center person stayed within one meter of the actual line and two flankers roamed from 0 to 10 meters from the line, one on each side. Desert tortoises, fitted with transmitters, were relocated daily during distance sampling to estimate the number of visible tortoises and the probability of detection (g0). The Milpitas habitat type is in a range that cannot be accessed prior to the spring sampling period, thus, all tortoises used for g0 were in the Chuckwalla Bench strata. Unfortunately only one tortoise was transmittered on the first day of distance sampling and the tenth tortoise was not fitted with a transmitter until the fourth day of distance sampling. All of the tortoises fitted with transmitters were adults, seven males and three females. During the sampling period the number of visible tortoises, g0, was 0.68 (n=10; SE=0.11).

Thirty-one desert tortoises were observed on the 41 transects, 25 on the Bench and six on the Milpitas. The encounter rate was 0.21 in the Chuckwalla Bench and 0.12 in the Milpitas strata. Nine of the 31 tortoises were above ground and the other 22 were visible in burrows. Twenty-four of the tortoises were adults (7 male, 10 female, 7 unknown), four were subadults (1 male, 1 female, 2 unknown), and three were immatures. None of the eight tortoises we were able to inspect had signs of Upper Respiratory Tract Disease. We could not inspect the plastrons of the tortoises observed, but three of the eight tortoises whose carapaces were inspected had shell lesions indicative of cutaneous dyskeratosis. The density for both strata was 0.17 tortoises per hectare with a 95% confidence interval from 0.08 to 0.27 and a cv(D) 30.02%. This project was funded by Marine Corps Air Station, Yuma via Contract No. N68711-97-M-8808. Ron Pearce (MCAS, Yuma) and Bill Fisher (Southwest Division, Naval Facilities Engineering Command) provided invaluable assistance.