West Mojave Plan FEIR/S - Desert Managers Group

1998 –2001 Survey Results: During the three years 3,362 transects covering 3,378 mi 2were surveyed, typically at a density of one transect per square mile. Of the 3,362 transects,1,405 (42%) did not have any tortoise sign, with some tortoise sign found on the remaining 1,957(58%) transects. The distribution of above-average sign counts reveals that higher densitytortoise areas occur on a northeast-southwest axis, between Fort Irwin and south of Edwards AirForce Base (Map 3-8). There were three higher concentration areas in the Ord-Rodman DWMA,and none was observed in the Pinto Mountain DWMA. No higher density tortoise areas werefound in the northern and western portions of the Fremont-Kramer and Superior-CroneseDWMAs, respectively.Tortoise Occurrence in Burrows versus Aboveground: During sign count surveys in thesummer-fall, 275 tortoises were observed, including 202 (73%) in burrows and 73 (27%)aboveground. During distance sampling in the spring, 216 tortoises were observed, including 86(40%) in burrows and 130 (60%) aboveground. For the survey period of 1998 to 2002, tortoiseswere mostly found in burrows (73%) in the summer-fall and mostly found aboveground (60%) inthe spring. These data indicate that tortoises were relatively more active (i.e., aboveground, outof burrows) in the spring and relatively less active in the fall, which is consistent with theliterature.Male versus Female Tortoise Encounters: The seasonal activity patterns described abovealso affected the number of male versus female tortoises encountered. Of the 108 sign counttortoises observed where gender could be determined, 73 (68%) were males and 35 (32%) werefemales, which is a 2:1 ratio of males to females. Comparatively, of the 156 distance-samplingtortoises where gender could be determined, 87 (56%) were males compared to 69 (44%)females, which is a 1.26:1 ratio of males to females. These data suggest that, relative to females,male tortoises are twice as likely to be encountered in the fall, and males and females areencountered at about equal rates in the spring. The difference is apparently due to increasedmale (or decreased female) activity in the fall. The data indicate a 1:1 ratio of males to femalesencountered, which suggests a 1:1 ratio within the surveyed population.Adult versus Subadult Tortoise Encounters: Of the 275 tortoises encountered during signcount surveys, both in burrows and aboveground, 238 (87%) were adults and 37 (13%) weresubadults 6 . Of the remaining 178 (i.e., 83% of 216) tortoises where age class was determined,146 (82%) were adults and 32 (18%) were subadults, which is very similar to the sign countobservations. Combined, there were 453 tortoises where age class could be determined,including 384 (85%) adults and 69 (15%) subadults. The age classes for 38 of the 216 (17%)tortoises encountered during distance sampling were recorded as “unknown.”6 Surveyors used a cutoff of 180 mm to determine age class; carapace lengths of less than 180 mm characterizedsubadults, and adults were 180 mm or greater in length. Although age class determinations were affected byseasonal activity of tortoises, age class was also determined from burrow widths. In general, the width of a tortoiseburrow opening is equal to the length of the tortoise constructing the burrow. As such, the age class for tortoises inburrows could be ascertained by measuring the width of the burrow opening. However, it is well documented thatsubadult tortoises are notoriously difficult to find, and generally under-represented in regional surveys.Chapter 3 3-84