Survival and movements of head‐started Mojave desert tortoises

Abstract

Head‐starting is a conservation strategy in which young animals are protected in captivity temporarily before their release into the wild at a larger size, when their survival is presumably increased. The Mojave desert tortoise (Gopherus agassizii) is in decline, and head‐starting has been identified as one of several conservation measures to assist in recovery. To evaluate the efficacy of indoor head‐starting, we released and radio‐tracked 68 juvenile tortoises from a 2015 cohort in the Mojave National Preserve, California, USA. We released 20 tortoises at hatching (control) in September 2015, and reared 28 indoors and 20 outdoors in predator‐proof enclosures for 7 months before releasing them in April 2016. We monitored tortoises at least weekly after release until 27 October 2016, and documented survivorship, movement, and surface activity. We estimated survivorship by treatment and evaluated effects of treatment, proximity to a raven (Corvus corax) nest (predator) coincidentally established after release, distance moved between monitoring events, surface activity, and release size on individual fate in a generalized linear model. Although indoor head‐start tortoises reached the size of 5–6‐year‐old wild tortoises by release at 7 months of age, survival did not differ significantly among the 3 treatment groups. Combined annual survival was 0.44 (95% CI = 0.34–0.58). Tortoises that were closer to an active raven nest were significantly more likely to die, as were those seen more often outside their burrows and active aboveground. Predicted estimates for short‐term probability of survival approached 1.0 as distance from a raven nest exceeded approximately 1.6 km. Rearing treatment, movement distance, and body size were not significant predictors of fate over the 1‐year monitoring period. Head‐started tortoises released ≥1.6 km from areas of raven activity will likely have higher short‐term survival. Population recovery through head‐starting alone is unlikely to be successful if systemic ecosystem‐level issues, such as habitat degradation and conditions that promote human‐subsidized predators, are not ameliorated. © 2019 The Wildlife Society.

The Effect of Size on Postrelease Survival of Head-Started Mojave Desert Tortoises

“…Our results also corroborate previous findings that increased surface activity is a predictor of postrelease fate (Daly et al 2019); however, in both Daly et al (2019) and the present study, the negative relationship between surface activity and survival may have been an artifact of the numerous mortalities during the initial postrelease dispersal period when surface activity was unusually high. In addition to this possible negative relationship between surface activity and postrelease survival, our results also suggest a weak negative interaction between surface activity and MCL as they relate to survival.…”

“…These savings can increase the number of head-starts that can be reared each year or fund additional management efforts for the benefit of the species (i.e., restoration, habitat protection). Based on our releases to date, we have found no evidence that the accelerated growth rates associated with indoor rearing, at least under the husbandry practices we used, result in physiological impairment or differences in postrelease behavior or survival (Daly et al 2018(Daly et al , 2019.…”

“…In autumn 2018, we released all juveniles into Yucca woodland habitat at our site. We chose the specific release site based on habitat quality, its accessible yet remote location, and previously published recommendations on minimum distance from potential perch sites for ravens Corvus corax (Nafus et al 2015;Todd et al 2016;Daly et al 2019). Daly et al (2019) reported that predicted survival of released head-started desert tortoises approached 100% when release sites were 1.6 km from a raven nest.…”

“…We chose the specific release site based on habitat quality, its accessible yet remote location, and previously published recommendations on minimum distance from potential perch sites for ravens Corvus corax (Nafus et al 2015;Todd et al 2016;Daly et al 2019). Daly et al (2019) reported that predicted survival of released head-started desert tortoises approached 100% when release sites were 1.6 km from a raven nest. Using ArcGIS (version 10.5; ESRI, Redlands, CA), we first delineated a 0.6-km 2 rectangular release plot (300 m 3 2000 m) parallel to our lone sandy access road, with the nearest corner .1.9 km from a powerline near our site.…”

“…Since 2011, our team has been evaluating the potential role of head-starting in contributing to the recovery of the desert tortoise. During this time, we conducted a series of experimental studies (Nafus et al 2015;Todd et al 2016;Daly et al 2018Daly et al , 2019Tuberville et al 2019) focused on identifying combinations of both pre-and postrelease conditions that maximize tortoise survival and the efficiency of head-starting. Herein, our primary objective was to evaluate the importance of size at release on survival of head-started desert tortoises during the first year following release.…”