Effects of artificial rain on survivorship, body condition, and growth of head-started desert tortoises (Gopherus agassizii) released to the open desert

We subjected neonate Desert Tortoises (Gopherus agassizii) that hatched inside fenced, predator-resistant field enclosures containing natural vegetation to either a natural rainfall regime or a regime of natural rainfall plus irrigation (supplemental precipitation) over a five-year period, to test the hypothesis that mimicking an above-average rainfall regime in years of average or low natural rainfall will improve rates of survival and growth. We also tested the hypothesis that survivorship of released 1-yr olds will be high, due to a decline in predation susceptibility once the vulnerable nesting and hatchling phases are completed. Survivorship inside the enclosures during the first year of life was high (averaging 90%) in both groups, even during a record low rainfall year, but growth rates were always substantially higher (2 to 16-[1] greater) in rain-supplemented juveniles. Body condition index (CI) measurements indicated that first-year juveniles without added rain were able to maintain body conditions similar to rain-supplemented juveniles during two average rainfall years, but not during a drought year. Older juveniles without added rain died during the latter part of the 16 mo drought, suggesting that the high drought survivorship of first-year non-supplemented juveniles may be related to the yolk they carried after hatching, along with possible behavioral and physiological diffrerences. Nearly all yearlings that were set free (in autumn) were dead within 6 mo, regardless of whether they had supplemental rain or not during their first year inside enclosures, and regardless of whether they were released near the head-start enclosures or a kilometer away. The main cause of mortality was predation, primarily by ravens. The poor survival of released yearling tortoises and the drought-induced death of nearly all older captive juveniles raised without added precipitation lead us to recommend that rain supplementation and delayed release be incorporated in the protocol for head-starting Desert Tortoises

Head-started desert tortoises (Gopherus agassizii): Movements, survivorship and mortality causes following their release

We released and monitored 53 juvenile Agassiz’s Desert Tortoises (Gopherus agassizii), aged two to 15 y, that were hatched and head-started inside predator-resistant field enclosures. We set free these tortoises under a variety of conditions to evaluate effects of release distance, season of release, and age/body size on homing behavior and survivorship. Some juveniles moved large distances following release, but homing itself was undetectable. The lack of homing behaviors was likely due to release distance and selection of release sites out of the line-of-sight of natal enclosures. The use of small halfway-house enclosures to accustom some relocated juveniles to release sites for four months before release (i.e. “soft release”) had no effect on subsequent movements or survivorship during the first year following release. Survivorship was not affected by distance of release from natal enclosures, which ranged from 546 m to 1.4 km. Survivorship through one year was similar for juveniles released in spring or autumn. After two years, most small juveniles had been killed by predators, but survivorship increased with body size and age. Juveniles over approximately 100 mm MCL (midline carapace length) and nine years of age when released exhibited high survivorship. However, following a long drought the previous two years, predation by Coyotes (Canis latrans) was heavy on these larger juveniles in the third year after release. Thus, survivorship after three years was relatively low (34%) with the youngest, smallest cohort (two years old when released) exhibiting the lowest survivorship (4%). We recommend releasing headstarted tortoises after they attain a body size of larger than 100 mm MCL and selecting release sites at least 546 m from enclosures

Effects of artificial rain on survivorship, body condition, and growth of head-started desert tortoises (Gopherus agassizii) released to the open desert

© 2015 Kenneth A. Nagy. All Rights Reserved. We subjected neonate Desert Tortoises (Gopherus agassizii) that hatched inside fenced, predator-resistant field enclosures containing natural vegetation to either a natural rainfall regime or a regime of natural rainfall plus irrigation (supplemental precipitation) over a five-year period, to test the hypothesis that mimicking an above-average rainfall regime in years of average or low natural rainfall will improve rates of survival and growth. We also tested the hypothesis that survivorship of released 1-yr olds will be high, due to a decline in predation susceptibility once the vulnerable nesting and hatchling phases are completed. Survivorship inside the enclosures during the first year of life was high (averaging 90%) in both groups, even during a record low rainfall year, but growth rates were always substantially higher (2 to 16-[1] greater) in rain-supplemented juveniles. Body condition index (CI) measurements indicated that first-year juveniles without added rain were able to maintain body conditions similar to rain-supplemented juveniles during two average rainfall years, but not during a drought year. Older juveniles without added rain died during the latter part of the 16 mo drought, suggesting that the high drought survivorship of first-year non-supplemented juveniles may be related to the yolk they carried after hatching, along with possible behavioral and physiological diffrerences. Nearly all yearlings that were set free (in autumn) were dead within 6 mo, regardless of whether they had supplemental rain or not during their first year inside enclosures, and regardless of whether they were released near the head-start enclosures or a kilometer away. The main cause of mortality was predation, primarily by ravens. The poor survival of released yearling tortoises and the drought-induced death of nearly all older captive juveniles raised without added precipitation lead us to recommend that rain supplementation and delayed release be incorporated in the protocol for head-starting Desert Tortoises

Head-started desert tortoises (Gopherus agassizii): Movements, survivorship and mortality causes following their release

We released and monitored 53 juvenile Agassiz’s Desert Tortoises (Gopherus agassizii), aged two to 15 y, that were hatched and head-started inside predator-resistant field enclosures. We set free these tortoises under a variety of conditions to evaluate effects of release distance, season of release, and age/body size on homing behavior and survivorship. Some juveniles moved large distances following release, but homing itself was undetectable. The lack of homing behaviors was likely due to release distance and selection of release sites out of the line-of-sight of natal enclosures. The use of small halfway-house enclosures to accustom some relocated juveniles to release sites for four months before release (i.e. “soft release”) had no effect on subsequent movements or survivorship during the first year following release. Survivorship was not affected by distance of release from natal enclosures, which ranged from 546 m to 1.4 km. Survivorship through one year was similar for juveniles released in spring or autumn. After two years, most small juveniles had been killed by predators, but survivorship increased with body size and age. Juveniles over approximately 100 mm MCL (midline carapace length) and nine years of age when released exhibited high survivorship. However, following a long drought the previous two years, predation by Coyotes (Canis latrans) was heavy on these larger juveniles in the third year after release. Thus, survivorship after three years was relatively low (34%) with the youngest, smallest cohort (two years old when released) exhibiting the lowest survivorship (4%). We recommend releasing headstarted tortoises after they attain a body size of larger than 100 mm MCL and selecting release sites at least 546 m from enclosures