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

Published source details Nagy K.A., Scott Hillard L., Tuma M.W. & Morafka D.J. (2015) Head-started desert tortoises (Gopherus agassizii): movements, survivorship and mortality causes following their release. Herpetological Conservation and Biology, 10, 203-215.
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This study is summarised as evidence for the following.

	Action	Category
	Use holding pens or enclosures at release site prior to release of captive-bred reptiles Action Link

Use holding pens or enclosures at release site prior to release of captive-bred reptiles

A replicated, controlled study in 2005–2008 in desert scrubland in California, USA (Nagy et al. 2015) found that first-year survival rates of head-started released juvenile desert tortoises Gopherus agassizii were similar regardless of whether holding pens were used and that overall one third of head-starters survived at least three years in the wild. First-year survivorship of tortoises initially released into holding enclosures was similar (9 of 12, 75% tortoises survived) compared to those that were direct-released into the same sites (12 of 15, 80% tortoises survived). Overall survivorship of released head-started juvenile desert tortoises was 32% over three years (17 of 53 tortoises survived). In the first year after release, 42 of 53 (81%) tortoises survived, in the second year after release 32 of 42 (76%) tortoises survived and in the third year after release 17 of 32 (53%) tortoises survived. Survivorship also was similar between tortoises released in the autumn compared to the spring (see original paper for details). In autumn 2005, twelve head-started tortoises were initially placed in temporary predator-proof enclosures in three sites (4 tortoises/site), 15 head-started tortoises were direct-released in the same three sites (5/site), and a further 16 head-started tortoises were direct-released in a fourth site. In spring and autumn 2006, ten further head-started tortoises were released into the fourth site. Tortoises housed in predator-proof enclosures (each 45 m²) were enclosed from September 2005–January 2006. All tortoises were radio-tracked weekly-biweekly during active seasons and monthly during inactive seasons from release until autumn 2008 (up to three years). Tortoises were recaptured twice/year while radio tracked for a health check.

(Summarised by: Katie Sainsbury)