Use holding pens or enclosures at release site prior to release of captive-bred reptiles
Overall effectiveness category Awaiting assessment
Number of studies: 2
Background information and definitions
Holding pens or enclosures at release sites (sometimes termed ‘soft release’) may be used to enable reptiles to become accustomed to new surroundings before release and may contain some natural habitat and burrows. Pens or enclosures may increase the chance that released animals will settle at the release site, potentially increasing the chance that the release will be successful. Captive-bred, naïve animals in particular may benefit from the acclimation period that holding pens provide.
This action discusses studies that test the effectiveness of placing captive-bred individuals into holding pens at the release site prior to release. See also: Use holding pens or enclosures at release site prior to release of wild reptiles.
Supporting evidence from individual studies
A controlled study in 2010–2011 in a grassland site in Illinois, USA (Sacerdote‐Velat et al. 2014) found that captive-bred smooth green snakes Opheodrys vernalis released into holding pens before release (‘soft release’) had a similar chance of recapture as those released directly, and moved less than wild residents. Soft-released snakes were recaptured a similar number of times (13 recaptures/snakes) compared to hard-released snakes (6 recaptures/snake) over 3–5 months following release. Soft-released snakes moved less than residents (soft-released: 2 m/day; residents: 5 m/day), but movement of hard-released snakes (2 m/day) was similar to both soft-released and resident snakes. Eighteen captive-bred and reared snakes (≥9 g) were released in 2011 via soft-release (9 snakes; released in to 1.2 x 1.2 x 1.2 enclosure, held for 3 weeks before final release) or hard-release (9 snakes, released directly). Monitoring was completed by radiotracking and checking under coverboards on the ground at least 5 days/week for the first week, then 3 times/week for 3–5 months. Snake growth was also monitored, but only in captivity.Study and other actions tested
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.Study and other actions tested