Use holding pens or enclosures at release site prior to release of wild reptiles
Overall effectiveness category Awaiting assessment
Number of studies: 7
Background information and definitions
Holding pens or enclosures at release sites may be used (sometimes termed ‘soft release’) 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.
This action discusses studies that test the effectiveness of placing wild-caught translocated individuals into holding pens prior to release. See also: Use holding pens or enclosures at release site prior to release of captive-bred reptiles.
Supporting evidence from individual studies
A replicated, controlled study in 1980–1982 in five areas of pine forest in Mississippi, USA (Lohoefener & Lohmeier 1986) found that translocations of gopher tortoises Gopherus polyphemus using release pens with artificial burrows prior to release were more successful than those that were not held initially in a release pen with burrows. Results were not statistically tested. When translocated gopher tortoises were initially held in release pens with artificial burrows, more tortoises were resighted or dug burrows during the 3–4-month monitoring period (17 of 21 recaptured or dug burrows, see paper for details) than when tortoises were released without a holding pen (directly released: 0 of 11; released in abandoned burrow: 1 of 5; released in artificial burrow: 0 of 3). Forty individually-marked adult gopher tortoises (some may have been captive releases) were translocated in spring–summer 1980–1982. Tortoises were released into artificial burrows in release pens (21 tortoises), directly released with no specific management (11 tortoises), into abandoned existing burrows (5 tortoises) or into artificial burrows (3 tortoises). Artificial burrows were 1 m deep and 45 degrees to the surface. Most tortoises (35 of 40) were released into areas with existing tortoise populations. Release pens were circular (4–7 m diameter), with translucent vinyl sheet walls (buried 10 cm into the ground) attached to wooden posts. Most tortoises were held in release pens for 2–4 weeks. Tortoises were monitored until late summer or early autumn in the release year.Study and other actions tested
A replicated, controlled study in 1988–1993 in a recovering area of wetlands and mixed shrubland, grasses and trees in New York, USA (Cook 2004) found that releasing eastern box turtles Terrapene carolina carolina into holding pens prior to release did not affect post-release survival or dispersal. Annual survival was 71% and was not affected by spending time in a holding pen, neither was survival to two years (pen: 33%; no pen: 34%) or five years (pen: 27%; no pen: 24%). Post-release direction of dispersal (result presented as a bearing) and initial dispersal speed (days to disperse 100 m: average of 24–85 days) were also not affected by being in a holding pen prior to release. Nineteen gravid females, 11 clutches of 1–9 eggs, and 10 offspring were discovered following releases. In 1987–1990, a total of 335 turtles were collected from development sites or while crossing roads in suburban areas. Fifty-three turtles were fitted with radio trackers, and were either released immediately, or held in a pen for 15 days prior to release. The remaining 282 turtles were held in pens for 30 days before release. Originally a saltmarsh, the release site was created by dredge spoil deposition during 1928–1945. Radio tagged turtles were located daily for the first three days, then weekly until 1993. In 1993–1995, a trained dog Canis lupus familiaris was used to locate turtles.Study and other actions tested
A controlled study in 2001–2003 in a mixed forest site in South Carolina, USA (Tuberville et al. 2005) found that releasing translocated gopher tortoises Gopherus polyphemus into a holding pen prior to release resulted in less dispersal away from the release site and smaller activity areas compared to when no holding pen was used. More tortoises stayed at the released site when a release pen was used (9 months penning: 8 of 13, 62% stayed; 12 months penning: 11 of 12, 92% stayed) compared to when no pen was used (3 of 13, 23% stayed). In the year of release, tortoises held in pens for 12 months had smaller activity areas (2 ha) than those held for nine months (37 ha) or not held at all (94 ha), whereas in the year after release, activity areas were similar for all groups (5–40 ha). In 2001, tortoises were collected from an industrial development site. Groups of 12–13 adults and sub-adults were assigned either to a ‘soft release’ penning treatment (9 months or 12 months) or ‘hard release’ (no penning). All release areas contained 24 starter burrows. All turtles were released in 2002 and relocated in October–November 2002 and March–October 2003. Dispersers were retrieved and re-released at the release site.Study and other actions tested
A controlled study in 2009 in a grassland enclosure in South Australia, Australia (Ebrahimi & Bull 2013) found that translocated pygmy bluetongue lizard Tiliqua adelaidensis confined to holding pens with artificial burrows for one day after release dispersed to marginal habitat less frequently and basked more than lizards confined for five days. After translocated pygmy bluetongue lizards were released from holding pens, lizards confined to a pen for one day dispersed to marginal habitat less frequently (0.2 lizards/cage/day) and basked for longer (22 minutes/hour) than lizards confined for five days (dispersal: 0.8 lizards/cage/day; basking time: 13 minutes/hour). Activity levels, movements, burrow switching, and agonistic interactions were similar between lizards confined for one or five days (see original paper for details). In October 2009, sixteen pygmy bluetongue lizards were captured in the wild and released into one of four predator-proof cages in a zoo enclosure (4 lizards/cage). Each cage included a central grassy circle (4 m diameter) with artificial burrows, surrounded by a strip of bare ground (5 m wide), encircled by a strip of marginal habitat (0.5 m wide) containing artificial burrows (see original paper for details of burrows). When lizards were released, all cages had a holding pen around the central grass areas. The pen was removed from two cages after one day and from the remaining two cages after five days. Lizard activity was monitored by video cameras over 10 days and analysis of lizard behaviour was based on observations from days 6–10 of the study.Study and other actions tested
A replicated, controlled study in 2011–2012 in the Orokonui Valley, New Zealand (Knox & Monks 2014) found that keeping translocated jewelled geckos Naultinus gemmeus in a holding pen for up to ten months prior to release resulted in less movement away from their release site compared to unpenned geckos. None of the penned geckos (10 individuals) moved outside of the release area after the pen was removed (distance moved from release site: 1–16 m) compared to 67% (six of nine geckos) of the unpenned geckos (distance moved from release site: 4–39 m). Fourteen months after release, four females (all gravid) were found at the penned site and two (neither gravid) were found at the unpenned site. Forty-two geckos were translocated to Orokonui Ecosanctuary in December 2011 and January 2012 (21 females, six males and 15 unsexed juveniles) and held in a release pen (10–15 m wide, 55–60 m long and 0.5 m high) until September 2012, at which point the pen was removed. In September 2012, eleven individuals (six females, three males, two unsexed subadults) were released directly at a nearby site (200 m away). Ten penned and nine unpenned geckos were monitored by radio tracking (attached using a 22 x 3 cm self-adhesive fabric strip) one to two times daily for three weeks.Study and other actions tested
A replicated study in 2008–2010 in dry scrubland in Florida, USA (McCoy et al. 2014) found that populations of Florida sand skinks Plestiodon reynoldsi translocated away from a proposed mining site and released into enclosures with different habitat features (trees, shade, woody debris) survived at least three years. Estimates of overall survival of translocated skinks ranged from 49–79%, and 105 of 300 skinks were recaptured during the two years following release. Provision of shade may have been important in explaining skink survival (reported as model result but effect size not reported). Newborn skinks (19 in 2008, 13 in 2009) were captured in all enclosure types. A further 35 newborns were trapped in 2010 (unpublished data). Skinks were sourced in spring 2007 from a site scheduled for sand mining and released in to fifteen 20 m2 enclosures (20 lizards/enclosure). Enclosures had five experimental treatments (tree only, shade cloth only, tree and coarse woody debris added, coarse woody debris only, control with no shade or debris). Skinks were trapped in enclosures in spring 2008–2009 (16 drift fences and 76 bucket-traps/enclosure), and further trapping was carried out in 2010 (method not given).Study and other actions tested
A randomized, controlled study in 2016–2017 in an area of mixed grassland, scrub and woodland in Kent, UK (Nash 2017) found that more translocated viviparous lizards Zootoca vivipara were recaptured after release into an enclosure compared to those released in an unenclosed area, and that body condition was similar in the enclosed and unenclosed areas. More lizards were resighted after release in the enclosure (101 lizards) than in the unenclosed area (16 lizards). Body condition was similar for lizards in the enclosure and those in the unenclosed area (reported as condition index). Two adjacent sites in the wider release area (1.5 ha each) were selected, and one was randomly selected and enclosed with a reptile-proof fence (38 cm high, buried 30 cm deep). Both sites were provisioned with one hibernaculum and four earth banks. In 2016, lizards were translocated to both sites in the release area (total of 1,364 lizards) and 695 were released in the enclosure and 669 were released in the unenclosed area. In April–May 2017, translocated lizards were monitored at the two sites using visual encounter surveys and artificial cover boards (45/site).Study and other actions tested