Action

Release captive-bred reptiles into the wild: Tortoises, terrapins, side-necked & softshell turtles

How is the evidence assessed?
  • Effectiveness
    not assessed
  • Certainty
    not assessed
  • Harms
    not assessed

Study locations

Key messages

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (13 STUDIES)

  • Abundance (1 study): One global review found that when using recruitment to the adult population as a measure of success, 32% of reptile translocations/releases (releases of captive individuals were 7% of total projects) were successful.
  • Occupancy/range (1 study): One review in Australia found that two of three releases of captive-bred Western swamp tortoises were classified as successful.
  • Reproductive success (2 studies): Two studies (including one replicated study) in Italy reported evidence of a gravid female and successful reproduction following release of captive-bred European pond turtles.
  • Survival (11 studies): Six of nine studies (including two replicated, controlled studies) in Madagascar, the Seychelles, the USA and Italy reported that 66–100% of 5–80 captive-bred tortoises and turtles released into the wild survived over monitoring periods of six months to two years. Two studies reported that 16–20% of 5 and 246 individuals survived over two years. The other study reported that some of over 250 individuals (number not given) were recaptured over a year of monitoring. One study also found captive-bred alligator snapping turtles that were older at their time of release had higher survival than younger turtles. One replicated study in Italy found that annual survival of released captive-bred European pond turtles was 67–91%. One replicated study in Spain and Minorca found that survival of captive-bred Hermann’s tortoises was higher after three years after release into the wild compared to 1–2 years after release. The study also found that after three years, survival of released tortoises was similar to that of wild tortoises in one population, but lower in a second population.
  • Condition (2 studies): One of two controlled studies (including one replicated study) in the USA found that released captive-bred juvenile alligator snapping turtles grew at a similar rate and achieved higher body condition than juveniles that remained in captivity. The other study found that released alligator snapping turtles had similar body conditions compared to individuals that remained in captivity.

BEHAVIOUR (1 STUDY)

  • Behaviour change (1 study): One randomized study in the USA found that captive-bred Blanding’s turtles released into open water habitat had larger home ranges than those released into places dominated by cattail or willows.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A replicated, controlled study in 1986–1999 in dry deciduous forest in Madagascar (Pedrono & Sarovy 2000) found that released captive-bred subadult ploughshare tortoises Geochelone yniphora that were held in pens for four weeks prior to release and provided food and water survived at least one year in the wild. All five released captive-bred subadult ploughshare tortoises survived at least one year in the wild, and settled within 138–523 m of the release site. Released tortoises returned fewer times to the same locations over time (26% of daily locations were the same) compared to wild tortoises (44% of daily locations were the same). Over one year, daily movements were similar between released and wild tortoises (see original paper for details). A captive breeding programme was established in 1986 and five first-generation offspring (8–9 years old) were released in February 1998 and monitored using radio transmitters until January 1999. Five wild juvenile tortoises of a similar size and age in the same region were monitored at the same time. Released tortoises were placed in an acclimatisation pen for four weeks at the release site and initially provided with food and water.

    Study and other actions tested
  2. A review of releases of captive individuals to wetland reserves and an island off the coast of Western Australia, Australia (Mawson 2004) found that two of three releases of captive-bred western swamp tortoises Pseudemydura umbrina were classified as successful. Two of three tortoise releases were considered successful and the success of a further release could not be determined. In 1994–2001, 12–130 tortoises were released at three sites. Animals were translocated from a zoo. The definition of successful translocation was not stated but for other species in the review it included measures of population increase and persistence.

    Study and other actions tested
  3. A review of worldwide reptile translocation projects during 1991–2006 (Germano & Bishop 2009) found that a third of the projects, that included some releases of captive-bred animals, were considered successful with substantial recruitment to the adult population. Of the 47 translocation projects reviewed (39 species), 32% were successful, 28% failed and long-term success was uncertain for the remaining 40%. Projects that translocated animals due to human-wildlife conflicts failed more often (63% of 8 projects) than those for conservation purposes (15% of 38) and those for research purposes (50% of 5). Success was independent of the life-stage translocated/released, number of animals released and geographic region (see paper for details). Releases of captive-bred animals made up 7% of the projects, and individuals involved were adults in 75% of cases, juveniles and sub-adults in 64% of cases and eggs in 4% of cases. The most common reported cause of failure was homing and migration with the second most common reported cause being insufficient numbers, human collection and food/nutrient limitation all equally reported. Success was defined as evidence of substantial recruitment to the adult population during monitoring over a period at least as long as it takes the species to reach maturity.

    Study and other actions tested
  4. A study in 1997–2011 on tropical islands in the Seychelles (Gerlach 2011) found that captive-bred Arnold’s giant tortoises Dipsochelys dussumieri arnoldi released into the wild survived for at least five years. After being released from captivity on Silhouette Island, five of five Arnold’s giant tortoises survived in the wild for at least five years and nests were found one and five years after release. After the remaining giant tortoises from a captive-breeding programme were released in 2011, the author reported that the tortoises settled in their release environments. In 1997–2011, a giant tortoise captive breeding programme was carried out on Silhouette Island. In 2006, five adult Arnold’s giant tortoises (three male and two female) were released at Grande Barbe, Silhouette Island (2010 ha). After the programme closed in 2011, 38 juvenile Arnold’s giant tortoises were moved and released on North Island (210 ha) in February–March 2011 and 92 were released on Fregate Island (207 ha).

    Study and other actions tested
  5. A study in 1997–2011 in Silhouette Island, Seychelles (Gerlach 2011) found that captive-bred adult black mud turtles Pelusios subniger parietalis released into the wild survived at least six months to two years and yellow-bellied mud turtles Pelusios castanoides intergularis survived at least several months. In 2002, five of five adult black mud turtles survived at least six months. One of the adult black mud turtles was recaptured two years after release. The author reported that released captive-bred and captive-maintained yellow-bellied mud turtles were seen regularly after release in 2010. In 2002, five captive-bred adult black mud turtles were released at Grande Barbe, Silhouette Island. In 2003, eighteen captive-bred juvenile turtles were released and in 2009 the remaining adult black mud turtles from the captive breeding programme were also released (total number of adults not provided). Three captive-bred juvenile and several captive-maintained adult yellow-bellied turtles were released into a lake in 2010 (total number of adults not provided). Adult black mud turtles were monitored by occasional trapping and tracking using radio tags for six months.

    Study and other actions tested
  6. A controlled study in 2007–2008 along a river in southern Oklahoma, USA (Moore et al. 2013; same experimental set-up as Moore et al. 2014) found that some captive-bred alligator snapping turtles Macrochelys temminckii released into the wild were recaptured in the year following release. Following release of 16 juveniles, individuals were recaptured on 5 occasions in the year of release and on 18 occasions the year after release (number of individuals recaptured not given). Individuals from a group of translocated adults were recaptured on 50 occasions (249 released, number of individuals not given). Released juveniles grew at a similar rate to those that remained in captivity (released: 0.07 mm/day, captive: 0.09 mm/day), but obtained higher body condition (data reported as statistical model result). Sixteen captive-bred juveniles were release at one location in June 2007, and a further 26 juveniles remained in captivity. An additional 249 adult turtles were confiscated from a turtle farm and released in groups of 27–62 at seven pools adjacent to the river in April 2007. Turtles were recaptured with baited hoop nets in May–August 2007 and 2008.

    Study and other actions tested
  7. A study in 2007–2008 in along a river in Oklahoma, USA (Moore et al. 2014; same experimental set-up as Moore et al. 2013) found that following release into the wild, most juvenile captive-bred alligator snapping turtles Macrochelys temminckii and subadults recovered from a turtle farm survived at least one summer in the wild. After one year, 24 of 32 (75%) released alligator snapping turtles were still alive. Two turtles were lost within 45 days of release and a further six turtles were lost by the beginning of the second year in the wild (see original paper for details). Captive-bred juveniles dispersed similar distances after release (765 m) as released subadults from turtle farms (769 m), but had smaller home ranges (captive-bred: 730 m; turtle farm: 1,789 m). In June 2007, sixteen captive-bred juvenile turtles (bred and reared in the Tishomingo National Fish Hatchery; born in 2002 or 2004) were radio-tagged and released into a single site (a river reach adjacent to the Washita River). In 2006, sixteen subadult alligator snapping turtles recovered from a turtle farm were radio-tagged and released at the same location. A further 250 turtles from the turtle farm were released at six locations in 2007 (monitoring data not provided).  Turtles were monitored weekly during the summer months in 2007 and 2008.

    Study and other actions tested
  8. A replicated, controlled study in 2008–2012 in two rivers in Oklahoma, USA (Anthony et al. 2015) found that releasing captive-bred alligator snapping turtles Macrochelys temminckii resulted in some individuals surviving at least four years in the wild. At least 40 of 246 turtles (actual number not given) were recaptured at least once 1–4 years after release, and overall annual survival was estimated at 59%. Turtles that were older at their time of release were estimated to have higher annual survival than younger turtles (5 years old: 100%; 4 years old: 70%; 3 years old: 59%). Recaptured turtles all showed increases their shell size compared to when released (average of 7–29% growth/year). Body condition of released turtles was similar to that of turtles that remained in captivity (reported as statistical model output). In 2008–2010, a total of 246 turtles were released into two rivers. Turtles were captive-bred and raised in captivity for 3–7 years. Annual trapping was carried out in 2008–2012 during May–August for 60–189 trap nights/year. Size of recaptured turtles (number not given) was compared to 224 still in captivity.

    Study and other actions tested
  9. A study in 2003–2009 in a wooded wetland in northern Italy (Masin et al. 2015) found that a population of juvenile captive-bred European pond turtle Emys orbicularis galloitalica hatchlings released into an area where predators were removed and excluded survived in the wild for at least two years and bred. Ten of 12 (83%) nine-month-old captive-bred European pond turtles survived at least two years in the wild. Five years after the first releases, the first female turtle was confirmed to be carrying eggs. Authors reported that the two turtles that died in the first year after release were probably predated by terrestrial mammals. In September 2003 (eight individuals) and 2005 (four individuals) hatchling European pond turtles born in captivity (sourced from a private breeder) were reared in a terrarium for eight months. In May 2004 and 2006, juvenile turtles were moved to a predator-proof acclimatisation cage (1 x 2 m) for one month prior to release into a fenced pond inside a fenced 1 ha wetland complex in a regional park (see original paper for details). A resident population of largemouth bass Micropterus salmoides was controlled prior to release by catching with pole and line. Larger fish predators were excluded from shallow waters in the release pond using fences woven from branches. A sand and dirt nesting area (2.5 m high x 15 m long) was created in the release area.

    Study and other actions tested
  10. A replicated study in 2008–2015 in three locations on a river in Liguria, Italy (Canessa et al. 2016) found that three populations of released captive-bred European pond turtles Emys orbicularis survived in the wild for at least 8 years. After eight years, 80 of 200 (40%) captive-bred released European pond turtles were estimated to still be alive in three different sites. Annual survival rates of captive-bred turtles released were 67–91% (survival rates differed between release sites, see original paper for details). In 2000–2015, five-hundred captive-bred pond turtles were hatched in an outdoor breeding facility. Hatchlings were reared in an aquarium for two years before being returned to the breeding facility for outdoor acclimatization (duration not specified) prior to release. Approximately 60% of hatchlings survived 3–4 years in captivity. Captive-bred turtles were released annually in three sites in June–July in 2008–2015 (200 individuals released). Survival rates were estimated based on three trapping surveys carried out for three days at a time, in May–August 2008–2015.

    Study and other actions tested
  11. A replicated study in 2014–2015 in four desert scrub vegetation sites in Nevada, USA (Nafus et al. 2017) found that more than half of released captive-bred juvenile Mojave desert tortoises Gopherus agassizzii survived at least six months and settled into home ranges within two months of release. Six months to one year after release, 53 of 80 (66%) released captive-bred juvenile desert tortoises were still alive. The authors reported that of 25 known tortoise deaths, 14 were due to starvation or exposure, and the remaining 11 showed signs of predation or scavenging. Overall, 46 of 71 (65%) desert tortoises settled into a home range pattern within two weeks, all 71 had settled within two months, and nine died before establishing a home range. In September 2014 and April 2015, eighty desert tortoises were hatched and reared in captivity (ages ranged from 6 months to 4 years) and released into four different locations in the Mojave Desert (19–21 tortoises released/locations). Tortoises were released at least 20 m apart and radio tracked weekly during March–October and every two weeks during November–February from release until September 2015. Two tortoises lost their transmitters and were excluded from survival numbers.

    Study and other actions tested
  12. A replicated study in 1987–2015 in sand dune and mixed forest habitats in Spain and Minorca (Bertolero et al. 2018) found that many captive-maintained, released Hermann’s tortoises Testudo hermanni hermanni survived and that survival rates of released tortoises increased three years after release. During the first two years after release, average survival rates of translocated tortoises were 44–90% in Spain (66 individuals) and 79–85% in Minorca (48 individuals). In the third year after release, survival rates of translocated tortoises in Spain (98%) were similar to wild-born tortoises (98%) but survival rates of translocated tortoises in Minorca (89%) were lower than wild-born tortoises (97%). Body condition of tortoises before release did not affect whether or not a tortoise was subsequently found dead or alive (see original paper for details). Tortoises were maintained in captivity, though their origin and total time in captivity was unknown. In Spain, 66 captive tortoises were released into a protected reserve in four batches: September 1987, May–August 1988, March 1997 and September 1998. In Minorca, 48 tortoises were released in March–April 2004. The amount of time tortoises had spent in captivity prior to release was unknown. Tortoises were monitored for 4–10 days a year (Spain: 28 years; Minorca: 14 years) using capture-mark-recapture surveys.

    Study and other actions tested
  13. A replicated study in 2016–2017 in two semi-permanent clay ponds in Savona, north-west Italy (Ottonello et al. 2018) found that released captive-bred European pond turtles Emys orbicularis ingauna bred in the wild and that after release they had adapted their diets to eat food that was available to them. Three turtle nests with successfully hatched eggs were observed at one of the release sites in 2017 (year of release not provided). Captive-bred released turtles ate a range of invertebrates, seeds and plant matter in the wild, although they had been fed commercial shrimp pellets and frozen shrimp and fish in captivity (see original paper for details).  Turtles were bred and reared in captivity from 1999 for 2–5 years prior to their release into two sites (year of release not stated in the original paper). Dietary analysis was carried out on droppings from 25 released turtles that were recaptured in June 2016.

    Study and other actions tested
  14. A randomized study in 2014–2016 in a wetland complex in Michigan, USA (Starking‐Szymanski et al. 2018) found that all released captive-bred Blanding’s turtles Emydoidea blandingii survived their first winter hibernation and most survived at least one year after release. All 24 turtles survived to the spring after release (approximately 9 months) and at least 16 turtles survived for 17 months (only one turtle death was confirmed by the presence of a carcass). Survivorship was lower for turtles released into open water habitat (best case: 5 of 6 turtles survived, worst case: 2 of 6 turtles survived) compared to turtles released into areas vegetated with cattail Typha sp. or willow Salix sp. (best case: 18 of 18 turtles survived, worst case: 13 of 18 turtles survived; no statistical tests were carried out). Average home ranges were larger for turtles released into open water habitat (2.9 hectares) compared to turtles in cattail- or willow-vegetated habitat (0.4–0.6 hectares). In total 24 individuals were selected randomly from turtles bred, hatched and reared in captivity. Turtles were at least one year old and shell length was >10.2 cm. Turtles were released in June 2014 into four wetland habitats (6 individuals per group): open water, sparse cattail vegetation, dense cattail vegetation and willow (see original paper for details). Turtles were monitored by radio transmitter in June 2014–November 2015 (for 515 days in total) once a week during May-September and every two weeks during October–April. Turtles were recaptured autumn 2014, spring 2015, and autumn 2015 to replace/remove transmitters. Best case survival estimates are based on known mortality, worst case include turtles whose radio transmitters were lost or failed and turtles were presumed dead. 

    Study and other actions tested
Please cite as:

Sainsbury K.A., Morgan W.H., Watson M., Rotem G., Bouskila A., Smith R.K. & Sutherland W.J. (2021) Reptile Conservation: Global Evidence for the Effects of Interventions for reptiles. Conservation Evidence Series Synopsis. University of Cambridge, Cambridge, UK.

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Reptile Conservation

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