Release captive-bred reptiles into the wild: Crocodilians

Overall effectiveness category Awaiting assessment
Number of studies: 4
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not assessed
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Background information and definitions

Captive breeding is normally used to provide individuals that can then be released into the wild (often called ‘reintroduction’) to either re-establish a population that has been lost, or to augment an existing population (‘restocking’).

Release techniques vary considerably, from ‘hard releases’ involving the simple release of individuals into the wild to ‘soft releases’ which involve a variety of adaptation and acclimatisation techniques before release, or post-release feeding and care.

This action includes studies describing the effects of release programmes for captive-bred reptiles that do not specifically test the effectiveness of specific release techniques. For studies that compare specific release techniques see Use holding pens or enclosures at release site prior to release of captive-bred reptiles; Use holding pens or enclosures at release site prior to release of wild reptiles and Release reptiles into burrows.

Due to the number of studies found, this action has been split by species group, though no studies were found for amphisbaenians. See here for: Sea turtles; Tortoises, terrapins, side-necked & softshell turtles; Snakes & lizards or Tuatara.

Study locations

Key messages

Four studies evaluated the effects of releasing captive-bred crocodilians into the wild. Two studies were in China, one was in South Africa and one was a global review.

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (4 STUDIES)

Abundance (2 studies): 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. One study in South Africa reported that following releases of captive-bred Nile crocodiles, wild populations increased in size over 30 years, but then declined in the subsequent 15 years.
Reproduction (2 studies): Two studies (one replicated) in China reported that breeding or nesting was observed within four years of releasing captive-bred Chinese alligators.
Survival (1 study): One study in China reported that of nine captive-bred Chinese alligators, three survived for nine years and six survived for at least one year following release.

BEHAVIOUR (1 STUDY)

Use (1 study): One replicated study in China reported that after 10 years of releases of captive bred Chinese alligators to an area that had historically been occupied, 56% of constructed ponds were occupied.

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

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
Referenced paper
Germano J.M. & Bishop P.J. (2009) Suitability of amphibians and reptiles for translocation. Conservation Biology, 23, 7-15.
A study in 1967–2009 along two rivers and associated floodplains in KwaZulu-Natal, South Africa (Calverley & Downs 2014), found that after releasing captive-bred Nile crocodiles Crocodylus niloticus, the number of crocodiles counted in the wild increased over 30 years, but then began to decline. Results were not statistically tested. In the 1990s, thirty years after a programme to breed and release Nile crocodiles began, the crocodile population numbered 937–1066 individuals, compared to 344–351 individuals in the 1970s. In 2009, fifteen years later, the population numbered 128–846 individuals and the authors reported that it may have been declining after peaking in the 1990s. In January 1967–November 1974, a captive-breeding programme produced, reared and released 1,257 Nile crocodiles into a game reserve (10,000 ha). Crocodile abundance was monitored on two river systems using aerial surveys (carried out by helicopter or airplane) in 1971–1973, 1985–1986, 1989–1990, 1992–1994 and 2009. Results reported here were corrected for differences between survey methods (see original paper for details).
Study and other actions tested
Referenced paper
Calverley P.M. & Downs C.T. (2014) Population status of Nile crocodiles in Ndumo Game Reserve, Kwazulu-Natal, South Africa (1971–2012). Herpetologica, 70, 417-425.
A replicated study in 2006–2016 in an area of ponds and dense vegetation in Anhui Province, China (Manolis et al. 2016) found that after 10 years of releases of captive-bred Chinese alligators Alligator sinensis, alligators occupied over half of ponds in the area, and successful reproduction was occurring. Alligators were found in 28 of the 50 ponds (56%). Survivorship of released alligators was thought to be >85% (no formal analysis carried out). Successful reproduction was recorded two years after the first release (158 eggs, producing 80 hatchlings were discovered), though the full extent of nesting was unknown. Fifty ponds (30 ha total water area) were constructed in the release area, at a cost of around $US10,000 to construct and prepare the average-sized pond. Ponds were established with terrestrial (e.g. bamboo) and aquatic vegetation, and “seeded” with fish, amphibians, and snails. Prior to release, adult alligators were isolated for 3–4 months for health screening. In 2006–2016, eleven releases (during May–June) of 93 alligators were carried out (sex ratio 1 male:2 females). Population monitoring was carried out using spotlight surveys.
Study and other actions tested
Referenced paper
Manolis C., Shirley M.H., Siroski P., Martelli P., Tellez M., Meurer A. & Merchant M. (2016) Trip report: CSG visit to China, August 2016. IUCN-SSC Crocodile Specialist Group report, 13pp.
A study in 2007–2016 in a wetland in Shanghai Province, China (Manolis et al. 2016) reported that some released captive-bred Chinese alligators Alligator sinensis survived for 1–9 years and successfully reproduced. Three of six alligators survived for 9 years, and a further six survived at least one year following release. Nesting was reported in four years following release. In 2016, the population consisted of nine adults (released individuals), at least four wild born adults (offspring of released alligators) and around five juveniles/sub-adults. In 2007, six captive-bred alligators were released into a wetland park. In 2015–2016 a further six were released.
Study and other actions tested
Referenced paper
Manolis C., Shirley M.H., Siroski P., Martelli P., Tellez M., Meurer A. & Merchant M. (2016) Trip report: CSG visit to China, August 2016. IUCN-SSC Crocodile Specialist Group report, 13pp.

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.

Where has this evidence come from?

List of journals searched by synopsis

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This Action forms part of the Action Synopsis:

Reptile Conservation

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