Release captive-bred frogs

How is the evidence assessed?
  • Effectiveness
  • Certainty
  • Harms

Study locations

Key messages

  • Four of five studies (including one replicated study and one review) in Europe, Hong Kong and the USA found that captive-bred frogs released as tadpoles, juveniles or adults established populations or stable breeding populations at 88-100% of sites, and in some cases colonized new sites. One study found that stable breeding populations were not established. One before-and-after study in Spain found that released captive-bred, captive-reared and translocated frogs established breeding populations at 79% of sites.
  • Three replicated studies in Australia and the USA found that a high proportion of captive-bred frogs released as eggs survived to metamorphosis, some released as tadpoles survived at least the first few months or few released as froglets survived. Three studies (including two replicated studies) in Australia, Italy and the UK and a review in the USA found that captive-bred frogs reproduced at all or 31–33% of release sites, or that there was very limited breeding by released frogs.


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 before-and-after study in 1988–1997 in ponds on abandoned farmland in Liepâja, Latvia (Zvirgzds, Stašuls & Vilnìtis 1995, Zvirgzds 1998) found that released captive-bred European tree frog Hyla arborea froglets established stable breeding populations at release sites and frogs colonized new breeding sites. Males were recorded calling from 1990 and tadpoles were observed from 1991. From 1993, calling males were heard outside the release site. By 1994 there were seven ponds with calling males up to 2 km away and by 1997 this had increased to 48 ponds within a 20 km radius of the release site. Breeding was recorded in at least 10 of those ponds. At least four generations had been produced in the wild by 1997. A total of 4,110 froglets were released into ponds in a Nature Conservation Area (300 ha) in June–August 1988–1992. Ponds were monitored using call surveys in spring and by counting tadpoles and froglets in autumn.

    Study and other actions tested
  2. A before-and-after study of projects in 1986–1997 that released captive-bred amphibians into restored and created ponds in Denmark (Fog 1997) found that released European tree frogs Hyla arborea established populations. European tree frogs established populations in 10 restored and 13 created ponds. A questionnaire was sent to all those responsible for pond projects across Denmark to obtain data. Animals were reared in captivity and then released into ponds as tadpoles or juveniles. For a pond to be defined as ‘colonized’ a species had to be present but not breeding.

    Study and other actions tested
  3. A replicated, before-and-after study in 1994–1997 in Jersey, UK (Gibson & Freeman 1997) found that there was limited breeding by released captive-bred agile frogs Rana dalmatina. The first egg mass was recorded two years after the first release and eggs were head-started due to the risk of predation by palmate newts Triturus helveticus. However, there was no breeding at the site the following year, although adults were recorded. In 1994–1996, 100–200 well-developed tadpoles each year and in 1996 twenty young frogs were released into two ponds.

    Study and other actions tested
  4. A replicated, before-and-after study in 1992–1998 in Hong Kong (Dudgeon & Lau 1999) found that released captive-bred Romer’s frog Philautus romeri tadpoles and adults established and maintained populations at seven of eight sites for four to five years after release. However, populations remained small and only one expanded its range significantly. In 1992, a total of 230 adults, several eggs and tadpoles were collected from the wild. Thirty adults were sent to Melbourne Zoo and the remainder were housed at the University of Hong Kong. A total of 1,170 frogs and 1,622 tadpoles were released in 1993 at three sites and in 1994 at eight sites. Additional small ponds were constructed at some sites to provide fish-free habitat. Frogs were monitored annually by call and visual surveys.

    Study and other actions tested
  5. A before-and-after study in 1992–2000 in Jersey, UK (Racca 2002) found that released captive-bred agile frog Rana dalmatina tadpoles established a breeding population. The first egg mass was found in 1996, two years after the first release. Breeding also occurred in the release pond in 1998–2000. Mortality during the embryonic stage was 50% in captivity compared to 40% in the wild. One or two egg clumps were taken from the wild for captive breeding in 1992–1993 and 1997–2000. Captive-bred tadpoles were released into a pond in 1994–2000.

    Study and other actions tested
  6. A before-and-after study in 1998–2003 in Gipuzkoa province, Spain (Rubio & Etxezarreta 2003) found that released captive-bred and captive-reared stripeless tree frog Hyla meridionalis juveniles and translocated adults established breeding populations in 11 of 14 created ponds. Metamorphosis, mating, eggs and well-developed larvae were observed in 11 of the ponds, froglets were also recorded in some ponds. Translocated adults survived in good numbers and returned to 12 of 14 ponds. Introduced predators, dense vegetation, eutrophication and drying resulted in reduced survival and reproduction in some ponds. A small number of additional ponds were colonized by the species. Thirteen ponds were created and one restored, with vegetation planted in 1999–2000. In 2000–2003, a total of 5,767 tadpoles were bred in captivity and released (171–3,989/year). Eggs were also collected, reared in captivity (in outdoor ponds) and then released as 871 metamorphs and 19,478 tadpoles into eight of the ponds. In 1998–2003, a total of 1,405 adults were translocated to the ponds.

    Study and other actions tested
  7. A replicated, before-and-after study in 1999–2006 in 18 ponds in Lombardy, Northern Italy (Pellitteri-Rosa, Gentilli, Sacchi, Scali, Pupin, Razzetti, Bernini & Fasola 2008) found that captive-bred Italian agile frogs Rana latastei released as tadpoles reproduced in six of the ponds. At least one egg mass (1–14) and/or calling males (4–8 in two ponds) were recorded in six of the 18 ponds. Four the ponds with breeding were new ponds and two were unmanaged. Up to four adults were found in three of the ponds. Breeding success was negatively affected by human disturbance and predator presence and positively affected by woodland, shore incline and pond permanence. Human disturbance was noted at 89% of the sites and potential predators, mainly fish, were found in 39% of ponds. New ponds were excavated in six Natural Parks in 1999–2001. In 2000 and 2001, tadpoles were released in 13 new ponds and five existing unmanaged ponds that had not recently been used for breeding. In February–April 2006, ponds were monitored during 45 visual and call surveys (average 2.5/pond).

    Study and other actions tested
  8. A before-and-after study in 2008–2010 in New South Wales, Australia (McFadden, Hunter, Harlow, Pietsch & Scheele 2010) found that only four of 610 released captive-bred booroolong frogs Litoria booroolongensis frogs were found a year after release. A total of 105 frogs were captured after release, 29 of which survived to sexual maturity and engaged in breeding activity. At sexual maturity, released frogs were similar in size and condition to wild frogs at the site. A high infection rate of chytridiomycosis was recorded in the population. A total of 610 two- to four-month-old frogs were marked and released along a 1.5 km section of a creek in February 2008. The creek was surveyed four times during the two months following the release and six times in October and February 2008–2010.

    Study and other actions tested
  9. A replicated study in 2009–2011 at San Diego Zoo, California, USA (Medlin 2011) found that mountain yellow-legged frog Rana muscosa tadpoles survived for at least the first few months after release. All tadpoles survived in acclimation cages prior to release. In 2011, a number of tadpoles released that year survived at least until the autumn. In 2006, 82 tadpoles were rescued from a drying stream and breeding was attempted from 2009. In 2010, 30 eggs and 36 tadpoles and in 2011, 300 eggs and 300 tadpoles were released into screen cages in a stream within a reserve. Tadpoles were kept in cages to acclimatize for different periods of time before release. Regular monitoring was undertaken.

    Study and other actions tested
  10. A review of two release programmes of captive-bred chiricahua leopard frogs Lithobates chiricahuensis in Arizona, USA (Sredl, Akins, King, Sprankle, Jones, Rorabaugh, Jennings, Painter, Christman, Christman, Crawford, Servoss, Kruse, Barnitz & Telles 2011) found that one programme resulted in breeding at four of 13 release sites and at four new localities, whereas the other programme failed. In one programme, breeding was first observed 10 months after releases and a total of 32 egg masses were recorded. In the second programme, multiple releases at four sites over a number of years did not result in the establishment of populations as no frogs were detected from 2009. In the first programme, 3,542 metamorphs and late-stage tadpoles were released at 13 sites throughout a watershed in 2009–2010. In the second programme, frogs were released at three sites from 1996 and four from 2000 to 2011. Most releases comprised fewer than 100 frogs. Surveys were undertaken shortly after release and then two to three times annually.

    Study and other actions tested
  11. A replicated study in 2012 of southern corroboree frogs Pseudophryne corroboree at Taronga and Melbourne Zoo, Australia (McFadden 2012) found that a high proportion of captive-bred frogs that were released as eggs reached metamorphosis and exited the ponds. Over 750 eggs were released into ponds at three remote sites. Captive breeding was undertaken as fewer than 50 individuals remained in the wild, mainly because of chytridiomycosis.

    Study and other actions tested
Please cite as:

Smith, R.K., Meredith, H. & Sutherland, W.J. (2020) Amphibian Conservation. Pages 9-64 in: W.J. Sutherland, L.V. Dicks, S.O. Petrovan & R.K. Smith (eds) What Works in Conservation 2020. Open Book Publishers, Cambridge, UK.

Where has this evidence come from?

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Amphibian Conservation

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Amphibian Conservation
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