Release captive-bred mammals into fenced areas
Overall effectiveness category Beneficial
Number of studies: 14
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Background information and definitions
Captive-bred mammals may be released into fenced areas. This may be done to keep them within a certain area (e.g. a game reserve), or to keep predators or other problem species out of an area to increase their chances of survival. Here fenced areas refer to those that are large enough to cover the home ranges of the target species. Studies that use smaller holding or pre-release pens before releasing captive-bred mammals into the wild are covered in Use holding pens at release site prior to release of captive-bred mammals.
Supporting evidence from individual studies
A study in 1993–1999 on an arid peninsula in Western Australia, Australia (Short & Turner 2000) found that following release into a fenced area where invasive species had been eradicated, a population of burrowing bettongs Bettongia lesueur increased. In 1999, six years after initial releases, the population was estimated at 263–301 bettongs, with 340 individuals born between 1995 and 1999. The population died out due to fox incursion in 1994, but was re-established with further releases. In 1990, a 1.6-m tall wire mesh fence (with an external overhang, an apron to prevent burrowing and two electrified wires) was erected to enclose a 12-km2 peninsular, within which foxes Vulpes vulpes and cats Felis catus were eliminated by poisoning in 1991 and 1995, respectively. Outside the fence foxes were controlled by biannual aerial baiting with meat containing 1080 toxin, distributed at 10 baits/km2 over 200 km2. From October 1993, an additional 200 baits/month were distributed along the fence and roads across the study area. Cats were controlled by trapping and poisoning in a 100 km2 buffer zone. In May 1992 and September 1993, twenty-two wild-caught bettongs were transferred to an 8-ha in-situ captive-breeding pen. In September 1993 and October 1995, 20 wild-caught bettongs were translocated to range freely in the reserve. From 1993–1998, one hundred and fourteen captive-bred bettongs were released. Artificial warrens, supplementary food and water were provided in 1993, but not for later releases. Eighty released bettongs were radio-tagged. From 1991–1995, European rabbits Oryctolagus cuniculus were controlled within the fenced area using 1080 ‘one shot’ oats. Bettongs were monitored every three months using cage traps set over two consecutive nights, at both 100-m intervals along approximately 40 km of track, and at warrens used by radio-collared individuals.Study and other actions tested
A study in 1998–2000 in an arid protected area in Western Australia, Australia (Parsons et al. 2002) found that after releasing captive-bred burrowing bettongs Bettongia lesueur into a fenced area without predators, the population persisted for at least eight years. In 1992 an unspecified number of bettongs were released onto a 1,200-ha peninsula, fenced to exclude predators. In July 1998, February and August 1999, and February 2000, the population was surveyed using unspecified methods.Study and other actions tested
A controlled before-and-after study in 2001 in five shrubland sites in Western Australia, Australia (Hardman & Moro 2006) found that most captive-bred banded hare-wallabies Lagostrophus fasciatus and rufous hare-wallabies Lagorchestes hirsutus released into a fenced peninsula (with predator control, supplementary food and water and, in some cases, holding pens prior to release), survived at least two months, although rufous hare-wallabies lost body condition while awaiting release in holding pens. After 1-2 months, 10 of 16 rufous hare-wallabies and 12 of 18 banded hare-wallabies were still alive. Overall both rufous and banded hare-wallabies recaptured had similar body conditions to when they were released, although rufous hare-wallabies lost 12% of body condition while waiting for release in holding pens (data presented as a body condition index; see paper for details). Sixteen captive-bred rufous hare-wallabies and 18 captive-bred banded hare-wallabies were released at five sites in August 2001. Six rufous and nine banded-hare wallabies were placed in separate 3-ha enclosures with electrified fencing for 10–19 days before being released. Remaining animals were released directly into the wild. Supplementary food (kangaroo pellets, alfalfa) and water were made available to all hare-wallabies (those kept in holding pens and those not; duration of feeding not given). Hare-wallabies were monitored by radio tracking (once/ week for 1.5 years after release) and live-trapping (at 4 and 8-9 weeks after release). Release areas were within a fenced peninsula where multiple introduced mammals were controlled (cats Felis catus and goats Capra hircus) or eradicated (red fox Vulpes vulpes).Study and other actions tested
A study in 1978–1995 in a desert reserve in Jordan (Harding et al. 2007) found that following release into a fenced area, a population of captive-bred Arabian oryx Oryx leucoryx increased six-fold in 12 years. The herd numbered 186 animals in 1995, after being founded from 31 oryx in 1983. The project began in 1978, with 11 captive-bred founder animals (six females and five males) held in breeding pens. In 1983, thirty-one oryx were released from these pens into the 342-km2 Shaumari Nature Reserve, but were fenced into a 22-km2 sub-section of the reserve in 1984 to exclude domestic grazing animals. An additional three males were introduced in 1984. Release outside the fenced reserve was prevented by in influx of pastoralists displaced from a war zone. From 1997 to 2006, one hundred and five oryx were moved to other reserves to reduce overcrowding. By 2006, forty-three oryx remained in the reserve. Oryx numbers were obtained from the reserve records and independent reports.Study and other actions tested
A study in 1995–2005 in 12 dry savanna and temperate grassland sites in South Africa (Gusset et al. 2008) found that translocated and captive-bred African wild dogs Lycaon pictus released into fenced reserves in family groups had high survival rates and bred successfully. Eighty-five percent of released animals and their wild-born offspring survived the first six months after release/birth. Released animals which survived their first year had a high survival rate 12–18 months (91%) and 18–24 months (92%) after release. Additionally, groups which had more time to socialise in holding pens prior to release had higher survival rates (data presented as statistical models). Between 1995 and 2005, one hundred and twenty-seven wild dogs (79 wild-caught, 16 captive-bred, 16 wild-caught but captive-raised, 16 “mixed” pups) were translocated over 18 release events into 12 sites in five provinces of South Africa. Animals were monitored for 24 months after release, and the 129 pups which they produced after release were monitored up to 12 months of age. Forty characteristics of the individual animals, release sites and methods of release were recorded, and their impact on post-release survival was tested.Study and other actions tested
Referenced paperGusset M., Ryan S.J., Hofmeyr M., van Dyk G., Davies-Mostert H.T., Graf J.A., Owen C., Szykman M., Macdonald D.W., Monfort S.L., Wildt D.E., Maddock A.H., Mills M.G.L., Slotow R. & Somers M.J. (2008) Efforts going to the dogs? Evaluating attempts to re-introduce endangered wild dogs in South Africa. Journal of Applied Ecology, 45, 100-108.
A review of eight studies in 1989-2005 in eight grassland and woodland sites in Victoria, Australia (Winnard & Coulson 2008) found that three captive-bred eastern barred bandicoot Perameles gunnii populations that were released into fenced areas with associated management survived between 1 and 15 years, animals were breeding and populations increased in size at least initially. In two studies, bandicoots were released into fenced areas and populations increased for at least five years after releases began and there was evidence of breeding and wild-born pouch young maturing to adults. These populations subsequently declined to low numbers 12-15 years after the original releases began. A further population released into a fenced area survived at least one year and both pouch young and wild-born adults were observed. Of five studies where bandicoots were not released into a fenced area, one population survived over at least seven years, two populations were extinct after five years, and two populations declined and management ceased (due to low detection rates) after 9-10 years. Between 22 and 207 bandicoots were released into three fenced areas (100-585 ha) and 50 to 103 bandicoots were released into unfenced areas (85-500 ha) in 1989-2005. All bandicoots were captive-bred. Bandicoots were released in stages in each site. Red fox Vulpes vulpes were controlled in all three fenced areas and four of five unfenced areas. Supplementary food was provided in two of the fenced areas (in one for 6-10 days after release, the other was not specified). In most sites, bandicoots were monitored by live-trapping but frequency and methods are not detailed.Study and other actions tested
A replicated study in 1990–2001 in seven grassland, wetland and forest sites in Victoria, Australia (Cook et al. 2010) found that using predator-proof fencing alongside regular predator control increased abundance of captive-bred eastern barred bandicoots Perameles gunnii released into the wild and that bandicoots were recorded at five of seven sites up to three years after the last release. Greater amounts of predator control had a positive influence on the number of bandicoot signs found at each site (Sites with 0-2 methods of regular predator control: 0 bandicoots/site; sites with 3+ methods, including predator-proof fencing: 0.3-2 bandicoots/site). Bandicoot signs were found in five of the seven release sites (average 0.3–2 signs/quadrat) but no signs were detected in two sites. At each of seven sites (88–500 ha), 50–129 captive-bred eastern barred bandicoots were released between 1990 and 1999. Combinations of regular predator control methods were employed (e.g. poisoning, shooting, destruction of red fox Vulpes vulpes dens) differed between the sites (1 site: no predator control; 1 site: 2 methods used; 2 sites: 3 methods used (including 1 site with partial fencing); 3 sites: 4 methods used (including 1 site with full predator-proof fencing). Bandicoot signs (fresh diggings and scats) were collected at 10 randomly distributed 5-m2 quadrats/site on two occasions in 2000–2001.Study and other actions tested
A study in 1998-2010 in a desert site in South Australia (Moseby et al. 2011) found that four of five mammal populations released into a predator-free enclosure and one population released into a predator-reduced enclosure survived, increased their distribution and produced a second generation, whereas two populations released into an unfenced area with ongoing predator management did not persist. After release into a fenced enclosure where red foxes Vulpes vulpes, cats Felis catus and rabbits Oryctolagus cuniculus had been eradicated, greater stick-nest rats Leporillus conditor, burrowing bettongs Bettongia lesueur, western barred bandicoots Perameles bougainville and greater bilbies Macrotis lagotis were detected for eight years, increased their distribution within five years and reproduced within two years. Numbats Myrmecobius fasciatus were only detected for three years and did not produce a second generation. Burrowing bettongs released into a fenced enclosure with cats and rabbits but no foxes survived and increased their distribution over at least three years and produced a second generation within two years. Greater bilbies and burrowing bettongs released into an unfenced area with some predator management did not survive to produce a second generation or increase their distribution. In 1998–2005, five numbats, 106 greater stick-nest rats (6 captive-bred individuals), 30 burrowing bettongs, 12 western barred bandicoots and nine greater bilbies (all captive-bred) were released into a 14-km2 invasive-species-free fenced area. Rabbits, cats and foxes were eradicated within the fenced area in 1999. All western barred bandicoots and greater bilbies, and some greater stick-nest rats (8 individuals) and burrowing bettongs (10 individuals) were put into a 10-ha holding pen before full release after a few months. All other animals were released directly into the larger fenced area. In 2004-2008, thirty-two greater bilbies and 15 burrowing bettongs were translocated to an unfenced area (200 km2) where invasive predators (cats and foxes) were managed with lethal controls and dingoes Canis lupus dingo were excluded by a fence on one side. In 2008, sixty-six burrowing bettongs were translocated to a 26 km2 fenced area which contained small cat and rabbit populations as a result of previous eradication attempts. Between 2000 and 2010, animals were monitored using track counts, burrow monitoring and radio-tracking.Study and other actions tested
A replicated study in 1996–1997 in three grassland sites in South Dakota, USA (Poessel et al. 2011) found that at least half of captive-bred black-footed ferrets Mustela nigripes released into fenced areas where predators were managed survived more than two weeks. At each of the three sites, 48% (12 of 25), 50% (9 of 18) and 89% (32 of 36) of captive-bred ferrets released into the wild survived for at least two weeks (long-term survival is not reported). Overall, twenty-four ferrets were killed by native predators (mostly great-horned owls Bubo virginianus and coyotes Canis latrans) and the cause of death of two others could not be determined. A total of 79 captive-bred black-footed ferrets were released across three mixed-grass prairie sites (18–36 ferrets/site) in September–October 1996 and October–November 1997. A 107 cm high electric fence was installed in each release site (creating 2 km2 enclosures) and activated 1-2 weeks prior to ferrets being released. Ferrets were able to move in and out of the fenced areas. Low-to-moderate lethal coyote control took place for 2-3 weeks each year prior to ferrets being released. Each of the 79 ferrets was radio-tagged and tracked every 5–30 min/night for two weeks post-release in 1996–1997.Study and other actions tested
A study in 2011–2014 of a dry dwarf-scrubland site in Saudi Arabia (Islam et al. 2014) found that captive-bred Arabian gazelles Gazella arabica released into a fenced reserve after being kept in holding pens started breeding in the year following the first releases. Seven females gave birth in August–September of the year after the first releases and all calves survived to the year end at least. Of 49 gazelles released over three years, 10 had died by the time of the final releases. In 2011–2014, three groups of captive-born gazelles, totalling 49 animals, were released in a 2,244-km2 fenced reserve. They were moved from a wildlife research centre and kept for 23 days to a few months in holding pens (500 × 500 m) prior to release at the reserve. Water and food was provided for three weeks following release. Released gazelles were radio-tracked from the ground and air.Study and other actions tested
A study in 2006–2015 in two woodland and shrubland sites in Western Australia and Northern Territory, Australia (Short & Hide 2015) found that following release into fenced areas, a captive-bred population of red-tailed phascogales Phascogale calura survived for less than a year, whereas a translocated population survived for more than five years. A population of phascogales established from wild-caught animals survived longer (>5 years) than a population established from captive-bred animals (which had been kept in pre-release pens and given supplementary food; < 1 year). Authors suggest that the unsuccessful site may also have had a shortage of tree hollows for nesting. In July 2006 and January–February 2007, thirty-two captive-bred phascogales were released into a 26-ha fenced reserve (outside which feral cats Felis catus were abundant) after spending either 10 days or over four months in a pre-release pen (3×6×2 or 4.5×3×2.2 m). Eleven nest boxes were provided within 150m of the release pen, and supplementary food was provided for one week after release. In April 2009 and June 2010, twenty-seven wild-caught phascogales were released into a 430-ha fenced reserve with 22 nest boxes, but with no pre-release pen or supplementary food. From November 2010–January 2013, thirteen additional boxes were installed inside (four) and outside (nine) the fenced area at this site. Phascogales were monitored after each release using radio-collaring or Elliott live traps, and through periodic monitoring of the nest boxes.Study and other actions tested
A study in 2009–2013 in a restored savanna site in Katané, Senegal (Abáigar et al. 2016) found that a population of captive-bred dorcas gazelle Gazella dorcas neglecta released into a fenced area reproduced successfully and almost doubled in number over four years. Over four years after release, the gazelle population increased from 26 to 50 individuals. Thirty-one births and 15 deaths were recorded. Twenty-three (nine male, 14 female) captive-bred dorcas gazelles were released into a fenced enclosure in March 2009 and a further three males were released in November 2010. The enclosure was initially 440 ha but was enlarged by 200 ha in 2010. Released gazelles shared the enclosure with scimitar-horned oryx Oryx dammah, mhorr gazelles Nanger dama mhorr and red-fronted gazelles Eudorcas rufifrons. Small and medium-sized animals, including predators, could pass through the enclosure fence. Natural vegetation was restored prior to the release. Dorcas gazelles were ear-tagged and monitored through direct observations twice daily during 2–3 surveys/season from June 2009 to March 2013.Study and other actions tested
A randomized, controlled study in 2005 in a grassland and forest site in Victoria, Australia (De Milliano et al. 2016) found that captive-bred eastern barred bandicoots Perameles gunnii released into a fenced reserve after time in holding pens had similar post-release survival and body weight compared to bandicoots released directly from captivity. Four out of six bandicoots (67%) released after time in holding pens survived at least 22 days after release, which was similar to the five out of six bandicoots (83%) released directly that survived this period. Maximum weight loss (released from pen: 13%; released directly: 13% loss of weight when released) and final weight 3–4 weeks after release (released from pen: 97%; released directly: 98% of weight when released) were similar. Twelve adult captive-bred bandicoots were randomly divided into two groups of six. One group was kept in a 1-ha pre-release pen (500m from the eventual release site) for one week and provided supplementary food and water and the other group was released directly from captivity. Both groups were released simultaneously into a 170-ha fenced reserve, free of exotic predators. Bandicoots were radio-tracked daily, and were trapped and weighed every 4–5 days, for one month.Study and other actions tested
A study in 2011–2014 in a semi-arid area in South Australia, Australia (West et al. 2017) found that over half of captive-reared black-footed rock-wallabies Petrogale lateralis released into a large fenced area survived at least 20 months and most females reproduced. Ten (five males, five females) of 16 captive-raised black-footed rock-wallabies (63%) survived at least 20 months after release into a fenced area. All five females that survived reproduced within 2-6 months of release. Over three years, 28 births from nine females were recorded. Between March 2011 and July 2012, sixteen captive-reared black-footed rock-wallabies (eight males, eight females; 1-5 years old) were released into a 97-ha fenced area. The fence included a floppy overhang to deter predator entry. Ten of the 16 black-footed rock-wallabies were wild-born and fostered by yellow-footed rock-wallaby Petrogale xanthopus surrogate mothers in captivity. Introduced predators, common wallaroos Macropus robustus and European rabbits Oryctolagus cuniculus were removed from the enclosure by September 2012. Supplementary water was provided in five 8-l tanks that were monitored with camera traps in 2011–2014. Wallabies were fitted with radio-collars and tracked 1-7times/week in 2011-2014. Trapping was carried out on seven occasions in 2011-2014.Study and other actions tested
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This Action forms part of the Action Synopsis:Terrestrial Mammal Conservation
Terrestrial Mammal Conservation - Published 2020
Terrestrial Mammal Conservation