Action: Use holding pens at release site prior to release of translocated mammals
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- Thirty-five studies evaluated the effects of using holding pens at the release site prior to release of translocated mammals. Ten studies were in the USA, seven were in South Africa, four were in the UK, three studies were in France, two studies were in each of Canada, Australia and Spain and one was in each of Kenya, Zimbabwe, Italy, Ireland and India.
COMMUNITY RESPONSE (0 STUDIES)
POPULATION RESPONSE (31 STUDIES)
- Abundance (4 studies): Three of four studies (two replicated, one before-and-after study) in South Africa, Canada, France and Spain found that following release from holding pens at release sites (in some cases with other associated actions), populations of roe deer, European rabbits and lions increased in size. The other study found that elk numbers increased at two of four sites.
- Reproductive success (10 studies): A replicated study in the USA found that translocated gray wolves had similar breeding success in the first two years after release when adult family groups were released together from holding pens or when young adults were released directly into the wild. Seven of nine studies (including two replicated and one controlled study) in Kenya, South Africa, the USA, Italy, Ireland, Australia and the UK found that following release from holding pens at release sites (in some cases with other associated actions), translocated populations of roan, California ground squirrels, black-tailed prairie dogs, lions, four of four mammal populations, most female red squirrels and some pine martens reproduced successfully. Two studies found that one of two groups of Cape buffalo and one pair out of 18 Eurasian badgers reproduced.
- Survival (26 studies): Two of seven studies (five controlled, three replicated studies) in Canada, the USA, France, the UK found that releasing animals from holding pens at release sites (in some cases with associated actions) resulted in higher survival for water voles and female European rabbits compared to those released directly into the wild. Four studies found that translocated swift foxes, gray wolves, Eurasian lynx and Gunnison's prairie dogs released from holding pens had similar survival rates to those released directly into the wild. One study found that translocated American martens released from holding pens had lower survival than those released directly into the wild. Two of four studies (three controlled) in South Africa, Spain, and the USA found that translocated African wild dogs and European rabbits that spent longer in holding pens at release sites had a higher survival rate after release. One study found mixed effects for swift foxes and one found no effect of time in holding pens for San Joaquin kit foxes. Eleven studies (one replicated) in Kenya, South Africa, the USA, France, Italy, Ireland, India, Australia and the UK found that after release from holding pens at release sites (in some cases with other associated actions), translocated populations or individuals survived between one month and six years, and four of four mammal populations survived. Two studies in the UK and South Africa found that no released red squirrels or rock hyraxes survived over five months or 18 days respectively. One of two controlled studies (one replicated, one before-and-after) in South Africa and the USA found that following release from holding pens, survival of translocated lions was higher than that of resident animals, whilst that of translocated San Joaquin kit foxes was lower than that of resident animals. A study in Australia found that translocated bridled nailtail wallabies kept in holding pens prior to release into areas where predators had been controlled had similar annual survival to that of captive-bred animals.
- Condition (1 study): A controlled study in the UK found that translocated common dormice held in pens before release gained weight after release whereas those released directly lost weight.
BEHAVIOUR (5 STUDIES)
- Behaviour change (5 studies): Three studies (one replicated) in the USA and Canada found that following release from holding pens, fewer translocated sea otters and gray wolves returned to the capture site compared to those released immediately after translocation, and elk remained at all release sites. Two studies in Zimbabwe and South Africa found that following release from holding pens, translocated lions formed new prides.
Holding pens at release sites (sometimes termed “soft release”) may be used to enable mammals to become accustomed to new surroundings before release. They are often enclosures containing natural habitat and enabling views of surrounding land. Additionally, some wild translocated mammals may display a homing instinct after release and pens may therefore be used to reduce the chance of animals returning.
The use of holding pens may be employed both for translocations of wild mammals to new sites and releases of captive-bred mammals, here we focus on the first group. See also: Use holding pens at release site prior to release of captive-bred mammals.
For studies that held translocated mammals in captivity away from the release site before release, see: Hold translocated mammals in captivity before release.
Supporting evidence from individual studies
A study in 1970–1978 in a grassland and forest reserve in southeast Kenya (Sekulic et al. 1978) found that after being kept in a holding pen prior to release, a population of roan Hippotragus equinus translocated into an area outside their native range persisted and bred for more than six years. Only eight out of the original 38 translocated roan could be located 18 months after the last release. However, six years after the last translocations, roan numbers had increased to 22. From 1973–1976, at least 15 calves were born, of which one-third survived to nine months of age. Between 1970 and 1972, 38 roan were released in Shimba Hills National Reserve, where there is no evidence for their existence since at least 1885. Animals were captured in the Ithanga Hills, by funnelling them into a 2.5 acre corral using horses, trucks and a helicopter. Prior to release roan were kept in a 30-acre holding pen. Roan were monitored between June 1973 and January 1978, but no further monitoring details are provided.
A study in 1976–1978 in a pasture in California, USA (Salmon & Marsh 1981) found that following release from holding pens at the release site, translocated California ground squirrels Spermophilus beechey established a reproductive colony. Reproduction occurred within one of the holding cages, but the number of young was not determined. At least three of the eight ground squirrels released from cages were still alive 8–13 months after release. Four wire-mesh cages (1.2 × 2.4 × 0.6 m high) were part-filled with soil, to 41 cm depth, in a 7.5-ha pasture. Cages each had four pipes (20 cm long, 10 cm diameter) leading down into the soil, as refuges. Cages were positioned in two adjacent pairs. Pairs were 46 m apart. In November 1976, one pair of wild-caught California ground squirrels was released into each cage. Squirrels were allowed to exit from two of the cages in March 1977 and from the other two in June 1977. In February–April 1978, tagged and non-tagged squirrels were observed and/or live-trapped near the cages.
A randomized, controlled study in 1975–1976 in a temperate forest in Wisconsin, USA (Davis 1983) found that when using holding pens prior to releasing translocated animals, American marten Martes americana survival was lower than when animals were released immediately after translocation. Eight of 10 American martens released after being held in pens died within 154 days. Only one of 11 animals released immediately after translocation had died within 161 days. None of the martens reproduced in this time. Thirty days after release, martens that had been held in pens stayed closer to the release site than did those released immediately (data not reported). In January 1975–April 1976, 124 martens, captured in Ontario, Canada, were released at a forest site in Wisconsin, USA. Twenty-six animals were held in pens at the release site for seven days before release and 97 animals were released within 48 hours of being transported to the site. Individuals were randomly assigned a release method. Twenty-one of the martens were radio-collared. Their movements were monitored until June 1976.
A study in 1988–1989 in coastal waters of California, USA (Ralls et al. 1992) found that after being held in pens at the release site, fewer translocated sea otters Enhydra lutris returned to the capture site compared to those released immediately after translocation. No statistical analyses were performed. None of 10 sea otters held in release pens returned to the capture site and all remained within 27 km for the duration of monitoring. Five of nine released immediately on arrival returned to the capture site. Nineteen sea otters (18 male, one female) were caught between May 1988 and May 1989 and were released 291 km further north. Nine were released immediately on arrival and 10 were held for 48 hours in floating pens before release. Sea otters were radio-tracked from the ground or air for 16–87 days after release.
A study in 1988–1989 in forest and swamp habitats in Florida and Georgia, USA (Belden & Hagedorn 1993) found that after being held in holding pens at the release site, more than half of translocated mountain lions Puma concolor survived over three months. Four out of seven translocated mountain lions survived at least 124–303 days after release. Individuals that had been in the wild >35 days established 96–930-km2 home ranges. However, during the hunting season, these home ranges were abandoned. At least three mountain lions died during the study, including one that was shot. In 1988, seven mountain lions were captured in Texas and flown to Florida. They were released as a trial for evaluating the feasibility of translocating Florida panthers Puma concolor coryi. Animals were sterilized, radio-collared and kept in holding pens for one week before release. They were monitored six days/week for 306 days from an airplane. Before translocation, the study area (>12,000 km2) had no mountain lions but had a high abundance of deer and wild hog and a low density of humans.
A controlled study in 1992 in woodland edge in Somerset, UK (Bright & Morris 1994) found that translocated common dormice Muscardinus avellanarius held in pens before release gained weight after release, whereas dormice released directly into the wild lost weight. The body mass of dormice released from pre-release pens increased after release by 0.12 g/day, whereas dormice released directly into the wild lost 0.14 g/day. The study was conducted along a 9-ha strip of trees and shrubs in August–September 1992. Six wild-caught dormice were placed in pre-release pens and 10 wild-caught dormice were released directly into the wild on their day of capture. Pre-release pens (0.45 m width, 0.5 m depth and 0.9 m height) were constructed from 1-cm2 weldmesh. Nest boxes, food and water were provided. Dormice stayed in pens for eight nights before release. Dormice were monitored by radio-tracking and were recaptured and weighed 10–14 days after release.
A replicated, controlled study in 1983–1993 in three grassland sites in Alberta, Canada (Carbyn et al. 1994) found that translocated and captive-bred swift foxes Vulpes velox released after time in holding pens had similar survival rates to those released without use of holding pens. No statistical analyses were performed. At least six out of 45 (13%) swift foxes held in pens before release survived over two years post-release, compared with at least five out of 43 (12%) released without use of holding pens. In 1983–1987, forty-five translocated swift foxes were held in pens before release. Pens (3.7 × 7.3 m) were fenced for protection from cattle. Animals were placed in pens in October–November and released between the following spring and autumn. They were provided with supplementary food for 1–8 months after release. In 1987–1991, four hundred and thirty-three foxes were released without use of holding pens. Released foxes included both wild-born and captive-bred animals. All foxes released from pens and 155 of those released directly were radio-tracked, from the ground or air, for up to two years.
A replicated study in 1995–1996 in two forest sites in Idaho and Wyoming, USA (Bangs & Fritts 1996) found that translocated gray wolves Canis lupus had similar survival rates and breeding success in the first two years after release when adult family groups were released together from holding pens or when young adults were released directly into the wild. No statistical analyses were conducted. Thirty out of 35 young adult wolves released directly into the wild were still alive seven months after the last releases, and had produced up to 40 pups from 3-8 pairs. Thirty-one adult wolves released from holding pens in family groups had produced 23 pups four months after the last releases. From these 54 animals, nine had died. Six of the seven adult pairs released together from holding pens remained together, and five of these pairs established territories in the vicinity of the pens. Wolves were wild-caught from Canada in January 1995 and 1996. In Idaho, young adults were directly released in January 1995 and 1996. In Wyoming, family groups of 2–6 wolves spent 8–9 weeks in 0.4-ha chain-link holding pens before release in March 1995 and April 1996. Wolves were radio-tracked every 1–3 weeks until August 1996.
A study in 1994-1998 in a savannah reserve in North West province, South Africa (Hofmeyr & van Dyk 1998) found that after release from holding pens in groups, approximately half of translocated cheetahs Acinonyx jubatus survived at least 18 months, of which half died within three years. Nine of 19 cheetahs survived 19-24 months, of which six were cubs that matured to independence, but only four cheetahs were known to still be alive at the end of the study period. Six cheetahs survived in the reserve less than one year, of which one died after a few weeks and two were removed to a captive breeding facility. The fate of four released cheetahs was unknown. In total 19 cheetahs were released into a game reserve between October 1994 and January 1998. Cheetahs were initially placed in 1 ha holding pens with electrified fencing for 4 weeks to several months. The feeding regime is not specified, but cheetahs were provided with at least one carcass on being placed in the pen and were lured from the pen with a carcass. Cheetahs were mostly rescued wild-caught animals, except for one that was habituated to humans (and had to be removed after two weeks). Cheetahs were either held in family groups (mothers with cubs) or as coalitions (of adult males). One animal/group was radio collared for monitoring.
A study in 1993–1994 on a forested peninsula in Dorset, UK (Kenward & Hodder 1998) found that none of the translocated red squirrels Sciurus vulgaris released into holding pens (with supplementary food, water and nestboxes) survived over five months after release. Out of 14 translocated red squirrels, 11 (79%) survived over one week. Only three (21%) survived >3 months and none survived >4.5 months. At least half of the 14 squirrels were killed by mammalian predators. When intact carcasses were examined they showed signs of weight loss and stress (see original paper for details). Between October and November 1993, fourteen wild-born red squirrels were released into an 80-ha forest dominated by Scots pine Pinus sylvestris. The forest had no red squirrels but had introduced grey squirrels Sciurus carolinensis. Capture and release sites were similar habitats. Squirrels were kept in 1.5 × 1.5 × 1.5 m weldmesh pens surrounded by electric fencing for 3–6 days before release. Squirrels were kept individually except for two males who shared a pen. After release, squirrels continued to have access to food, water and nest boxes inside the pens and outside (20-100 m away). All squirrels were radio-tagged and located 1–3 times/day, for 10–20 days after release and thereafter every 1–2 days.
A replicated study in 1995–1997 in four grassland sites in New Mexico, USA (Truett & Savage 1998) found that after release from holding pens and provision of supplementary food, translocated populations of black-tailed prairie dogs Cynomys ludovicianusi persisted at least two years and reproduced in the wild. The number of black-tailed prairie dogs approximately doubled during the first spring after release from holding pens in one site on one ranch where supplementary food was provided. Between the second spring and summer, after all supplementary feeding had ceased, the number of animals associated with both release sites on the same ranch doubled. Precise numbers are not reported. One hundred and one prairie dogs were translocated to two ranches (Armendaris Ranch received 71 individuals; Ladder Ranch: 30 individuals) between June 1995 and June 1997. At each ranch, prairie dogs were released into two 0.4-ha holding pens (number of individuals/holding pen is not provided). Holding pens were fenced and surrounded by electric wire. Animals at Armendaris ranch were provided with supplementary food in pens for up to year. Information on population persistence at Ladder Ranch is not provided. The time individuals were kept in the holding pens before subsequent release varied between a few days and weeks (see original paper for details).
A controlled study in 1997 in a mixed pasture and cultivated fields farmland site in northern France (Letty et al. 2000) found that keeping translocated European rabbits Oryctolagus cuniculus in holding pens for three days prior to release (and carrying out associated management such as supplementary feeding) increased survival rates of female, but not male rabbits immediately following release compared to rabbits released directly into the wild. During the first day after translocations, the survival rate of female rabbits released from pre-release pens was higher (100%) than that of females released directly into the wild (83%) and male rabbits released from release pens (78%). The survival rate of male rabbits released from pre-release pens (78%) was not significantly different to that of male rabbits released directly into the wild (92%). One hundred and four rabbits were translocated from Parc-du-Sausset to a 150-ha area of cultivated fields and pasture in Héric, approximately 400 km away in January 1997. Of these, roughly half were acclimatised in eight 100-m² enclosures (fence height: 1 m), for three days prior to release. Rabbits were provided with supplementary food. Survival was estimated by night-time relocation of ear-tagged rabbits using a spotlight, daily in the first week after release and twice a week until late February 1997.
A study in 1997–1998 on a savanna estate in Zimbabwe (Hoare & Williamson 2001) found that a translocated lion Panthera leo family kept in a holding pen prior to release joined with immigrant lions and formed a new pride. A lioness was translocated with three cubs (one male, two female). Within 45 days, seven male lions were close by and the female mated with one of these. The male cub moved away and the pride then entailed the female and daughters with two adult male lions. A wild lioness joined the pride 1.7 months after release, but was killed by a snare after six months. After 12–13 months, the original lioness had three new cubs and her daughters each also had litters. Resident lions on the estate were eliminated in 1995. In January 1997, a lioness and three cubs were translocated from communal land to a holding pen and were released on the estate after 90 days. Lions were monitored through to May 1998 by radio-tracking and direct observation.
A study in 1996–1999 at a woodland reserve in Queensland, Australia (Pople et al. 2001) found that wild-born translocated bridled nailtail wallabies Onychogalea fraenata kept in holding pens prior to release into areas where predators had been controlled had similar average annual survival to that of captive-bred animals. Over four years, the average annual survival of wild-born translocated wallabies (77–80%) did not differ significantly from that of captive-bred bridled nailtail wallabies (57–92%). In 1996–1998, nine wild-born translocated and 124 captive-bred bridled nailtail wallabies were released into three sites across Idalia National Park. Ten captive-bred wallabies were held in a 10-ha enclosure within the reserve for six months before release, and 85 were bred within the 10-ha enclosure. All of the 133 released wallabies were kept in a holding pen (30-m diameter) for one week at each site before release. Mammalian predators were culled at release sites. A total of 67 wallabies (58 captive-bred, nine wild-born) were radio-tagged and tracked every 2–7 days in 1996–1998. Wallabies were live-trapped at irregular intervals with 20–35 wire cage traps in 1997–1999.
A study in 1998–2002 in a shrubland wildlife reserve in Limpopo, South Africa (Kilian & Bothma 2003) found that after being held together in a pen for three months before release, five translocated African lions Panthera leo eventually formed two separate prides. Two months after release, there was aggression between two males and a female, which had sustained injuries shortly after release. Aggression continued intermittently for 10 weeks until the injured lion mated. Subsequently, over the following 3.5–4 years, two prides established territories. One pride comprised of a male and female half-siblings with an additional related female. The second pride was a looser association between a male and female sibling. Thus, inbreeding was likely to occur between mated pairs. Two male and three female wild-caught lions (from two locations) were released on 16 January 1998 into a 33,000-ha fenced reserve, after being held for three months in a 50 × 50-m pen. Lions were monitored by radio-tracking through to February 2002.
A study in 2000–2003 in a mixed karoo grassland reserve in Northern Cape Province, South Africa (Venter 2004) found that following release from a holding pen in groups into a fenced reserve, one out of two translocated Cape buffalo Syncerus caffer groups scattered and escaped the reserve while the other formed a single herd and stayed in the reserve and bred. One month after release, a group of four buffalo had split into two solitary animals and a pair formed by one male and one female. One of the solitary animals was not seen again, the second solitary male animal was located two years after release on a neighbouring farm and released into the second group of translocated animals in May 2003. The pair escaped the reserve three times in 13 months. After the third escape, the male was moved to a different reserve and a new male introduced to form a herd with the remaining female. A second group of 10 translocated animals formed a single herd (along with the two remaining animals from the previous introduction) and over 10 months no animals died or escaped. A year after the introduction, five calves were born. Four subadult buffalo (2 male, 2 female) were placed in a holding pen in July 2000 and released in August into a fenced 12,000-ha reserve. A second group of seven adult and three subadult animals (4 male, 6 female) was placed into a holding pen in August 2002 and released into a 200 ha area in September before being completely released in October 2002. Both groups were monitored weekly with telemetry until October 2003.
A study in 1989–2002 in 25 temperate forest sites in Montana, Idaho, and Wyoming, USA (Bradley et al. 2005) found that holding translocated wolves Canis lupus in pens at the release site before release (soft release) increased the chance of wolves not returning to their capture site relative to direct (hard) release. A lower proportion of soft-released wolves returned to their capture site (8%) than of hard-released wolves (30%). Soft-releases entailed confinement at release sites for ≥28 days after capture. Hard-releases were those occurring ≤7 days following capture. Eighty-eight wolves were translocated 74–515 km in 1989–2001 in response to livestock predation (75 wolves) or pre-emptively to avoid such conflict (13 wolves). Translocated wolves were radio-collared, and were monitored through to the end of 2002.
A study in 1995–2002 in a mixed oak forest reserve in the south of France (Calenge et al. 2005) found that following translocation using holding pens prior to release and associated actions, approximately half of female roe deer Capreolus capreolus survived over one year after release and that overall the deer population increased six years after the translocations began. Twenty-six out of 49 (53%) translocated female roe deer survived over one year post-release. Of the animals that died in the first year, 35% of mortality occurred within the first month after release. After six years the deer population had increased to 0.47 deer/km2 compared to 0.06 deer/km2 in the first year after translocation began. In February 1995–1997, fifty-two male and 52 female roe deer were translocated from Northern France into a 3,300-ha forest reserve in Southern France in seven release sessions. Animals were placed into enclosures in groups of approximately 15 individuals for 2-10 days and provided with food (pellets and fresh vegetables) during this time prior to release. Forty-nine females (21 <1 year old and 28 >1 year old) were radio-tagged and were located from a vehicle once or twice each week, over one year post-release. In addition, surveys were carried out on foot (6 transects, each 5-7 km long) eight times a year in February-March 1996-2002 to estimate population growth. Deer were present in low numbers prior to translocation.
A study in 2001–2005 in a mixed forest and farmland site in northern Italy (Balestrieri et al. 2006) found that just over half of translocated Eurasian badgers Meles meles released from holding pens (with supplementary food) in groups survived at least one month after release and one pair reproduced. Seven out of 12 badgers survived for 1–9 months, after which monitoring equipment stopped operating. One badger died almost immediately after release due to unknown causes. Two badgers escaped (one after the first month, the other after an unknown period). The fate of three other badgers was unknown. One pair of translocated animals reproduced in the wild 4 years after release. From March 2001 to May 2004, twelve badgers were captured at four sites in northern Italy. Badgers were fitted with radio-collars and transported 20-40 km to the release site where they were kept in a 350 m2 enclosure in a wooded area in their release groups (2001: 2 individuals, 2002: 4 individuals, 2003: 2 individuals; 2004: 4 individuals) and provided supplementary food for 3–10 weeks before release. Seven of the 12 badgers were located once/week, for up to nine months after release.
A review of a study in 2001–2002 at a restored wetland in London, UK (Mathews et al. 2006) found that using holding pens prior to release of translocated and captive-bred water voles Arvicola terrestris resulted in greater post-release survival than did releasing them directly into the wild. Voles released from pens were three times more likely to be recorded during the initial follow-up survey than were those released without use of pens (result presented as odds ratio). A total of 38 wild-caught and 109 captive-bred water voles were released in groups of 6–15 animals in May–July 2001. Prior to release, no water voles were present at the site. An unspecified number of animals were placed in an enclosure with food and shelter and allowed to burrow out at will. The remainder were released directly into the wild. Animals were monitored by live-trapping over three periods of five days, between autumn 2001 and early-summer 2002.
A controlled study in 1983–2002 in a temperate forest in Vosges massif, France (Vandel et al. 2006) found that survival of translocated Eurasian lynx Lynx lynx that were held in captivity before release was similar between animals kept in holding pens at the release site and animals which were released directly. Four of eight animals which were kept in enclosures at the release site prior to release survived for 10–11 years, compared to six of 13 animals that survived 2–7 years after being released without holding pens. The distribution of lynx increased from 1,870 km2 (six years after the first releases) to 3,160 km2 (12 years later). At least two females, both of which were released without holding pens, produced litters. In 1983–1993, twenty-one adult lynx were brought to France from European zoos. The program sought wild-caught lynx for releases, however the exact origin of each animal, and the length of time that each spent in captivity, are unclear. Lynx were released at four sites in the Vosges mountains. The first eight animals were held in cages at the release site for 4–45 days prior to release, but the remainder were released immediately upon arrival. Animals were radio-tracked for 1–847 days. The presence of lynx was also established through sightings, lynx footprints, detection of faeces or hair and reports of attacks on domestic animals.
A study in 1992–2004 in a grassland reserve in KwaZuluNatal Province, South Africa (Hunter et al. 2007) found that most translocated lions Panthera leo held in pens before release survived for more than one year and established stable home ranges and that the population grew. Of 15 lions released, all except three, which were removed after killing a tourist, survived ≥398 days post-release. Average post-release survival was ≥1,212 days. At least 95 cubs from 25 litters were documented among translocated lions and descendants over the 13-year study. Excluding cubs translocated to other sites or those still <18 months old at the end of the study, 51 of 65 cubs (78%) survived past 18 months old. Nine lions were released in May 1992, six in February 1993 and two in January 2003. Releases were into a fenced reserve (initially 176 km2, then extended to 210 km2). Before release, lions were held in groups, each in an 80-m2 acclimation pen, for 6-8 weeks, during which time socialization occurred and stable prides were formed. Eleven of the founder lions were monitored by radio-tracking and other animals were monitored by direct observations.
A replicated study in 1998–2004 within four largely forested areas in Ontario, Canada (Rosatte et al. 2007) found that following translocation elk Cervus canadensis, most of which had been kept in holding pens in groups, remained present at all release sites and numbers had increased at two of four sites. By 3–6 years after translocations, elk populations had grown at two sites and fallen at two. From 443 elk translocated, the population at the end of the study was estimated at 375–440 animals. Between 1998 and 2004, forty-one percent of translocated elk died. Causes of death included 10% lost to wolf predation, 5% to emaciation and 5% to being shot. Elk were translocated from a site in Alberta, Canada in 1998–2001 in nine releases. Transportation took 24–58 hours. Elk were held in pens at recipient sites for up to 16 weeks before release (some were released immediately) but the effect of holding pens was not tested. Of 443 elk released, 416 were monitored by radio-tracking. The overall population was estimated in March 2004.
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 that spent more time in holding pens in groups had a higher survival rate after release. Wild dog families that had more time to socialise in holding pens prior to release into fenced areas had a higher survival rate than groups which spent less time in holding pens (data presented as model results). Overall, 85% of released animals and their wild-born offspring survived the first six months after release/birth. Released animals that survived their first year had a high survival rate 12–18 months (91%) and 18–24 months (92%) after release. 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. Individuals were kept in pre-release pens for an average of 212 days, but groups were given between 15 and 634 days to socialise in pens prior to release. 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.
A replicated, controlled study in 1999–2004 in three mixed savanna and woodland sites in KwaZulu-Natal, South Africa (Trinkel et al. 2008) found that after translocation to a fenced reserve with holding pens, survival of released lions Panthera leo was higher than that of resident lions, and that translocated animals reproduced successfully. No statistical tests were performed. After five years, a higher proportion of introduced animals survived (eight of 16 animals, 50%) than of resident animals (20 of 84 animals, 24%). Seven translocated females reproduced successfully. Between August 1999 and January 2001, sixteen lions were translocated to an enclosed reserve to improve genetic diversity. They were held at release sites in 0.5–1.0-ha pens for 4–6 weeks before release. Nine translocated lions were fitted with radio-collars. From August 1999 to December 2004, translocated animals were located at least every 10 days. Resident lions were also tracked at least every 10 days.
A study in 2005–2007 in a mixed conifer forest in Galway, Ireland (Poole & Lawton 2009) found that following release from holding pens (with nest boxes and supplementary food), over half of translocated red squirrels Sciurus vulgaris survived over eight months after release and most females reproduced during that period. At least 10 out of 19 (53%) translocated squirrels survived over eight months post-release and five out of nine translocated females (56%) were lactating five-seven months after release. In August 2006, seven juvenile squirrels were caught. At least one squirrel was still alive in the release location in two years after the original release. Two squirrels died while in the release pen or shortly afterwards. Another four squirrels died 1-2 months after release. Nineteen squirrels were translocated to a nature reserve (19 ha) in the middle of a 789-ha commercial pine plantation, 112 km from the capture site. Individuals were marked and radio-tagged. Squirrels were kept on average for 46 days in one of two pre-release enclosures (3.6 × 3.6 × 3.9 m high). Enclosures contained branches, platforms, nest boxes, and supplementary feeders (containing nuts, maize, seeds and fruit). Supplementary food (50/50 peanut/maize mix) was provided in six feeders in the nature reserve until July 2006. Twenty nest boxes were also provided Squirrels were radio-tracked in September and November 2005 and February and May 2006, and were trapped in February, May and August 2006 and observed once in October 2007.
A study in 2007 at rocky outcrops on a reserve in KwaZulu-Natal Province, South Africa (Wimberger et al. 2009) found that all translocated rock hyraxes Procavia capensis kept in a holding pen and released as a group died (or were presumed to have died) within 18 days of release. Eight of nine wild translocated hyraxes died within 18 days of release and the other was presumed to have died. The group split up and were not seen together after release. In October 2007, nine hyraxes (one juvenile, three sub-adults and five adults) were caught in baited mammal traps (900 × 310 × 320 mm) in an area where they were abundant, and moved 150 km to a 656-ha reserve where the species was nearly extinct. Hyraxes were kept together in a holding cage (1850 × 1,850 × 1850 mm) for 14 days before release. They were monitored daily for one week, and then every few days by direct observation and radio-tracking.
A replicated, randomized, controlled study published in 2010 of a grassland site in Andalucía, Spain (Rouco et al. 2010) found that holding translocated wild European rabbits Oryctolagus cuniculus for longer in acclimation pens before release improved subsequent survival rates. A lower proportion of rabbits enclosed for six nights before release was killed by mammalian predators over the following 10 days (9%) than of rabbits enclosed for three nights before release (38%). Rabbits were translocated to a 4-ha grass field with artificial warrens. Food and water were provided. Of 181 rabbits released (average 10/warren), 38 randomly selected rabbits (2–5 in each of 15 warrens) were radio-collared. Twenty-three of these were released on the seventh day, following six nights of confinement and 14 were released on the fourth day, following three nights of confinement. The date of the study is not stated. Rabbits were monitored daily during confinement and for 10 days following release.
A study in 2008–2009 in a subtropical forest in Rajasthan, India (Sankar et al. 2010) found that three translocated tigers Panthera tigris tigris that were kept in holding pens prior to release survived for at least 3–11 months after release and established home ranges. The annual home range of a released male was 169 km2 and that of a female was 181 km2. The summer home range of a later released female was 223 km2. Home ranges overlapped by 54–99 km2. Mating was observed between the male and each female. Of 115 recorded kills by tigers, 12 were of domestic animals. Thirty-two villages were located within the 881-km2 reserve. Tigers had been absent since 2004. One male and one female wild-caught tiger were released on 6 and 8 July 2008, respectively. A further female was released on 27 February 2009. Tigers were held in 1-ha enclosures at release sites for 2–8 days before release. They were satellite- and radio-tracked from release until June 2009.
A study in 1998-2010 in a desert site in South Australia (Moseby et al. 2011) found that after being kept in a holding pen, all four mammal populations released into an invasive-species-free fenced enclosure survived and bred. After being kept in a holding pen prior to 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 range within five years and produced a second generation within two years. In 1998–2005, eight wild-born greater stick-nest rats, 10 wild-born burrowing bettongs, 12 wild-born western barred bandicoots and nine captive-bred greater bilbies were translocated into a 14-km2 invasive-species-free fenced area. Rabbits, cats and foxes were eradicated within the fenced area in 1999. Animals were kept in a 10-ha holding pen before full release after a few months. Between 2000 and 2010, tracks were surveyed annually along eight 1 km × 1 m transects.
A replicated, controlled study in 2008–2009 of grassland at two sites in Arizona, USA (Nelson & Theimer 2012) found that following translocation of Gunnison's prairie dogs Cynomys gunnisoni into burrows that were topped with acclimation cages for one week, survival was not greater than that of prairie dogs released into uncaged burrows. Among prairie dogs whose identity could be established in the second year, 10% of both those released into borrows topped with acclimation cages and those released into uncaged burrows survived for at least one year. Additionally, pups were seen at both sites a year after release (39 and 37 pups at the two sites). No definite immigrants to the recipient colonies were recorded. Prairie dogs were trapped from 7 July to 5 August 2008 at one urban and one suburban site (74 and 75 prairie dogs, respectively) and moved approximately 50 km to two abandoned colonies (6 km apart) in a rural area. Approximately half at each colony was released directly into open burrows and half into borrows topped, for one week, with acclimation cages. Survival monitoring, from 10 June to 25 August 2009, entailed live-trapping, PIT-tag reading and direct observations.
A replicated, before-and-after study in 2008–2012 in 32 shrubland sites in Andalusia, Spain (Guerrero-Casado et al. 2013) found that following release from holding pens with artificial warrens to boost a local population, translocated European rabbit Oryctolagus cuniculus abundance was higher after three years. Rabbit abundance was around nine-fold higher three years after translocations (9.3 latrines/km) than before translocation (1.0 latrines/km). In autumn and winter of 2008–2009, between 75 and 90 rabbits/ha were released into artificial warrens located in 32 electric-fenced 0.5-7.7 ha plots (fencing was 0.5 m below ground and 1.7 m above ground). At the end of the 2009–2011 breeding season, small gates on the fences were opened and the rabbits were allowed to disperse into adjacent areas. Rabbit abundance was estimated by latrine counts along four 500-m transects (128 total transects) around each plot, in the summers of 2008–2009 before gates were opened and in 2012 after gates were opened. Wooden branches and artificial warrens were added within a 500-m radius of some plots and, in some, scrub was cleared to create pasture.
A controlled study in 2002–2007 on a large area of prairie in South Dakota, USA (Sasmal et al. 2015) found that using holding pens at release sites affected survival rates of translocated swift foxes Vulpes velox. A higher proportion of foxes released after 14–21 days in holding pens survived for ≥60 days post-release (76%) than of foxes held in pens for >250 days (66%) or released after 14–21 days in kennels at a field station (61%). A total of 179 foxes (85 males and 94 females; 91 adults and 88 sub-adults) were translocated in 2002–2007. Holding pens provided acclimatisation at release sites, with food provided at pens following release. Foxes released from short stays in holding pens, and those released having been held in kennels, were released in August–October. Long-stay foxes were released in mid-July. Survival was monitored by radio-tracking and visual observations at dens.
A controlled, before-and-after study in 1989–1992 on a hilly grassland and scrubland site in California, USA (Scrivner et al. 2016) found that the survival of translocated San Joaquin kit foxes Vulpes macrotis mutica kept in holding pens in pairs prior to release was lower than that of resident animals, but did not change with the length of time in holding pens. The survival of 40 translocated foxes in the first year after release (six alive, 32 dead, two unknown) was lower than that of 26 resident foxes (13 alive, 13 dead), but did not change with the length of time spent in holding pens. Eleven pups born in the holding pens and released with their parents all died within 17 days of release. Only four foxes were known to breed after release, all with resident foxes. At the end of the study (in 1992) one fox was known to be alive and 36 (out of 40) were known to have died. Causes of death were predation (20 foxes), road accidents (two foxes) and death during trapping operations (one fox). The cause of death was unknown for 13 foxes. In August and December 1988 and January 1989, and from June–October 1989, foxes were caught and translocated up to 50 km to a 19,120-ha reserve. Foxes were kept in male–female pairs in holding pens (6.1 × 3.1–6.1 × 1.8 m) for 32–354 days before release in spring and summer 1990 (12 adults, 1 pup) and 1991 (28 adults, 10 pups). Foxes were monitored by radio-tracking 4–5 days/week after release.
A study in 2015–2016 in a wooded mountain region in central Wales, UK (MacPherson 2017) found that some translocated pine martens Martes martes held in pre-release pens and then provided with supplementary food and nest boxes survived and bred in the first year after release. At least four out of 10 females that had been kept in pre-release pens survived and bred the year after release. Around 10–12 months after release, 14 out of 20 martens were alive and in good condition. Twelve were within 10 km of their release site. Six martens died in the first year, two had a fungal infection two weeks after release. Authors suggest this may have been due to damp conditions in November. From September–November 2015, twenty breeding age (>3-years-old) pine martens were caught in Scotland, health checked, microchipped and fitted with a radio-collar, and in some cases a GPS logger. Martens were transported overnight to Wales, and held in individual pre-release pens (3.6 × 2.3 × 2 m) for up to seven nights. Males’ pens were within 500 m of a female, but >2 km from the nearest male. Releases took place in autumn, and supplementary food was provided for 2–6 weeks after release (for as long as it continued to be taken). Den boxes were provided within 50 m of each release pen. Martens were radio-tracked until home-ranges were established, then located daily–weekly. Intensive tracking of females was carried out in March to locate breeding sites. Hair tubes and camera traps were used to monitor breeding success. A further 19 martens were released using the same procedure in September–October 2016.
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