Hand-rear orphaned or abandoned marine and freshwater mammal young
Overall effectiveness category Trade-off between benefit and harms
Number of studies: 12
Background information and definitions
Young marine and freshwater mammals believed to be orphaned or abandoned are sometimes taken into captivity by wildlife rehabilitators, to be reared and released back into the wild. Often this is done more for animal welfare reasons than for species conservation, although for rare species, release of such animals may provide opportunities to augment wild populations. The success of such programmes can be difficult to judge without long-term survival data or benchmark data for survival of wild-reared mammals.
See also Reunite abandoned marine and freshwater mammal young with parents and Place orphaned or abandoned marine and freshwater mammal young with foster parents. For studies that hand-reared captive-born mammals, see Breed marine and freshwater mammals in captivity. For studies that brought mammal young into captivity for rehabilitation (as opposed to hand-rearing), see Rehabilitate and release injured, sick or weak marine and freshwater mammals.
Supporting evidence from individual studies
A study in 1991–1998 of a pelagic area in the North Atlantic Ocean off the coast of Puerto Rico, USA (Mignucci-Giannoni 1998) found that one of three West Indian manatee Trichechus manatus calves reared in captivity was released back into the wild and survived for at least four years with supplemental feeding. One stranded male manatee calf was released back into the wild after 27 months in captivity. The calf survived for at least four years in the wild and was observed feeding, visiting freshwater sites and interacting with wild manatees. Supplemental food was periodically provided from two years after release when the calf was observed to be underweight. The other two calves (one female, one male) died in captivity (after two weeks and 20 months respectively). The three calves (102–122 cm in length) were found stranded at coastal sites in 1991, 1993 and 1995. They were taken to rehabilitation facilities, housed in saltwater pools and given medical treatment. The surviving calf was fitted with a satellite tag and released in a protected bay used by wild manatees after a six-month period of adaptation in an enclosed sea-pen. The calf was tracked and sighted for four years after release in 1994–1998.Study and other actions tested
A study in 1997–1998 at an aquarium in San Diego, USA (Bruehler et al. 2001) found that an orphaned California gray whale Eschrichtius robustus calf reared in captivity survived for over 14 months and increased in body weight and length. Between September 1997 and March 1998, the whale calf increased in body weight (4,800–8,200 kg) and length (7.5–9.2 m). The female calf was brought into captivity in September 1997 and fed warm water and dextrose via a stomach tube followed by an artificial milk formula every 2 h for the first three days. This was replaced with a mixture of herring Clupea spp., milk formula, amino acid supplements, water and cream, which the calf suckled through a tube during seven feeding sessions/day. After 7–8 months, the calf was weaned onto solid food (small fish, squid and krill) fed at least four times/day. The calf was kept in a holding pool (9.1 m deep) and enrichment was provided (kelp and marine invertebrates).Study and other actions tested
A study in 1997–1998 of a pelagic area in the North Pacific Ocean, off the coast of San Diego, USA (Stewart et al. 2001) found that a gray whale Eschrichtius robustus calf reared in captivity and released back into the wild survived for at least three days. The rehabilitated whale calf was successfully tracked for three days after release and was observed swimming strongly before the satellite transmitter became detached. The female calf was found stranded in 1997 and taken to a rehabilitation facility where she was given formula and weaned onto fish and invertebrates. After 14 months in captivity, the calf was satellite-tagged and released several kilometres offshore on 31 March 1998. The calf was tracked and observed from a boat for three days after release before the satellite transmitter dislodged and was found washed ashore.Study and other actions tested
A controlled study in 1995–1996 and 1998 at a beach in the North Pacific Ocean, California, USA (Lander et al. 2002) found that Pacific harbour seal Phoca vitulina richardsi pups reared in captivity and released back into the wild had similar survival estimates and diving behaviour to wild pups. Overall, survival estimates did not differ significantly between captive-reared seal pups and wild seal pups during the first 15 weeks after release for (data reported as statistical model results). Captive-reared and wild pups also dived for similar durations (average 1.2 vs 1.3 minutes respectively) and surfaced at similar intervals (average 0.4 minutes for both). Twenty-nine stranded seal pups were taken to a rehabilitation facility during March–May 1995, 1996 and 1998. They were treated with antibiotics, fed milk formula, and weaned onto herring Clupea spp. The 29 pups were radio-tagged and released in pairs/groups of three at a beach in 1995, 1996 and 1998 once they had reached a weight of at least 20 kg and had suitable behaviour. Twenty-four newly weaned, wild Pacific harbour seal pups were captured in 1995, 1996 and 1998 at three locations along the same coast and fitted with radio-tags. Each of 53 pups was radio-tracked for 3–5 months and dive behaviour monitored for 9–24 h (captive-reared pups) or 15–22 h (wild pups) after release during 1995, 1996 or 1998.Study and other actions tested
A replicated, controlled study in 1995–1996 and 1999–2000 at two islands in the North Pacific Ocean, off the coast of California, USA (Lander et al. 2003) found that three Steller sea lion Eumetopias juba pups reared in captivity and released back into the wild survived for at least 1–3 months and had similar diving behaviour to wild sea lions. The three captive-reared sea lion pups were successfully tracked for 1–3 months after release back into the wild. All three pups dived to similar average depths (18–30 m) and for similar average durations (0.9–1.6 minutes) compared to 25 wild juvenile Steller sea lions (depth: 18 m; duration: 1.1 minutes) although statistical significance was not assessed. Three Steller sea lion pups (aged 2 weeks old) found stranded, dehydrated and underweight were taken to a rehabilitation facility in June 1995 (two males) and June 1999 (one female). The pups were fed formula and weaned onto fish at 3 months old. After 10 months in captivity, the pups were tagged and fitted with satellite time-depth recorders and released at sea near two islands in April 1996 (two sea lions) and April 2000 (one sea lion). Each of three sea lions was tracked for 1–3 months after release in 1996 and 2000. Data for the 25 wild sea lions were from a previous study.Study and other actions tested
A study over nine years (dates not stated) at a rehabilitation facility in Florida, USA (Manire et al. 2004) found that five orphaned pygmy and dwarf sperm whale Kogia spp. calves reared in captivity increased in body weight but died after 3–20 months. Four captive-reared pygmy sperm whales Kogia breviceps increased in body weight from 33–57 kg to 60–232 kg but died after 91–631 days in captivity due to intestinal problems (three calves) or liver failure (one calf). One captive-reared dwarf sperm whale Kogia sima increased in body weight from 27–75 kg but died after 465 days in captivity due to an impacted colon. Each of the five calves was found stranded, transported to a rehabilitation facility and treated for dehydration and constipation. The calves were fed artificial formula mixed with electrolytes through a stomach tube. Squid was fed from six months of age. Ulcers were treated with antibiotics and anti-fungal drugs. The calves were considered unsuitable for release due to their inexperience in the wild.Study and other actions tested
A study in 2003 in an estuary in the Indian River Lagoon, Florida, USA (Mazzoil et al. 2008) found that an orphaned common bottlenose dolphin Tursiops truncatus calf reared in captivity and released back into the wild survived for at least seven days. The orphaned male calf was successfully tracked for seven days after release before contact was lost with his transmitter. During this time, the calf remained within 10 km of the release site and was observed foraging and interacting with other dolphins. The orphaned calf (one year old) was found stranded, underweight and dehydrated in August 2003 and transported to a rehabilitation facility. He was treated with antibiotics and provided with appropriate nutrition. After three months in captivity, the calf was radio-tagged and held in a temporary enclosure (7 x 12 x 2 m) within the estuary for 1 h before release. The calf was tracked daily for seven days in October 2003. Attempts to locate the calf were made for a further 10 days after contact was lost, including multiple vessel and aerial surveys.Study and other actions tested
Referenced paperMazzoil M.S., McCulloch S.D., Youngbluth M.J., Kilpatrick D.S., Murdoch M.E., Mase-Guthrie B., Odell D.K. & Bossart G.D. (2008) Radio-tracking and survivorship of two rehabilitated bottlenose dolphins (Tursiops truncatus) in the Indian River Lagoon, Florida. Aquatic Mammals, 34, 54-64.
A study in 2006–2007 on an atoll in the North Pacific Ocean, Hawaii, USA (Norris et al. 2011) found that most Hawaiian monk seal Neomonachus schauinslandi pups reared in captivity gained weight, but none survived after release back into the wild. Six of seven seal pups reared in captivity increased in body weight by 31–141% but died within 3–5 months after release back into the wild, aged <2 years old. The other seal pup lost body weight and died after 23 days in captivity. Two of three wild seal pups born on the same atoll in the same breeding season survived to at least four years of age. In May–December 2006, seven female juvenile Hawaiian monk seal pups were captured in the wild and kept in shoreline net pens (9 x 40 m) to increase their survival over winter. The pups were given multivitamins and fed frozen Pacific herring Clupea pallasii 2–3 times/day and larger live reef fish. In March 2007, the six surviving seal pups were radio-tagged and released after 89–279 days in captivity. Three wild seal pups (two males, one female) from the same atoll were radio-tagged in March 2007. Tagged seals were tracked for 37–146 days (released pups) or 74–311 days (wild pups) in 2007–2008. Visual sightings were made during annual surveys in 2007–2010 (methods not reported).Study and other actions tested
A controlled study in 2010–2011 on an island in the Salish Sea, San Juan County, USA (Gaydos et al. 2013) found that harbour seal Phoca vitulina richardii pups reared in captivity and released back into the wild travelled greater distances and further from the release site than wild pups born at the same site and in the same season. On average, captive-reared seal pups travelled greater total distances (562 km), greater daily distances (7.5 km/day) and further from the release site (212 km) than wild pups (total 309 km; 2.6 km/day; 65 km from the site). Ten stranded seal pups that were rescued (at 3–8 days old) and captive-reared were fitted with satellite and radio tags and released at a seal weaning site on an island in September–October 2010 (at an average age of 81 days old). Ten wild seal pups (estimated to be 33 days old) were captured at the same site in August 2010 and fitted with identical tags. Tracking was carried out for an average of 77 days (captive-reared pups) or 133 days (wild pups) in 2010–2011.Study and other actions tested
A study in 2003–2009 at a coastal site in Guerrero Lagoon in Quintana Roo, Mexico (Mercadillo-Elguero et al. 2015) found that an orphaned Antillean manatee Trichechus manatus manatus calf reared in captivity and released back into the wild was unable to survive on its own and had to be returned to captivity. Five months after release, the male manatee calf (aged 2.5 years) had lost 33% of his body weight (30 kg) and had a skin condition (hyperkeratosis). An earlier release attempt also failed. The calf was returned to semi-captivity, in which food was provided (fruit and vegetables) and the calf could move freely between a captive facility and the wild. In 2009, the manatee (aged 6 years) was reported to be dependent on human care. The manatee calf was rescued in September 2003 and reared for eight months in a plastic pool. The calf was then transferred to an enclosure within a lagoon inhabited by wild manatees. Release was attempted in July 2005, but the manatee followed people and returned to the enclosure. The manatee was finally released in September 2005 before being returned to captivity in February 2006. The manatee was monitored in 2005–2009. Behavioural observations were carried out in 2008–2009.Study and other actions tested
A replicated study in 1994–2012 at three coastal sites in the South Atlantic Ocean, northeast Brazil (Normande et al. 2015) found that more than three-quarters of orphaned Antillean manatees Trichechus manatus manatus reared in captivity and released back into the wild survived for at least one year, and most manatees monitored for longer periods reproduced. Twenty-one of 26 orphaned, captive-reared manatees (81%) survived for at least one year in the wild, although five had to be rescued and re-released. Four males and two of three females monitored for an average of seven years bred with wild or released manatees. The other five captive-reared manatees died in the first year after release or had to be returned permanently to captivity. One captive-reared manatee died before release. Twenty-seven stranded manatee calves (16 males, 11 females) were rescued and reared in captivity. They were kept in pools and fed soya milk compound, algae and sea grass, supplemented with vegetables and vitamins. After 1–17 years, 26 manatees were fitted with satellite tags and released at three sites within marine protected areas between 1994 and 2012. Manatees were kept in enclosures at release sites for 15 days or 3–12 months prior to release. Released manatees were tracked for an average of 972 days. Seven of the 26 released manatees (four males, three females) were tracked and observed for an average of seven years.Study and other actions tested
A replicated study in 1988–2013 at multiple freshwater, marine and brackish water sites in Florida, USA (Adimey et al. 2016) found that 24 of 40 (60%) orphaned Florida manatee Trichechus manatus latirostris calves reared in captivity and released back into the wild survived for at least one year. Twenty-four of 40 orphaned, captive-reared manatee calves survived for at least one year in the wild after release, occupied appropriate habitats, did not require additional rescue and were in good condition. The other 16 manatee calves required intervention or died within the first year (number for each not reported). All of 40 manatees were rescued as calves (<235 cm in length) and kept in captivity for between <1 and >10 years before release back into the wild. Release sites were warm freshwater, marine or brackish water near rescue locations or alternative locations used by wild manatees (number of sites for each not reported). Each of 40 released manatees was monitored with radio-tracking and visual observations once or twice/week for at least one year in 1988–2013.Study and other actions tested