Rescue and release stranded or trapped marine and freshwater mammals
Overall effectiveness category Beneficial
Number of studies: 11
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Background information and definitions
Marine and freshwater mammals that are found stranded or trapped without significant injury or impairment may be rescued 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 rescuing and releasing mammals can be difficult to judge without long-term survival data. It is also important to note that most studies summarised below have small sample sizes, and that unsuccessful attempts are unlikely to have been reported.
Evidence is summarised below for studies that released trapped or stranded mammals immediately. For studies that rescued and rehabilitated mammals in captivity prior to release, see Rehabilitate and release injured, sick or weak marine and freshwater mammals.
Supporting evidence from individual studies
A replicated study in 1990–1999 at multiple pelagic sites in the North Atlantic Ocean, Cape Cod Bay, USA (Wiley et al. 2001) found that all stranded and rescued Atlantic white-sided dolphins Lagenorhynchus acutus and most short-beaked common dolphins Delphinus delphis and long-finned pilot whales Globicephala melas were successfully released and did not re-strand. All 16 white-sided dolphins, six of eight (75%) common dolphins, and 38 of 53 (72%) pilot whales were successfully released and were not found re-stranded. One common dolphin and three pilot whales died during transport to release sites. One common dolphin and 12 pilot whales re-stranded and died after release. The 77 dolphins and whales were rescued after mass stranding events in 1990–1999. They were either released at stranding sites or transported to sites up to 40 km away. Most were released within 3–10 h of stranding. All animals were individually marked with tags prior to release. Re-stranded animals were reported by members of the public and national stranding records were searched.Study and other actions tested
A study in 2001 at a pelagic site in the South Atlantic Ocean, near Salvador, Brazil (Bastos at al. 2002) found that a trapped rough-toothed dolphin Steno bredanensis was successfully rescued and released. An adult female rough-toothed dolphin found trapped between wooden house stilts was successfully rescued and released in the open sea. The dolphin was observed swimming alongside the boat after release, although long-term survival was not reported. On 29 October 2001, the dolphin (2.3 m long) was found trapped in a suburban area within a bay. The next day after 4 h of observation, a nylon net (150 m long x 3 m deep, 0.4 mm nylon wires, 2 cm mesh size) and a silk cable net with foam floats attached (5 x 5 x 5 m, 8 mm silk cables, 10 cm mesh size) were dragged by small boats to encircle and capture the dolphin. Local fishers helped with the rescue, which took 3 h. The dolphin was treated with Dexametason, Diazepam, and Enrofloxacin just after capture to reduce the effects of stress. The dolphin was covered with wet white cloths and transported by motorboat to waters adjacent to a beach at a museum. She was contained in the silk net and treated with an anti-parasitic drug (Ivermectin). The dolphin was monitored for 1 h before being released at sea, one nautical mile offshore.Study and other actions tested
A study in 2003 in the Cachoeira River estuary, southern Bahia, Brazil (Batista et al. 2005) found that two stranded tucuxi dolphins Sotalia fluviatilis were successfully rescued and released. Two young, male tucuxi dolphins found trapped in a natural pool were successfully rescued and released in deep water nearby. Both dolphins were observed feeding within an hour of release and were not found re-stranded, although long-term survival was not reported. The dolphins were trapped within a natural pool (7 m deep x 50 m diameter) along a river for nine days. Nylon nets (120 m long x 6 m deep with 80 mm mesh) dragged by canoes were used to capture the dolphins in March 2003. Both dolphins were injected with a drug to prevent shock (4 mg of Dexamethasone), transported individually to the release site by a motorboat, and observed for 1 h after release.Study and other actions tested
A study in 2007 in an inlet of the Indian Ocean, west Tasmania (Thalmann et al. 2008) found that seven stranded sperm whales Physeter macrocephalus were successfully rescued and released. Seven male sperm whales found stranded in shallow water were successfully rescued and released back into the sea within 1–4 days of stranding. All seven whales were observed swimming away after release, although long-term survival was not reported. Five other sperm whales that stranded on the same day died before rescue could be attempted. On 7 March 2007, seven whales (11–15 m long) were found stranded in shallow water (approximately 1 m deep) separated from a deep channel by a sand bar. Wet sheets were placed over the whales and respiration rates monitored during rescue. Two of the seven whales were moved back into deep water using jet wash propulsion. The other five whales were re-floated and released using a modified panel-shaped net (20 m long x 4.5 m deep) towed between two vessels. Vessel engine noises and acoustic devices were used to deter released whales from the stranding site and direct them out to sea.Study and other actions tested
A study in 2008–2009 on an island off the coast of South Africa (Hofmeyr et al. 2011) found that at least nine of 52 stranded Cape fur seal Arctocephalus pusillus pusillus pups that were rescued and released survived for at least three months. At least nine of 52 (17%) rescued and tagged seal pups were observed alive three months after release. The authors estimated a survival rate of 23% to account for the potential loss of tags. A total of 200 seal pups (aged several days to five weeks old) were stranded on the mainland after a severe storm and taken to rehabilitation facilities in December 2008. Thirty-one seal pups died in captivity and 169 pups were released back into the wild within 1–5 days at a seal colony on a nearby island. Fifty-two of the 169 released pups were fitted with tags that were appropriate for post-release monitoring. The release site was visited on five occasions in January–April 2009. Tagged seals were identified and observed from a vessel 1–5 m from the shore.Study and other actions tested
A replicated study in 2005–2011 of multiple pelagic sites in the North Atlantic Ocean, near Cape Cod, USA (Sampson et al. 2012) found that all stranded Atlantic white-sided dolphins Lagenorhynchus acutus and a third of stranded short-beaked common dolphins Delphinus delphis that were rescued and released survived for at least 1–7 months. All of eight Atlantic white-sided dolphins survived for at least 33–218 days after release. One of three short-beaked common dolphins survived for at least 65 days after release. Contact was lost with the two other common dolphins (including one juvenile) 8 h and 9 days after release, and it was not known if they died or satellite-tags failed. Eight Atlantic white-sided dolphins and three short-beaked common dolphins were rescued, satellite-tagged and released during seven mass stranding events in 2005–2010. Stranded dolphins were kept moist, shaded and comfortable. Behavioural observations, physical examinations and blood tests were carried out prior to release. The dolphins were released singly or in pairs at various locations on the same day as stranding. The 11 released dolphins were tracked for between 8 h and 218 days in 2005–2011.Study and other actions tested
A review of seven case studies in 2006–2010 in the North Atlantic Ocean, USA (Wells et al. 2013) found that five of seven trapped common bottlenose dolphins Tursiops truncatus that were rescued and released survived for at least 1–3 years. Five of seven rescued dolphins were successfully tracked for 365–1,040 days after release. Two other trapped and rescued dolphins were tracked for <1–2 days after release. One stranded and died, the other was considered unlikely to have survived. The dolphins (three males, three females, one unknown) were found trapped out of their natural habitats in 2006, 2007 and 2010. They were rescued, treated, transported to appropriate habitats, and released immediately. All seven dolphins were radio-tracked after release. Details of monitoring methods were not reported. Data were from published and unpublished studies.Study and other actions tested
A study in 2011 of a pelagic area in the North Atlantic Ocean, off the coast of Florida, USA (Wells et al. 2013) found that one of two stranded short-finned pilot whales Globicephala macrorhynchus that was rescued and released survived for at least two months. One of two stranded whales survived for at least 67 days after release. The whale occupied appropriate habitats (warm waters in high relief areas) and had dive depths (maximum average 1,000–1,500 m) and durations (99% of dives <30 minutes) within or greater than reported ranges for the species. Contact was lost with the other whale 16 days after release. A sudden decline in travel rates and dive depths suggested the whale died. The two adult male whales were released following a mass stranding event in May 2011. Both were considered healthy following assessment of body condition, behaviour and blood samples. The whales were satellite-tagged and released together 16 km offshore within two days of stranding. They were tracked to 118–319 locations during 16–67 days in 2011.Study and other actions tested
A review of interventions in 1980–2012 for Hawaiian monk seals Monachus schauinslandi in the North Pacific Ocean, Hawaii, USA (Harting et al. 2014) found that rescuing and releasing stranded or trapped seals, along with at least seven other interventions to enhance survival, resulted in 139 of 532 (26%) seals surviving and reproducing. The study did not distinguish between the effects of rescuing stranded or trapped seals and the other interventions carried out. The 139 surviving seals (including 71 females) produced at least 147 pups, which also went on to reproduce (15 pups). In 2012, the number of surviving seals and their offspring were estimated to make up 17–24% of the seal population (198–271 of 1,153 seals). In 1980–2012, a total of 885 intervention events of seven types were carried out: rescue of stranded or trapped seals (37 events), translocation (284 events); removal of derelict fishing gear from seals (275 events); pups reunited with mothers (113 events); umbilical cord removed or other medical treatment (84 events); other actions, such as deterring aggressive male seals (120 events). Field biologists monitored the seal population in 1980–2012. Data were analysed for 532 individual seals facing severe mortality risks and involved in 698 of the 885 intervention events.Study and other actions tested
A study in 2012–2013 in the Shannon Estuary, western Ireland (O'Brien et al. 2014) found that a stranded common bottlenose dolphin Tursiops truncatus that was rescued and released survived for over a year and was observed with a calf. The rescued adult female dolphin survived for at least 482 days after being released at sea in June 2012. She was observed with a dependent calf on 18 occasions in May–September 2013. The dolphin (3.5 m long) was found stranded on a beach during a receding tide on 1 June 2012. Wet towels were used to cool the dolphin. A transport box attached to a tractor was used to move the dolphin back into the water where she was directed by hand to the open sea. The rescue took 70 minutes to complete and the dolphin was observed swimming away. Researchers on board tour boats recorded sightings of the dolphin within the estuary during daily photo-identification surveys in 2012–2013.Study and other actions tested
A study in 2010–2012 of a pelagic area in the North Atlantic Ocean, near Cape Cod, USA (Sharp et al. 2016) found that more than half of stranded and rescued dolphins Delphinidae spp. released back into the sea survived for at least 3–11 weeks. Eighteen of 34 dolphins were successfully tracked for 21–79 days after release and travelled an average of 1,395 km during that time. Eight dolphins were tracked for 6–14 days before contact was lost with their transmitters. Eight dolphins were tracked for only one day (one of which re-stranded and died). In 2010–2012, a total of 34 dolphins of three species (28 short-beaked common dolphins Delphinus delphis, five Atlantic white-sided dolphins Lagenorhynchus acutus, one long-finned pilot whale Globicephala melas) were rescued during 33 stranding events. Dolphins were provided with medical treatment at the stranding site before being satellite-tagged and released, either singly or in groups. Health assessments and blood tests were carried out prior to release. The 34 dolphins were tracked for 1–79 days after release in 2010–2012.Study and other actions tested
Referenced paperSharp S.M., Harry C.T., Hoppe J.M., Moore K.M., Niemeyer M.E., Robinson I., Rose K.S., Sharp W.B., Landry S., Richardson J. & Moore M.J. (2016) A comparison of postrelease survival parameters between single and mass stranded delphinids from Cape Cod, Massachusetts, U.S.A. Marine Mammal Science, 32, 161-180.
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This Action forms part of the Action Synopsis:Marine and Freshwater Mammal Conservation
Marine and Freshwater Mammal Conservation - Published 2021
Marine and Freshwater Mammal Synopsis