Translocate marine and freshwater mammals to re-establish or boost native populations

How is the evidence assessed?
  • Effectiveness
    50%
  • Certainty
    50%
  • Harms
    15%

Study locations

Key messages

  • Four studies evaluated the effects of translocating marine mammals to re-establish or boost native populations. The four studies were in the North Pacific Ocean (USA).

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (4 STUDIES)

  • Reproductive success (2 studies): One replicated study and one review in the North Pacific Ocean found that after translocating Hawaiian monk seals, along with rehabilitation or at least seven other interventions to enhance survival, more than a quarter of the seals reproduced.
  • Survival (4 studies): Two studies (including one replicated and one controlled study) in the North Pacific Ocean found that 50–83% of translocated, and 52% of rehabilitated and translocated, Hawaiian monk seal pups survived for at least one year. One of the studies and one review in the North Pacific Ocean found that translocated seal pups had similar survival rates to non-translocated pups born at release sites or greater survival rates than non-translocated pups remaining at the original site. One review in the North Pacific Ocean found that translocating Hawaiian monk seals, along with at least seven other interventions to enhance survival, resulted in more than a quarter of the seals surviving.

BEHAVIOUR (1 STUDY)

  • Behaviour change (1 study): One review in the North Pacific Ocean found that translocated Hawaiian monk seal pups had similar dispersal times to non-translocated seal pups born at release sites.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A review of multiple translocations in 1994–2009 in the North Pacific Ocean, Hawaii, USA (Baker et al. 2011) found that translocated Hawaiian monk seal Monachus schauinslandi pups had similar survival rates and dispersal times to non-translocated seal pups born at release sites. The first-year survival rate of 161 translocated seal pups (45%) was similar to that of non-translocated seal pups born at release sites (43%). The average minimum time between weaning and dispersal of seal pups to other sites was also reported to be similar for 72 translocated pups (43 days) and non-translocated pups born at release sites (data not provided). Hawaiian monk seal pups were translocated between islands in 1994–2009 to reduce the risk of shark predation and male aggression, or to be fostered. Survival was estimated for 291 pups (161 translocated; 130 non-translocated) born in 1997 and 2001–2008. Dispersal times were estimated for 72 seal pups translocated in 1994–2009 and non-translocated pups born at release sites (number not reported). All translocations were part of a long-term research programme. Seal populations were monitored during annual field camps for 2–5 months in the spring and summer in 1994–2009.

    Study and other actions tested
  2. A replicated study in 1984–2005 of multiple sites on islands in the North Pacific Ocean, Hawaii, USA (Gilmartin et al. 2011) found that nearly all translocated Hawaiian monk seal Monachus schauinslandi young, and approximately half of rehabilitated and translocated monk seal young, survived for at least one year after release and some reproduced. Five of six translocated monk seal young (83%) were known to survive for at least one year. Thirty-five of 68 rehabilitated and translocated monk seal young (52%) were known to survive for at least one year after release, 18 of which reproduced in the wild (at least 68 pups in 1984–2005). Thirty other rescued monk seal young died in captivity (17 seals) or were kept permanently in captivity for health or behavioural reasons (13 seals). In 1984–1995, a total of 104 weaned, female seals (aged <3 years old) that were underweight, ill or threatened (by human disturbance, shark predation or aggressive adult male seals) were translocated directly to new sites (six seals) or brought into captivity for 3–14 months before release at new sites (98 seals). Captive seals were given medical treatment and fed milk formula or fish with multivitamins. The seals were transported between islands by plane or ship, and either released immediately or held in beach enclosures before release. All released seals were tagged and observed annually in 1984–2005.

    Study and other actions tested
  3. 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 translocating 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 translocation and the other interventions carried out. The 139 surviving seals (including 71 females) produced at least 147 pups, which 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: translocation (284 events); removal of derelict fishing gear from seals (275 events); rescue of stranded or trapped seals (37 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
  4. A controlled study in 2008–2011 at two islands in the North Pacific Ocean, Hawaii, USA (Norris et al. 2017) reported that at least half of translocated Hawaiian monk seal Neomonachus schauinslandi pups survived their first year, and survival rates were greater than those of non-translocated pups remaining at the original site. Results are not based on assessments of statistical significance. At least six of 12 seal pups (50%) survived to one year of age after translocation. Survival of translocated seal pups was higher than that of non-translocated seal pups remaining at the original site (11 of 36 pups, 31%). However, the authors state that survival estimates may not be reliable due to small sample sizes and low survey effort at the release site (<1% of that at the original site). In August 2008 and 2009, twelve newly weaned seal pups (average 78 days old) were translocated 450 km to an island with better foraging conditions to improve their chances of survival. Attempts were made to re-sight the 12 translocated seal pups and 36 non-translocated seal pups of the same age during surveys. Biannual surveys were carried out at the release site during a total of 12 days in 2009–2011. Surveys of non-translocated pups were carried out at the original site in 2009–2011 (details not reported).

    Study and other actions tested
Please cite as:

Berthinussen, A., Smith, R.K. and Sutherland, W.J. (2021) Marine and Freshwater Mammal Conservation: Global Evidence for the Effects of Interventions. Conservation Evidence Series Synopses. University of Cambridge, Cambridge, UK.

Where has this evidence come from?

List of journals searched by synopsis

All the journals searched for all synopses

Marine and Freshwater Mammal Conservation

This Action forms part of the Action Synopsis:

Marine and Freshwater Mammal Conservation
Marine and Freshwater Mammal Conservation

Marine and Freshwater Mammal Conservation - Published 2021

Marine and Freshwater Mammal Synopsis

What Works 2021 cover

What Works in Conservation

What Works in Conservation provides expert assessments of the effectiveness of actions, based on summarised evidence, in synopses. Subjects covered so far include amphibians, birds, mammals, forests, peatland and control of freshwater invasive species. More are in progress.

More about What Works in Conservation

Download free PDF or purchase
The Conservation Evidence Journal

The Conservation Evidence Journal

An online, free to publish in, open-access journal publishing results from research and projects that test the effectiveness of conservation actions.

Read the latest volume: Volume 18

Go to the CE Journal

Discover more on our blog

Our blog contains the latest news and updates from the Conservation Evidence team, the Conservation Evidence Journal, and our global partners in evidence-based conservation.


Who uses Conservation Evidence?

Meet some of the evidence champions

Endangered Landscape Programme Red List Champion - Arc Kent Wildlife Trust The Rufford Foundation Save the Frogs - Ghana Bern wood Supporting Conservation Leaders National Biodiversity Network Sustainability Dashboard Frog Life The international journey of Conservation - Oryx British trust for ornithology Cool Farm Alliance UNEP AWFA Butterfly Conservation People trust for endangered species Vincet Wildlife Trust