Remove derelict fishing gear from mammals found entangled

Overall effectiveness category Beneficial
Number of studies: 2
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How is the evidence assessed?

Effectiveness

70%
Certainty

70%
Harms

10%

Calculating overall effectiveness category

Background information and definitions

Abandoned, lost or otherwise discarded fishing gear (or ‘ghost’ gear) is a major threat to marine and freshwater mammals. Mammals may become entangled in ‘ghost’ gear, such as nets, lines and ropes resulting in injury or death (Stelfox et al. 2016). Attempts may be made to remove derelict gear from mammals found entangled to improve survival. This may require specialist techniques, tools and training. Injuries or wounds caused by entanglement may also require treatment. Evidence is summarised below for studies that removed derelict gear from mammals in the wild. For studies that removed fishing gear from mammals in captivity as part of rehabilitation, see Rehabilitate and release injured, sick or weak marine and freshwater mammals.

Stelfox M., Hudgins J. & Sweet M. (2016) A review of ghost gear entanglement amongst marine mammals, reptiles and elasmobranchs. Marine Pollution Bulletin, 111, 6–17.

Study locations

Key messages

Two studies evaluated the effects of removing derelict fishing gear from mammals found entangled. One study was in the North Pacific Ocean (USA) and one in the North Atlantic Ocean (USA).

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (2 STUDIES)

Reproductive success (1 study): One review in the North Pacific Ocean found that after removing derelict fishing gear from Hawaiian monk seals, along with at least seven other interventions to enhance survival, more than a quarter of the seals reproduced.
Survival (2 studies): One review in the North Pacific Ocean found that removing derelict fishing gear from Hawaiian monk seals, along with at least seven other interventions to enhance survival, resulted in more than a quarter of the seals surviving. One review in the North Atlantic Ocean found that three common bottlenose dolphins survived for at least 1–4 years after they were disentangled from derelict fishing gear and released.

BEHAVIOUR (0 STUDIES)

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

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 removing derelict fishing gear from 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 removing derelict fishing gear 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: removal of derelict fishing gear from seals (275 events); translocation (284 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
Referenced paper
Harting A.L., Johanos T.C. & Littnan C.L. (2014) Benefits derived from opportunistic survival-enhancing interventions for the Hawaiian monk seal: the silver BB paradigm. Endangered Species Research, 25, 89-96.
A review of three case studies in 2003–2010 in the North Atlantic Ocean, USA (Wells et al. 2013) found that three common bottlenose dolphins Tursiops truncatus entangled in derelict fishing gear that were rescued and released survived for at least 1–4 years. All of three rescued and disentangled dolphins (including one calf) were successfully tracked for 365–1,541 days after release. The dolphins (two males, one female calf) were found entangled in derelict fishing gear in 2003, 2006 and 2008. They were disentangled, treated, transported to appropriate habitats, and released immediately. All three 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
Referenced paper
Wells R.S., Fauquier D.A., Gulland F.M.D., Townsend F.I. & DiGiovanni R.A. (2013) Evaluating post-intervention survival of free-ranging odontocete cetaceans. Marine Mammal Science, 29, 463-483.

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?

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This Action forms part of the Action Synopsis:

Marine and Freshwater Mammal Conservation

Related Actions

Actions	Effectiveness	Category
Recover lost or discarded fishing gear Action Link	No evidence found (no assessment)	Synopsis Link
Improve methods for locating fishing gear Action Link	No evidence found (no assessment)	Synopsis Link
Use biodegradable fishing gear Action Link	No evidence found (no assessment)	Synopsis Link