Install exclusion and/or escape devices for mammals on fishing nets

How is the evidence assessed?
  • Effectiveness
    not assessed
  • Certainty
    not assessed
  • Harms
    not assessed

Source countries

Key messages

  • Seven studies evaluated the effects on marine mammals of installing exclusion and/or escape devices on fishing nets. Four studies were in the Indian Ocean (Australia, Tasmania) and/or Tasman Sea (Tasmania) and three studies were in the South Atlantic Ocean (South Georgia).

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (2 STUDIES)

  • Survival (2 studies): One study in the Indian Ocean found that less than one third of common bottlenose dolphins exited escape hatches on trawl nets alive. One replicated study in the Tasman Sea and Indian Ocean found that fewer fur seals died in exclusion devices with large escape openings than in those with small openings.

BEHAVIOUR (0 STUDIES)

OTHER (5 STUDIES)

  • Reduction in entanglements/unwanted catch (5 studies): Three studies (including two controlled studies) in the South Atlantic Ocean and Indian Ocean found that installing exclusion and/or escape devices on trawl nets reduced the number of trapped or entangled Antarctic fur seals and common bottlenose dolphins. One before-and-after study in the Indian Ocean found that installing exclusion and escape devices on trawl nets reduced common bottlenose dolphin entanglements for three of four fishing vessels. One study in the South Atlantic Ocean found that modifying an exclusion and escape device by enlarging and relocating the escape panel resulted in fewer Antarctic fur seal entanglements.

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 controlled study in 2004 of a pelagic site in the South Atlantic Ocean, South Georgia (Hooper et al. 2005) reported that installing exclusion and escape devices on trawl nets resulted in fewer Antarctic fur seal Arctocephalus gazella entanglements compared to when no devices were installed. Results are not based on assessments of statistical significance. Fewer seals were entangled when exclusion and escape devices were installed within trawl nets (total 28 seals; 0.2 seals/trawl) than when no devices were installed (total 157 seals, 1.9 seals/trawl). A commercial fishing vessel (‘Top Ocean’, USA) trawling for krill (Euphausiacea) with two nets carried out 118 trawls with exclusion and escape devices on both nets, and 81 trawls without devices installed. Exclusion and escape devices on each net were modified throughout the study (see original paper for details). The final design had an inclined mesh barrier (140 mm mesh size) within the net, 1–3 escape openings (1–1.6 m diameter) in the roof of the net, and a large mesh barrier (14 x 12 m) inserted 47 m from the mouth of the net. Seal entanglements were recorded by fishers (30 trawls) and an independent observer (169 trawls) in June–July 2004.

    Study and other actions tested
  2. A study in 2004 of a pelagic site in the South Atlantic Ocean, South Georgia (Hooper et al. 2005) reported that installing two designs of exclusion device on trawl nets resulted in fewer Antarctic fur seal Arctocephalus gazella entanglements compared to when no device was installed. Results are not based on assessments of statistical significance. Fewer seals were entangled in trawl nets with a mesh barrier (total 5 seals, 0.8 seals/trawl) or a mesh ‘bag’ (total 2 seals, 0.06 seals/trawl) installed at the mouth of the net compared to when no exclusion device was installed (total 76 seals, 1.4 seals/trawl). The mesh barrier also reduced target fish catches (data not reported). A commercial fishing vessel (‘InSung Ho’, Republic of Korea) trawling for krill (Euphausiacea) carried out six trawls with a mesh barrier, 42 trawls with a mesh ’bag’ and 55 trawls without an exclusion device installed. The mesh barrier comprised two mesh panels (44 x 20 m; 240 mm mesh size) attached to the head rope and ground rope at the mouth of the trawl net. The mesh ‘bag’ comprised one mesh panel (240 mm mesh size) attached to the mouth of the trawl net and extending 20 m into the body of the net to form a large ‘bag’. Seal entanglements were recorded by fishers (eight trawls) and an independent observer (95 trawls) in August 2004.

    Study and other actions tested
  3. A study in 2004 of a pelagic site in the South Atlantic Ocean, South Georgia (Hooper et al. 2005) reported that modifying an exclusion and escape device by enlarging and relocating the escape panel resulted in fewer Antarctic fur seal Arctocephalus gazella entanglements. Results are not based on assessments of statistical significance. A total of 11 seals (6 seals/trawl) were entangled in trawl nets with an exclusion device angled towards a small escape panel in the roof of the net. However, after the exclusion device was angled towards a larger escape panel in the floor of the net, no seals were found entangled. A commercial fishing vessel (‘Atlantic Navigator’, Vanuatu) carried out a total of 15 trawls for krill (Euphausiacea) with a sloping metal grid angled towards an escape panel within the trawl net. In the first two trawls, a smaller escape panel (size not reported) was located within the roof of the net. In the following 13 trawls, the escape panel was larger (size not reported) and located in the floor of the net. During trawls, the net was kept at fishing depths for long periods and a pump used to remove krill. Seal entanglements were recorded by fishers (two trawls) and an independent observer (13 trawls) in June–July 2004.

    Study and other actions tested
  4. A controlled study in 2005–2006 of a pelagic area in the Indian Ocean, Western Australia (Stephenson et al. 2006; same study area as Allen et al. 2014) reported that trawl nets with exclusion and escape devices installed had fewer trapped common bottlenose dolphins Tursiops truncatus than those without devices, but some dolphins exited the net dead or in distress. Results are not based on assessments of statistical significance. Eight dolphins/1,000 trawls were trapped in nets with exclusion devices, whereas 15–22 dolphins/1,000 trawls were trapped in nets without exclusion devices. Three of seven dolphins were observed exiting nets alive, through the escape opening or the mouth of the net. The other four dolphins died or were in distress and fell through the escape opening dead. Six commercial fishing vessels deployed trawl nets with exclusion devices (2006: total 1,156 trawls) and without (2005: 659 trawls; 2006: 229 trawls). Exclusion devices were semi-flexible metal grids (1.2 x 2 m; 15.5 cm vertical bar spacing) attached to the start of the net extension, 10 m from the end of the net, with an escape opening below. Onboard observers recorded trapped dolphins during each haul in 2005–2006. Underwater video cameras recorded the activity of seven dolphins in trawl nets with exclusion devices in 2006.

    Study and other actions tested
  5. A before-and-after study in 2003–2009 of a pelagic area in the Indian Ocean, Western Australia (Allen et al. 2014; same study area as Stephenson et al. 2006) found that installing exclusion and escape devices on trawl nets reduced common bottlenose dolphin Tursiops truncatus entanglements for three of four fishing vessels. Dolphin entanglement rates reported by skippers were significantly lower for three of four fishing vessels after exclusion and escape devices were installed (2.4–6.8 dolphins/1,000 trawls) than before (7.1–11.3 dolphins/1,000 trawls). The difference was not significant for the other vessel (after: 5.6 dolphins/1,000 trawls; before: 5.1 dolphins/1,000 trawls). Exclusion and escape devices were introduced to a trawl fishery in March 2006. A semi-flexible metal grid with vertical bars was fitted on trawl nets, either just before the ‘cod-end’ or at the start of the net extension. The grid was angled towards a bottom-opening escape hatch. Numbers of entangled dolphins were extracted from skippers’ logbooks for periods before (August 2003–March 2006; total 11,168 trawls) and after (March 2006–September 2009; total 16,736 trawls) exclusion and escape devices were fitted.

    Study and other actions tested
  6. A replicated study in 2006–2007 of three pelagic sites in the Tasman Sea and Indian Ocean, Tasmania, Australia (Lyle et al. 2015) found that exclusion devices on trawl nets with large escape openings had lower fur seal Arctocephalus spp. mortality than those with small escape openings. Fewer fur seals died in exclusion devices with large escape openings (6 of 90 seals, 7%) than in exclusion devices with small escape openings (14 of 56 seals, 25%). Midwater trawls were carried out by a commercial fishing vessel at each of three sites using exclusion devices with a small escape opening (1 m2; total 30 trawls) or large escape opening (1.9 m2; total 48 trawls). Exclusion devices had two vertical steel grids (2.3 m2) angled forwards with an escape opening at the base. An underwater video camera recorded behaviour and mortality of seals within the exclusion devices during each of the 78 trawls in 2006–2007.

    Study and other actions tested
  7. A study in 2012 of a pelagic area in the Indian Ocean, Western Australia (Wakefield et al. 2017) found that less than 30% of common bottlenose dolphins Tursiops truncatus that entered exclusion and escape devices on trawl nets escaped alive through hatches. Two of seven dolphins that entered exclusion and escape devices on trawl nets escaped alive through an escape hatch in the roof of the net within 18 seconds and five minutes. The five other dolphins were retained at the grid of the exclusion device, one of which died and was expelled through an escape hatch. The seven dolphins were recorded interacting with exclusion and escape devices during five of 774 day-trawls carried out by a commercial fishery targeting groundfish. Exclusion and escape devices were installed between the body and ‘cod-end’ extension panel of each trawl net. The devices consisted of a steel grid angled either up or down towards an escape hatch and/or slit in the roof or floor of the net. Underwater video cameras recorded dolphins within the nets during each of the five trawls in June–September 2012.

    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.

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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

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